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Taxonomy of the weed species of the genus Echinochloa
(Poaceae, Paniceae) in Southwestern Europe:
Exploring the confused current state of affairs
Ivan Hoste1, Filip Verloove1
1Meise Botanic Garden, Nieuwelaan 38, B-1860 Meise, Belgium
Corresponding author: Ivan Hoste (ivan.hoste@botanicgardenmeise.be)
Academic editor: Marcin Nobis|Received 17 December 2021|Accepted 5 April 2022|Published 23 May 2022
Citation: Hoste I, Verloove F (2022) Taxonomy of the weed species of the genus Echinochloa (Poaceae, Paniceae) in
Southwestern Europe: Exploring the confused current state of aairs. PhytoKeys 197: 1–31. https://doi.org/10.3897/
phytokeys.197.79499
Abstract
e taxonomy of Echinochloa, a predominantly tropical to warm-temperate genus of 40–50 species, in-
cluding some of the world’s worst weeds, is still poorly understood. is is because some species, including
the extremely widespread E. crus-galli, show a wide range of morphological, physiological and ecological
variation, in part the result of a complex recent evolutionary history. Furthermore, there is often a dearth
of clear distinguishing features among species. e same applies to the species established in Southwestern
Europe, where unintentionally introduced populations have now established themselves as important
weeds of crops, especially maize and rice. Taxonomic and nomenclatural confusion hampers progress in
weed science. In this study, we give an identication key that covers the weedy taxa encountered in South-
western Europe, followed by notes on taxonomy and nomenclature. Moreover, a lectotype is designated
for Echinochloa frumentacea. It is argued that current confusion cannot be overcome without including
populations of Eastern Asian origin in taxonomic studies and without the joint eorts of experts in the
elds of weed science, morphology-based taxonomy, genomics and phylogenetics.
Keywords
Echinochloa, evolutionary history, lectotypication, nomenclature, Poaceae, Southwestern Europe, taxonomy
Copyright Ivan Hoste & Filip Verloove. This is an open access article distributed under the terms of the Creative Commons Attribution License
(CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
PhytoKeys 197: 1–31 (2022)
doi: 10.3897/phytokeys.197.79499
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Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
2
Introduction
Echinochloa P. Beauv. is a predominantly tropical to warm-temperate genus of 40–50
species that are usually associated with wet or damp places (Michael 2003). Echinochloa
crus-galli (L.) P. Beauv., by far the most widespread species of the genus, is among the
worst weeds worldwide (Holm et al. 1977). However, in Western and Southern Europe,
E. crus-galli is not the only troublesome species of this genus of Poaceae. Together
with several other C4 grasses of the genera Digitaria Haller, Panicum L. and Setaria P.
Beauv., Echinochloa muricata (P. Beauv.) Fernald, too, has become a widespread weed,
especially in maize elds, in the past few decades (Jauzein and Montégut 1983; Scholz
1995; Hoste 2004). Furthermore, the spread of a few additional taxa of Asian origin in
rice elds in the Mediterranean area has increasingly challenged the identication skills
of botanists and agronomists (Jauzein 1993; Viggiani and Tabacchi 2017; Martínez-
Azorín and Crespo 2021).
e taxonomy of Echinochloa is still poorly understood, resulting in strongly di-
verging interpretations of its classication and nomenclature. ese divergent inter-
pretations can be attributed to several reasons, such as the wide range of within-species
variation (not in the least in ill-dened and polymorphic E. crus-galli), the recurrent
absence of unequivocal qualitative and quantitative distinguishing features among spe-
cies, insucient joint research by taxonomists and agronomists and the often extended
lag time between the introduction of an exotic taxon in a new geographic region and
its detection and correct identication by local botanists and weed scientists. As a result
of the description of numerous taxa with probably little or no taxonomic value, quite
a few species may be overvalued.
In Southwestern Europe, taxonomically widely divergent treatments of the genus
Echinochloa are available for the British Isles (Hubbard 1968; Cope and Gray 2009;
Stace 2019), the Netherlands (Duistermaat 2020), Belgium (Lambinon and Verloove
2012; Verloove 2021), France (Jauzein 1993, 1995; Tison and de Foucault 2014),
Central Europe (Conert 1998; Parolly and Rohwer 2019), the Iberian Peninsula
(Martínez-Azorín and Crespo 2021) and Italy (Pirola 1965; Pignatti 1982; Viggiani et
al. 2003; Ban 2017; Viggiani and Tabacchi 2017). Nothing better illustrates the con-
fusion and changing views on taxonomy and nomenclature of Echinochloa in South-
western Europe during the past half-century than the ve references given for Italy.
e tangled web of confusion is also revealed in a quote by Tabacchi et al. (2006)
about Early watergrass (E. oryzoides) as “never been reported before in Italy,” whereas
the species was described (as Panicum oryzoides) on the basis of material that was in all
likelihood collected in Italy (see below).
In an overview of the weedy species of Echinochloa in Southwestern Europe, Car-
retero (1981) concentrated on presence in rice elds in Italy, Southern France, Spain
and Portugal. He mentioned two indigenous species, E. crus-galli and E. colona (L.)
Link, plus three introduced taxa of Asian origin. Two decades later, Costea and Tardif
(2002), in a paper on “the most common weedy European Echinochloa species,” never
mentioned E. muricata. However, by then, this American species had been recorded
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 3
as a weed from Camargue, France, and Jauzein (1993) urged botanists to be watchful
of E. muricata, which he warned had recently started spreading quickly in other parts
of France. Unfortunately, to date, this species has hardly ever been mentioned in bo-
tanical and weed science papers dealing with Southern Europe: although the species is
denitely not common, it may have been overlooked. ere is also doubt about its sta-
tus in the British Isles. Cope and Gray (2009) claimed that some races of E. crus-galli
“have been considered worthy of recognition at species level, but there is no general
agreement on this.” Echinochloa muricata is not included in the keys by Stace (2019),
yet the author observed that some specimens keying out as E. crus-galli would belong
to E. muricata subsp. microstachya (Wiegand) Jauzein.
Distinguishing between American E. muricata and European E. crus-galli
based on morphology is relatively easy, yet separating the latter from persistent and
morphologically variable Echinochloa introduced from Asia and today thriving in rice
elds in Southern Europe proves much more dicult. e contrasting treatments of
Echinochloa in Japanese (Ohwi 1965; Ibaragi 2020) and Chinese (Shouliang 1990;
Shouliang and Phillips 2006) oras only accentuates the confusion.
To develop superior control methods in crops, including rice and maize, basic
knowledge of the classication, morphology, physiology and ecology of specic weeds
is essential (Yabuno 1983). Recent advances in molecular techniques have created
new opportunities to study the weedy species of genus Echinochloa. New research
combining morphological and molecular data has been undertaken with the aim to
better understand the species’ classication and establish useful morphological traits
that allow weed scientists and farmers to reliably identify the dierent taxa. To date, it
has been shown that E. muricata and E. crus-galli are clearly distinct (Claerhout et al.
2016); however, studies dealing with the taxa of Asian origin and specically aspiring
to integrate morphological and molecular data have, so far, yielded only limited
success (e.g., Yasuda et al. 2002; Yamaguchi et al. 2005; Ruiz-Santaella et al. 2006;
Tabacchi et al. 2006; Aoki and Yamaguchi 2008; Lee et al. 2014a, 2014b; Ye et al.
2014; Yasuda and Nakayama 2019). Often with E. oryzicola (Vasinger) Vasinger as the
exception, matching the data from genetic research with the multitude of names and
descriptions from the morphology-based literature remains ridden with diculties.
Nomenclatural confusion resulting in the same name being applied to dierent taxa
in dierent studies is a source of uncertainty and may render the interpretation of
published research results precarious, especially when no herbarium specimens have
been deposited (Yamaguchi et al. 2005). Moreover, the naming of specimens based on
the two dierent and widely diverging identication keys from Carretero (1981) and
Pignatti (1982) has also not been helpful to link molecular data with morphology-
based taxa (Tabacchi et al. 2006; Kaya et al. 2014). Claerhout et al. (2016) warned that
using incorrectly identied seeds accessed from institutes or companies in experiments
is a potential source of errors. is probably explains the position of ‘E. muricata’
among a cluster of E. crus-galli accessions in the phylogenetic tree proposed by Lee et
al. (2016; g. 2). For the same reason, an accession from a Spanish rice eld (Seville)
identied as ‘E. crus-pavonis’ (Ruiz-Santaella et al. 2006) seems doubtful as this species
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
4
is not mentioned by Martínez-Azorín and Crespo (2021) and is probably not present
as a weed in rice elds anywhere in Southern Europe (Michael 1983).
Morphology-based distinguishing traits frequently used in keys and descriptions
often nd no conrmation in molecular data. An attempt to bridge the gap with a
modied “simple and eective morphological key” (Tabacchi et al. 2006) was not con-
vincing and has been replaced later with a highly modied version (Viggiani and Ta-
bacchi 2017). Most of the authors dealing with the problem declare a stalemate and
put their hopes in future research. With this paper, we do not have the ambition to
resolve the taxonomic and nomenclatural puzzle posed by Echinochloa in Southwestern
Europe. Instead, our goal is twofold. On the one hand, we present a provisional key
that makes it possible to identify the weedy species occurring in Southwestern Europe
(from the British Isles to Portugal and Italy); critical comments are added to explain
our choice of accepted taxa. On the other hand, we wonder why matching the results
of recent molecular studies on Echinochloa in Europe and the Far East with those ob-
tained by morphological research is so problem-ridden. e current variation of taxa
in the genus Echinochloa, including some that were recently inadvertently introduced
to Southwestern Europe, is partly the result of a complex evolutionary history, the
traces of which are visible in the morphological and genetic characteristics of currently
existing taxa. We include the timescales of both geological epochs and human history
to frame the future study of the taxonomy and phylogeny of weedy Echinochloa in
Southwestern Europe.
Result
An identification key for the species of Echinochloa in Southwestern Europe
Identication keys for Echinochloa in oras or weed science papers are often restricted
to a rather small geographical area. Covering a larger area and more taxa may lead to
more attention being paid to taxa which, so far, could have been overlooked. As far as
the reviewed European literature is concerned, this paper is mainly restricted to South-
western Europe, roughly stretching from the British Isles in the north to the Iberian
Peninsula and Italy in the south. e key should, however, prove useful to identify
the established weedy species of the genus Echinochloa in most of Europe. Owing to
nomenclatural and taxonomic uncertainties, the key is considered provisional; for a
dierent recent interpretation, see Martínez-Azorín and Crespo (2021).
A number of rare casuals that have been reported from Europe in the past, for in-
stance, as wool aliens, have been omitted. ese include Echinochloa inundata Michael
& Vickery and E. jubata Stapf from Belgium (Verloove 2021), E. turneriana (Domin)
J.M.Black from Germany (Conert 1998) and E. crus-pavonis (Kunth) Schult., E. py-
ramidalis (Lam.) Hitchc. & Chase (a perennial species) and E. turneriana from Great
Britain and Ireland (Ryves et al. 1996; Reynolds 2002). Adding these species – the
exact identity of some of which requires conrmation – would have made the key
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 5
unnecessarily dicult. Moreover, there is currently no indication for these ephemerals
establishing as troublesome weeds in crops.
Echinochloa crus-pavonis has been excluded from the key since the records from
rice elds in Southern Europe seem to be based on erroneous identications (Ban
2017); the photographs given by Viggiani et al. (2003, pages 242–243) show a form
of E. crus-galli s.l.
ose who run into problems when using the key given below or suspect they are
dealing with a species missing from the key are referred to the keys to the annual and
perennial species of Echinochloa produced by P.W. Michael (1983), with updates, in-
cluding those from Michael (2019). In Europe, the known weedy species are all annuals.
In combination with the wide variation within individual species, the dearth of
strong qualitative and quantitative features precludes easy identication in the genus
Echinochloa. Within the same inorescence, the spikelets may show considerable vari-
ation. e number, size, position and direction of hairs and bristles is often strongly
inuenced by competition for space among the closely packed spikelets. e length of
the lower glume and the shape of the sterile lemma (occasionally part of them shiny
and convex) can be assessed only by examining several spikelets. e length of the
spikelet – excluding the awn of the sterile lemma – is an important feature (Michael
1983; Jauzein 1993). Especially when awned or having an elongated tip, measuring
the length of the spikelet may prove dicult as deciding where the spikelet passes into
the awn is rather arbitrary. e presence of spikelets in which the upper glume has an
elongated tip or a short awn (as sometimes occurs in several taxa) renders a correct
measurement more uncertain.
1 Fertile oret not disarticulating at maturity. Spikelets unawned. Fertile oret and
caryopsis markedly humped. Inorescence compact, usually contracted and with
the axis often hardly visible, sometimes with spreading branches (Fig. 1) ..........2
1' Fertile oret disarticulating at maturity. Spikelets awned or not. Fertile oret and
caryopsis not markedly humped. Inorescence not strongly contracted when fully
developed, with the axis showing through (but compare with clearly dierent E.
muricata var. wiegandii when in doubt) .............................................................3
2 Spikelets dark brownish or purplish at maturity (Fig. 2), ca. 3–4 mm long. Cary-
opsis brownish .................................................................................E. esculenta
2' Spikelets pale (yellowish or greenish) at maturity (Fig. 1), ca. 3–3.5 mm long.
Caryopsis whitish ....................................................................... E. frumentacea
3 Spikelets < 3 mm long and lower glume ca. 1/2 length of the spikelet, which
is always unawned. Axis of the inorescence branches (almost) without bristles
(except at the base). Inorescence without secondary branches (Fig. 3). Leaves
narrow, usually not exceeding 6 mm. Caryopsis whitish ....................... E. colona
3' Spikelets usually ≥ 3 mm long, awned or not. (If spikelet < 3 mm, then lower
glume only ca. 1/3 length of the spikelet.) Axis of the inorescence branches with
bristles. Inorescence often with secondary branches. Leaves usually wider. Cary-
opsis usually darker, yellowish or brownish .......................................................4
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
6
4 Spikelets ≥ 4 mm long and at least some spikelets with lower glume up to 2/3
length of the spikelet (Fig. 4D). Mature inorescence more or less erect (Fig. 5).
Spikelets unawned or with an awn up to 20 mm long. Caryopsis 2–2.4 mm long.
Embryo at least 0.75 to over 0.9 length of the caryopsis. (An obligate weed of
rice.) ................................................................................................E. oryzicola
4' Spikelets ≥ 4 mm long and lower glume not longer than 1/2 length of the spikelet.
Mature inorescence drooping (Fig. 6). Spikelets usually awned, with an awn up
to 50 mm long. Caryopsis 2.2–2.8 mm long. Embryo 0.65–0.75(–0.85) length
of the caryopsis. (An obligate weed of rice.) ..............E. crus-galli var. oryzoides
4" Spikelets ≤ 4 mm long and lower glume usually clearly less than 1/2 length of
the spikelet. (If spikelets > 4 mm, see 6, E. muricata var. muricata. Solely a rare
casual?)..............................................................................................................5
5 Lemma of the fertile oret with a membranous tip that is clearly dierentiated
from the coriaceous body of the lemma (Fig. 7A); the membranous tip demarcat-
ed from the coriaceous body by a line of minute hairs (the latter, however, not or
hardly visible with a hand lens). Palea of the fertile oret with a blunt, soft, frayed
looking, usually strongly recurved tip (Fig. 7B). Spikelets unawned or awned; awn
length extremely variable (up to 40 mm long or more). e leaf subtending the
distal inorescence with the demarcation between blade and sheath more or less
semicircular or forming a slightly elongated upside-down U; blade usually patent
from the base. (A complex taxon with several dicult-to-distinguish intergrading
forms that are not keyed out here; see comments below.) ................. E. crus-galli
5' Lemma of the fertile oret with a sti, smooth tip, not clearly dierentiated from the
coriaceous body of the lemma (Fig. 8A). Palea of the fertile oret with a sti, (nearly)
straight tip; the tip (in mature orets!) appressed against the lemma (Fig.8B). Spike-
lets unawned or awned, with the awn usually shorter than 10 mm (but longer in the
rare var. muricata). e leaf subtending the distal inorescence with the demarcation
between blade and sheath forming an elongated upside-down U (Fig. 9); blade sti
upright (esp. when short) or recurved higher up (E. muricata) ............................. 6
6 Spikelets ≤ 3.5 mm long, with strongly spreading papilla-based bristles (which
give the spikelet a rugged appearance), unawned or at most with an elongated tip
(Fig. 10). Tip of both the lemma and palea of the fertile oret short. Inorescence
often large (not uncommonly > 20 cm long), when mature with widely spreading
lower branches ....................................................E. muricata var. microstachya
6' Spikelets ≤ 3.5 mm long; the papilla-based bristles not strongly spreading. Nu-
merous spikelets in the inorescence with a short awn (sometimes up to ca. 10
mm) (Fig. 11). Tip of the palea of the fertile ower ne and elongated, tting
with the elongated tip of the lemma. Inorescence usually smaller, its branches
usually not spreading when mature .............................................. var. wiegandii
6" Spikelets ≥ 3.5 mm long; numerous spikelets in the inorescence with a longer
awn (up to 16 mm). (Apart from the presence of awns, the rugged spikelets look
like a more robust version of var. microstachya.) (Probably only a rare casual.) .....
..................................................................................................... var. muricata
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 7
Notes on the species included in the key
Echinochloa colona (L.) Link, Hort. Berol. 2: 209. 1833.
Basionym. Panicum colonum L., Syst. Nat. (ed. 10) 2: 870. 1759.
Type. LINN-80.23 (lectotype, designated by Hitchcock 1908). Image available at
http://linnean-online.org/1255/.
Remarks. Echinochloa colona is usually easy to identify, yet care should be taken
to distinguish it from forms with small spikelets of E. crus-galli (Martínez-Azorín and
Crespo 2021). In the Mediterranean region, it occurs as a persistent weed in crop
elds; elsewhere, it has only been recorded as a usually ephemeral alien.
Echinochloa crus-galli (L.) P. Beauv., Ess. Agrost. 1: 53, 161, 169, pl. 11, f. 2. 1812.
Basionym. Panicum crus-galli L., Sp. Pl. 1: 56. 1753.
Type. Herb. Burser 1: 303, sine dato (UPS).
Notes. ere has always been a great deal of confusion about the type; see, e.g.,
Hitchcock (1908) or Gould et al. (1972). In fact, all original material came from North
America and belongs to E. muricata; in 1753 E. crus-galli, from Eurasia, was not yet a
widespread introduced species in North America. Crespo et al. (2020b) formally pro-
posed to conserve the binomial P. crus-galli with a conserved type based on the specimen
Herb. Burser I: 103 (UPS), the one previously chosen as “lectotype” by Michael (1983).
Echinochloa crus-galli var. crus-galli
= Echinochloa crus-galli subsp. spiralis (Vasinger) Tzvelev, Zlaki SSSR 662. 1976. Basio-
nym: Echinochloa spiralis Vasinger, Flora SSSR 2: 739–740. 1934. Type: Caucasus:
Kuban: Krasnodar vic., 28 Oct 1931, A.V. Vazinger-Alektorova s.n. (holotype; LE).
= Echinochloa crus-galli var. praticola Ohwi, Acta Phytotax. Geobot. 11: 37 1942. Type:
Kiushiu, m. Kujusan, U. Faurie 2646 (holotype; KYO). Image available at http://
www.museum.kyoto-u.ac.jp/collection/PlePlant/PlePlant00001775_1.htm.
= Echinochloa crus-galli var. hispidula (Retz.) Honda, Bot. Mag. (Tokyo) 37: 122. 1923.
Basionym: Panicum hispidulum Retz., Observ. Bot. 5: 18. 1789. Type: India: “In-
dia orientali”, without data, König s.n. (LD 1219266) (lectotype, designated by
Fischer 1932: 71). Image available at https://plants.jstor.org/stable/10.5555/al.ap.
specimen.ld1219266.
= Echinochloa erecta (Pollacci) Pignatti, Arch. Bot. 15(1): 2. 1955. Basionym: Panicum
erectum Pollacci, Atti Ist. Bot. Univ. Pavia 13: 228, t. 5. 1908. Type: Italy: Lom-
bardia, Presso Pavia, Oct 1907, G. Pollacci s.n. (lectotype, designated by Ardenghi
et al. 2015: 135, PAV-Erbario Lombardo “118”, isolectotypes PAV-Erbario Lom-
bardo “121”, “141”, “123”, “137” (2 sheets), “139”, “140”).
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
8
Echinochloa crus-galli var. oryzoides (Ard.) Lindm., Svensk Fanerogamora 69. 1918.
Basionym. Panicum oryzoides Ard., Animadv. Bot. Spec. Alt. 2: 16, pl. 5. 1764.
Type. LINN 80.68. Image available at https://linnean-online.org/1302/.
Note. According to Carretero (1981), LINN 80.68 is a plant sent by Arduino to
Linnaeus, possibly collected in Italy. It was designated as the lectotype for that name
by Crespo et al. (2020a).
Remarks on E. crus-galli. Echinochloa crus-galli s.l. is taxonomically the most complex
Echinochloa occurring as a weed in Southwestern Europe. As we understand, this species
occurs in a number of varieties, but E. oryzicola is not one of them and is accepted as a
separate species (see below). As a result of a long and complex evolutionary history, in-
cluding signicant modications in the recent past (after the introduction of agriculture),
the dierences among the varieties are often slight. Furthermore, introductions of several
taxa as weeds in a range of crops far outside their natural range have contributed to ob-
scuring their original geographical distribution. Rather than aiming at precisely describ-
ing the limits and dening features of varieties of E. crus-galli occurring in Southwestern
Europe, we restrict ourselves primarily to indicating where unsolved problems remain.
Figure 1. Inorescence of Echinochloa frumentacea. (Photograph: Nico Wysmantel).
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 9
Being extremely polymorphic, numerous varieties of E. crus-galli have been de-
scribed, many of them based on the presence or absence of awns. As the development
of awns is inuenced by environmental conditions (Michael 1983), the value of varie-
ties or forms based on such characteristics as the presence or absence or the length of
awns is quite limited. Inorescences that develop later in the season frequently dif-
fer from the terminal inorescence. Other characteristics on which the description
of varieties has been based include the coloration of the plant (inorescence, leaves,
stem nodes, etc.), structure and position of the inorescence (erect, bent or nodding;
primary branches more or less patent or not, alternately positioned on the main axis
or whorled), the arrangement of the spikelets on the branches and the dimensions of
the spikelet. e importance of the length of the spikelets is emphasised by Michael
(1983), who in his identication key for the annual Echinochloa separates the spe-
cies characterised by spikelets measuring 3–5 mm from those with either shorter or
longer spikelets. Applied to specimens collected in Southwestern Europe, the criterion
of spikelet length works well to separate only the two rice mimics, E. crus-galli var.
oryzoides (Ard.) Lindm. (syn.: E. oryzoides [Ard.] Fritsch) and E. oryzicola, from the
remaining taxa of E. crus-galli s.l. with smaller spikelets.
ere is a broad consensus that E. crus-galli var. crus-galli occurs in large parts of
Europe and Asia, but authors dier on how to appropriately dene it. Ibaragi (2020)
stated that Asian var. crus-galli slightly diers from plants in Europe, “but the dier-
Figure 2. Inorescences showing the variation of Echinochloa esculenta. e apex of the spikelet varies
from usually obtuse (A) to less often shortly acute (B). (Photographs: Bart Mortier).
A B
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
10
ences are dicult to formally distinguish.” us, the need for additional research on the
morphological and genetic variation of the type variety throughout its range is evident.
According to Michael (2019), Echinochloa crus-galli var. hispidula (Retz.) Honda is
the appropriate name for E. crus-galli with non-pyramidal panicles and usually promi-
nently awned spikelets that are widespread in sub-tropical areas of Japan and Southern
China. With slightly larger spikelets than var. crus-galli, this taxon is often treated as a
separate species, Echinochloa hispidula (Retz.) Nees ex Royle; however, Ibaragi (2020)
completely ignored it, and Shouliang and Phillips (2006) interpreted it as synonym of
var. crus-galli. Its extreme variability (Carretero 1981, as E. hispidula) makes it hard to
distinguish var. hispidula from var. crus-galli, which is characterised by a usually more
or less procumbent habit (the lower nodes often rooting), oppy leaves, the whole
plant or parts of it more often than not purple-tinged, erect to strongly bent pyramidal
inorescences with the branches alternately placed or sometimes whorled, with at least
the lower branches usually more or less patent, and spikelets with or without awns, the
length of the awns and the percentage of awned spikelets within a single inorescence
exhibiting considerable variation (Fig. 12; description based on material from maize
elds in Belgium, where var. hispidula, a taxon of sub-tropical climates, is considered
not established as a persistent weed). If accepted as a separate taxon, the strongly bent
inorescence with appressed branches, the green colour of the plants and the stier
Figure 3. Inorescence of Echinochloa colona. (Photograph: Rutger Barendse).
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 11
leaves (the latter two features shared with var. oryzoides and E. oryzicola) might help to
separate E. hispidula from E. crus-galli var. crus-galli (Jauzein 1993). To this could be
added the less bristly spikelets of var. hispidula (Martínez-Azorín and Crespo 2021).
Whether the branches of the inorescence are whorled or not (Michael 1983) seems
to be a less reliable trait to use. e synonymizing of E. erecta (Pollacci) Pignatti, char-
acterised by an erect inorescence, with E. hispidula (see, e.g., Ardenghi et al. 2015)
underscores the wide morphological variation of hispidula and the diculty to dene
it as a unit clearly dierent from the equally variable var. crus-galli. Interestingly, the il-
lustration of E. hispidula given in Shouliang (1990) shows a spikelet with both the up-
per glume and lower (sterile) lemma with a short awn, a feature not mentioned in the
recent literature; however, apparently, it corresponds with the specication “calycibus
hispidis biaristatis” in the original description of the species (Retzius 1789).
Within E. crus-galli as interpreted here, var. oryzoides is the most easily identied
variety, clearly distinguished by the large size of its spikelets. Although the descrip-
tions given in triplet 4 in the key above may suggest otherwise, it is not always easy to
distinguish between var. oryzoides and E. oryzicola; see the discussion about the latter
species below. At one time, the name Echinochloa hostii (M. Bieb.) Link was used by
Italian botanists (Pignatti 1982). Previously, Pirola (1965) merely cited this name as
a synonym of E. crus-galli subsp. oryzoides, but Pignatti (1982) accepted the name
at species rank for the taxon that is here named E. crus-galli var. oryzoides. However,
from Pignatti’s identication key, it is clear that the name E. hostii was used for the
species that today can only be identied as E. oryzicola, based on the quite diagnos-
tic glume characteristics. Unfortunately, we were not able to trace type material of
Panicum hostii M. Bieb. which according to Tsvelev (1984) is preserved in LE. us,
Figure 4. Spikelets of Echinochloa oryzicola A fertile lemma with the tip dierentiated from the coria-
ceous body of the lemma (upper glume removed) B, C two spikelets with convex shiny sterile lemma
D spikelet with long lower glume and non-shiny sterile lemma. (Photograph: André De Kesel, Meise
Botanic Garden).
A B C D
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
12
we do not know whether P. hostii is indeed identical with E. crus-galli var. oryzoides
as stated by nearly all contemporary authors. Nonetheless, we are certain that the
binomial E. hostii was wrongly applied by Italian authors (particularly Pignatti 1982)
for E. oryzicola.
Recently, Martínez-Azorín and Crespo (2021) accepted E. crus-galli var. oryzoides
as a species, just like the similar-looking E. oryzicola. e strong similarities shared by
these two taxa are explained by a shared ancestor – tetraploid E. oryzicola being one of
the parent species of hexaploid E. crus-galli – and recent convergent evolution as rice
mimics derived from E. oryzicola and E. crus-galli (Fig. 13). is evolutionary trajec-
tory provides an argument for assigning the rank of variety to E. oryzoides. Further, it
would seem logical to reduce the rice mimic E. oryzicola to the rank of variety (pro-
visionally ‘var. infestans’ in Fig. 13) as well, but since it is not known whether or how
the ‘original’ E. oryzicola of pre-agricultural times diered from today’s E. oryzicola –
because it is now extinct or goes undetected – this is not an option.
Among the forms with small spikelets, subsp. spiralis (Vasinger) Tzvelev (no com-
bination available as a variety) and var. praticola Ohwi have been mentioned as occur-
ring in Europe. Apparently solely based on the small spikelets, both names were syn-
onymised by Scholz (2002), who noted that subsp. spiralis – a taxon with a huge distri-
bution area and possibly indigenous to Europe – and subsp. crus-galli are polymorphic
and that no sharp distinction between the two is possible solely based on the spikelet
length. Martínez-Azorín and Crespo (2021) interpreted var. praticola as probably no
Figure 5. Echinochloa oryzicola as an ephemeral casual in the Antwerp port area, Belgium A plant with
young inorescences B part of an inorescence with fully developed spikelets. (Photographs: Filip Verloove).
AB
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 13
more than an impoverished form of E. crus-galli, and Tison and de Foucault (2014)
seriously doubted the taxonomic value of subsp. spiralis and var. praticola. In Belgium
also, E. crus-galli with small spikelets has been recorded, but nowhere have such plants
been known to establish as noxious weeds.
A rather distinct form of E. crus-galli with spikelets ca. 3 mm long or a little longer
(somewhat smaller than average var. crus-galli) has occasionally been observed in Bel-
gium, including in the border of maize elds where, however, it seems not to establish
easily and disappears after only a few years. ese plants usually have an erect habit and
rather sti leaves. e inorescence is erect, with patent branches. e purple-tinged
spikelets are usually unawned (but a few spikelets may have a long awn), and some have
a glabrous, convex and shiny sterile lemma. e lower leave sheaths vary from glabrous
to densely covered with short retrorse hairs. Scholz (2002) included specimens with
small spikelets with a convex, shiny sterile lemma in subsp. spiralis, mentioning that
the spikelet morphology resembles Echinochloa glabrescens Munro ex Hook.f. Another
name for E. glabrescens is E. crus-galli var. formosensis Ohwi (Yabuno 1983; Ibaragi
2020); this name was used by Japanese authors for a weed of wetland rice elds. e
habitats in which the plants were found in Belgium stand in contrast to those preferred
by var. formosensis in Japan. Adding to the confusion, the name E.glabrescens has also
Figure 6. Habit of Echinochloa crus-galli var. oryzoides cultivated from seeds collected in rice, Italy. (Pho-
tograph: Maurizio Tabacchi, ISIDRO, Italy).
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
14
been applied to plants with spikelets 3.5–5 mm long by Bor (1960), Shouliang and
Phillips (2006; as E. glabrescens Kossenko) and Xia et al. (2011). Echinochloa with small
spikelets and a shiny lower lemma collected in Europe requires more study in order to
reveal its true identity and its relation with similar taxa having small spikelets in Asia
and E. oryzicola; see, e.g., Yasuda and Nakayama (2019). Although a quite distinctive
feature, the convex and shiny sterile lemma might prove to be of little value taxonomi-
cally. Bor (1960) wondered whether the “most peculiar” feature of the indurated sterile
lemma in E. glabrescens was sucient to make it a good species, and Yabuno (1966)
indicated that in E. oryzicola, the convex lemma is a simple dominant characteristic
(Fig. 4B, C).
Considering the preceding discussion, we accept, for the present, only few varieties
of E. crus-galli as occurring in Southwestern Europe. Indigenous and quite variable var.
crus-galli, usually with a less erect habit and more oppy leaves, is by far the most wide-
Figure 7. Spikelets of Echinochloa crus-galli A fertile lemma with the tip dierentiated from the co-
riaceous body of the lemma (upper glume removed) B spikelet with tip of the fertile palea frayed and
strongly recurved (lower glume and sterile ower removed).
A B
Figure 8. Spikelets of Echinochloa muricata A fertile lemma with the tip not clearly dierentiated from
the coriaceous body of the lemma (upper glume removed) B spikelet with tip of the fertile palea sti and
straight (lower glume and sterile ower removed).
A B
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 15
spread variety, especially towards the north. Part of the variation observed in Europe is
perhaps due to the involuntary introduction and establishment of populations of var.
crus-galli, with slightly dierent morphological features, from Asia. Echinochloa crus-
galli var. oryzoides, characterised by large spikelets, a more erect habit and stier leaves,
is a rice mimic in rice elds of Southern Europe. Echinochloa crus-galli var. hispidula, in
some respects resembling var. crus-galli and in others var. oryzoides, appears to us not to
deserve a separate status and is, therefore, included in var. crus-galli.
Plants with small spikelets are the most dicult to interpret. Probably represent-
ing more than one taxon – quite possibly including taxonomically irrelevant forms of
var. crus-galli – they require additional study, which will need to include material of
Asian origin.
Finally, it can be argued that Echinochloa esculenta (A. Braun) H. Scholz, a culti-
vated taxon derived from E. crus-galli, should be included in E. crus-galli (Ban and
Galasso 2021). Here, in line with most recent oras, it is pragmatically accepted as a
well-dened separate species. However, species rank is justiable based on morpho-
logical features, as the two taxa clearly dier from each other and identication of E.
esculenta is usually not much of a problem.
Figure 9. Echinochloa muricata. e uppermost leaf (or leaves) subtending the distal inorescence have
the demarcation between blade and sheath in the form of an elongated upside-down U. (Photograph:
Rutger Barendse).
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
16
Echinochloa esculenta (A. Braun) H. Scholz, Taxon 41(3): 523. 1992.
Basionym. Panicum esculentum A. Braun, Index Sem. [Berlin] 1861(App.): 3. 1861.
Type. Koernicke s.n., Cult. Hort. Bonn-Poppelsdorf, 28 Oct 1875 (B) (neotype,
designated by Scholz 1992: 523). Image available at https://ww2.bgbm.org/Herbari-
um/specimen.cfm?Barcode=B100366144.
Remark. See the combined comments below, under E. frumentacea.
Echinochloa frumentacea Link, Hort. Berol. 1: 204. 1827.
Type, lectotype designated here. India, Roxburgh s.n. (K000215131, the specimen
on the extreme right on the sheet). Image available at http://specimens.kew.org/her-
barium/K000215131.
Note. e protologue refers to a Roxburgh collection from India (“Roxb. ind. 1.
307. R. S. m. 2. 250. Hab. in India orientali ubi colitur”). e Kew herbarium houses
two original but undated Roxburgh collections (sheets K000215131 and K000215132)
that can serve for a proper typication. None exactly matches the information provid-
ed in the protologue, but since Link described the species in 1827, i.e. well after Rox-
burgh’s (1751–1815) death, these collections are supposed to have been at his disposal
when describing the species. In the apparent absence of other original material, one
of the two above-mentioned Kew collections could be chosen as the lectotype for that
name. Digital images of both are easily accessible via online resources such as the Kew
Herbarium Catalogue, JSTOR or POWO. Sheet K000215131 comprises ve stems,
four of which have an inorescence. e extreme left specimen is atypical and might as
well represent a dierent species. e other owering specimens are representative for
the species, and the specimen on the extreme right is here designated as the lectotype
for the name E. frumentacea. According to Staeu and Cowan (1983) considerable sets
of duplicates of Roxburgh specimens are stored at BM, BR, E, G and LIV. In some of
these herbaria isolectotypes could thus be found although a quick online search did
not yield further specimens.
Remarks on E. esculenta and E. frumentacea. Echinochloa esculenta (syn.: E. utilis
Ohwi & Yabuno) and E. frumentacea are cultivated species. Neither is considered a
persistent weed in Southwestern Europe. Still, they are included in the key since they
are the most frequently occurring non-weedy representatives of the genus in South-
western Europe, frequently recorded as bird-seed aliens in and along the border of
crop elds (Hanson and Mason 1985). ey look similar, and young specimens can
be dicult to identify, yet mature ones are easily distinguished by the colour of the
spikelets. In both species, the inorescence varies. In the more typical specimens,
the branches are tightly clustered and appressed against the axis, creating a com-
pact inorescence. Often, however, the inorescence is rather lax, with the distal
part of the branches somewhat curved towards the axis; such specimens are easily
mistaken for an awnless form of E. crus-galli. Yabuno (1966) describes the distinct
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 17
Figure 10. Spikelet of Echinochloa muricata var. microstachya showing the lower glume and unawned sterile
lemma (left) and upper glume (right). Scale bar 1 mm. (Drawing: Sven Bellanger, Meise Botanic Garden).
Figure 11. Spikelet of Echinochloa muricata var. wiegandii showing upper glume (left) and lower glume
and awned sterile lemma (right). Scale bar 1 mm. (Drawing: Sven Bellanger, Meise Botanic Garden).
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
18
characteristics of the two species, and recent genetic studies have conrmed that
they are quite distinct, E. esculenta being derived from E. crus-galli and E. frumen-
tacea from E. colona (Yamaguchi et al. 2005; Ye et al. 2014). ere are arguments
for reducing these two taxa to variety rank or, following Ban and Galasso (2021),
subspecies rank under E. crus-galli and E. colona. Yabuno (1966) insinuated that E.
esculenta shows more variation, and Michael (1983) added that in this species, the
spikelets may be awned (although awned spikelets seem to be rare); this reects the
highly polymorphic nature of the parent species.
Echinochloa muricata (P. Beauv.) Fernald, Rhodora 17(198): 106. 1915.
Basionym. Setaria muricata P. Beauv., Essai Agrostogr. 51, 170, 178. 1812.
Type. Canada: Quebec Lac Champlain, s.d., A. Michaux s.n. (holotype: P-
MICHX, isotype: US-80768).
Echinochloa muricata var. muricata
Figure 12. Habit of Echinochloa crus-galli var. crus-galli growing as a roadside weed, Belgium. Although
extremely variable, the usually more or less procumbent habit (the lower nodes often rooting) and the
oppy leaves are among the features that distinguish var. crus-galli from the obligate rice weeds E. crus-
galli var. oryzoides and E. oryzicola which are characterised by a more erect and stier habit. (Photograph:
Luc Audenaerde).
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 19
Echinochloa muricata var. microstachya Wiegand, Rhodora 23(267): 58–60. 1921.
Type (lecto-). USA: New York, Tompkins Co., Ithaca, between Fall Creek, In-
let and city, waste soil, border of west marsh, open alluvial and marshy ats, 19
Jul 1913, E.L. Palmer 097 (GH). Image available at https://s3.amazonaws.com/
huhwebimages/755E8AFFFFF6435/type/full/303931.jpg.
Echinochloa muricata var. wiegandii (Fassett) Mohlenbr., Ill. Fl. Illinois (ed. 2)
396. 2001.
Basionym. Echinochloa pungens (Poir.) Rydb. var. wiegandii Fassett, Rhodora 51(601):
2. 1949.
Type. USA: Oregon, Hayden Island, sandy roadside, J.C. Nelson 1974, 8 Sep 1915
(holotype GH). Image available at https://kiki.huh.harvard.edu/databases/specimen_
search.php?mode=details&id=126740.
Remarks on E. muricata. Echinochloa muricata is native to North America. Its status
as separate from E. crus-galli, which was inadvertently introduced there long ago from
Europe, was contested by Hitchcock (1920, 1935, 1950). Hitchcock (1920) rejected
the separate status stating that he was unable to distinguish the two species based on the
distinguishing features given by Fernald (1915). However, further studies by Wiegand
(1921) and especially by Fassett (1949) conrmed the separate status of E. muricata
(Gould et al. 1972). Probably, largely due to Hitchcock’s inuential publications, a sig-
nicant share of American authors have for decades combined native and introduced
taxa under E. crus-galli in oras and weed-control publications (Maun and Barrett
1986). In the 21st century, some researchers still refer to New World E. crus-galli – not
to be confused with E. crus-galli introduced in North America from Europe – rather
than using the name E. muricata (Aoki and Yamaguchi 2008). By now, however, mo-
lecular research has conrmed E. muricata as a separate species, clearly distinct from
E. crus-galli (Claerhout et al. 2016; Mascanzoni 2018). is should put an end to the
confusion that goes back to the days of Linnaeus, as it has been demonstrated that the
type specimen of E. crus-galli in fact belongs to E. muricata (Crespo et al. 2020a).
Echinochloa muricata is a highly variable species, though less so than E. crus-galli.
is, combined with its resemblance to E. crus-galli, has added to the diculty for
agronomists and botanists on both sides of the Atlantic to detect and correctly name its
introduced populations. Early records of introduced E. muricata from France revealed
morphologically very uniform populations (as E. pungens [Poir.] Rydb. var. microstach-
ya [Wiegand] Fernald & Griscom; Deschatres et al. 1974). is resulted in identica-
tion keys that made it harder to correctly identify clearly deviating forms of E. muricata
that had established in maize elds in Belgium (Hoste 2004).
e European populations of E. muricata exhibit only part of the variation
found in the natural range of the species. So far, three morphologically distinct
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
20
varieties have been recorded from Belgium and France. Echinochloa muricata var.
muricata, with larger spikelets, seems to occur only as an ephemeral alien (Hoste
2004). References to this variety in France require conrmation as they are prob-
ably based on misidentications (Jauzein 1995; Tison and de Foucault 2014). e
specimens with smaller spikelets recorded from Belgium are of two clearly dierent
types, apparently with very few intermediates. e characteristics given in the key are
mainly based on observations on European-origin plants. Specimens with unawned
spikelets with strongly spreading bristles are assigned to var. microstachya Wiegand,
and those with shortly awned spikelets with more appressed bristles to var. wiegandii
(Fassett) Mohlenbr.; see Hoste (2004) and Bomble (2016) for illustrations of the
inorescences and spikelets. Genetic research on specimens collected from maize
elds in Belgium has resulted in two clusters of E. muricata collections (Claerhout
et al. 2016). From the study of the morphological features of three of these collec-
tions, we tentatively conclude that the two clusters C and D identied by Claerhout
et al. (2016) correspond to var. wiegandii and var. microstachya, respectively (IH,
unpublished data). Nonetheless, more genetic studies are needed to conrm whether
the three morphologically distinct varieties are indeed genetically well-dened taxa.
Both within and outside North America, forms of E. muricata with smaller spikelets
have shown a stronger tendency to spread as weeds outside their natural range (Dore
and McNeill 1980; Michael 2001).
Echinochloa muricata is a species of moist, disturbed sites. It is not an important
weed of rice elds (Michael 2001, 2003) and in Europe it mainly occurs as a weed in
maize elds (Hoste 2004; Bomble 2016).
Echinochloa oryzicola (Vasinger) Vasinger, Fl. SSSR 2: 33. 1934.
= Echinochloa phyllopogon auct., non (Stapf) Stapf ex Kossenko in Botanicheskie Ma-
terialy Gerbariia Botanicheskogo Instituta imeni V. L. Komarova Akademii Nauk
SSSR 8(12): 208. 1940.
= E. hostii auct. ital., non (M. Bieb.) Link, Hort. Berol. 2: 209. 1833.
Basionym. Panicum oryzicola Vasinger, Trudy Prikl. Bot. 25(4): 125. 1931.
Type. Vladivostok region, left bank of Santakheza, 4 km east of Lake Hanka, 23
Aug 1928, A. Venzinger-Alexandrova (lectotype, designated by Tzvelev 1976: 664,
LE01010882). Image available at http://herbariumle.ru/?t=occ&id=15824&rid=ima
ge_0036250.
Remarks. Although sometimes included in E. crus-galli, several features justify ac-
cepting E. oryzicola as a separate species. Echinochloa oryzicola is tetraploid (2n = 36),
whereas E. crus-galli is hexaploid (2n = 54) (Yabuno 1966, 1981). e length of the
embryo is a reliable feature to distinguish E. oryzicola from E. crus-galli var. oryzoides
(which also has large spikelets) and from specimens of the very poorly dened E. crus-
galli var. hispidula. If carefully applied, the shape of the mature inorescence and the
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 21
length of the lower glume may help separate it from E. crus-galli var. oryzoides. It is
rather surprising that the seemingly distinctive feature of the length of the lower glume
is not mentioned in Vasinger’s original description (Vasinger in Komarov 1934).
Yabuno (1966) distinguished two morphological forms of E. oryzicola: the F-form,
in which the lemma of the sterile ower is at and has a coarse surface texture, and
the C-form, in which the lemma is convex, coriaceous and shiny. e latter form has
only rarely been recorded from Southwestern Europe. Specimens with spikelets much
too small for E. oryzicola but with a lemma that morphologically closely resembles
Yabuno’s C-form have been recorded from Germany (as E. crus-galli subsp. spiralis;
Scholz 2002) and Belgium (IH, unpublished records).
e treatment of the rice mimics E. oryzicola and E. crus-galli var. oryzoides in
taxonomic and agronomic publications has been extremely confusing. In the past, the
name E. phyllopogon, often without author citation and thereby adding to confusion,
was used separately for each of the two taxa as well as for both of them together; see,
e.g., the shifting interpretation in successive publications by Michael (1983, 1994,
2001) and Yabuno’s (1981) discussion of European E. phyllopogon as a synonym for E.
oryzicola. Echinochloa phyllopogon is a very confusing name, whose identity has been re-
cently summarised and discussed by Crespo et al. (2020a). Its basionym, Panicum phyl-
lopogon, was described by Stapf (1901). e accompanying plate shows a specimen that
seems to combine features of at least two species. It was said to have been collected by
Arcangeli in rice elds near Pisa (Italy). Arcangeli’s herbarium is located in PI and FI, at
least for the most part. A targeted search in the Arcangeli Herbarium (PI-ARC) did not
yield any Echinochloa specimen collected in the rice elds near Pisa (comm. F. Roma-
Marzio, 09.2018). In the Herbarium Generale of PI, there is a specimen labelled as
P. phyllopogon, which was part of Flora Italica Exsiccata. e herbarium label states that
this species was collected in Italy for the rst time in Novara and that Stapf erroneously
indicated it to be from Pisa. In fact, the species was collected by Jacometti near Novara
but was originally, erroneously so, attributed to a collection of Arcangeli from near Pisa
(comm. N. Ardenghi 10.2018). A lectotype for this name was designated by Kossenko
(1940) based on one of Jacometti’s collections (K000958854; image available at http://
www.kew.org/herbcatimg/638594.jpg). is collection includes both vegetative and
owering material that, according to P.W. Michael, refers to two dierent species. e
non-owering part, with very characteristic hair tufts at the junction of leaf blade and
leaf sheath, was said to represent P. phyllopogon and was recommended to serve as (sec-
ond step) lectotypication for that name (Michael 1983). However, the presence or ab-
sence of such hair tufts is a non-diagnostic feature that can be observed (although not
so frequently) in various species of Echinochloa, including E. oryzicola and E. crus-galli
var. oryzoides. Since both these taxa occur in the Novara area in Italy, it is impossible to
assign Stapf’s P. phyllopogon to one of these taxa. erefore, it is a confusing name that
should be abandoned. However, lectotypication of P. phyllopogon was eected later
by Kossenko (1940) himself, though under the combination “E. phyllopogon subsp.
stapana Kossenko”, a superuous, illegitimate name that explicitly included the type
of the species (subsp. phyllopogon). Crespo et al. (2020a) argued this lectotype is to be
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
22
followed; this made the later lectotype proposal by Michael (1983) ineective. Conse-
quently, E. phyllopogon should be included as synonymy of E. oryzoides, as suggested by
Crespo et al. (2020a) and Martínez-Azorín and Crespo (2021).
e separate status of E. oryzicola has been corroborated by molecular studies (e.g.,
Yamaguchi et al. 2005; Ye et al. 2014), although Yasuda and Nakayama (2019) have
shown that relying solely on cpDNA may result in misidentication of E. crus-galli var.
formosensis as E. oryzicola.
Unfortunately, the structure of the tip of the fertile lemma, which clearly distin-
guishes E. crus-galli from E. muricata (Hoste 2004), has received little attention in
studies on the weed ora of rice elds in Europe and Asia. In E. oryzicola, the tip more
closely resembles E. crus-galli, although the line of tiny hairs is usually more dicult to
see than in E. crus-galli (based on specimens from Italian rice elds seen by us; Fig. 4A).
Taxonomy of Echinochloa: morphology, genetics and evolutionary history
Dened as “an ubiquitous plant, with variation you can’t get your teeth into, which
clutters up herbaria” (Anderson 1952), the complex of Echinochloa crus-galli and a few
closely related taxa ts the denition of a weed perfectly. In the decades after the pub-
lication of tentative keys for the annual and perennial species of the genus worldwide
(Michael 1983), numerous studies have tried to solve the taxonomic problems relating
to this genus. In general, these studies were mostly intended to give an overview of the
species that occur in a restricted geographical area (e.g., in country oras) or to help
nd remedies to lower the impact of Echinochloa as noxious weeds in crops (such as rice
and maize), which presupposes a correct identication of the taxa involved. Molecular
Figure 13. Schematic reconstruction of the evolutionary history of Echinochloa oryzicola, E. crus-galli
and two rice mimics derived thereof. It is hypothesised that the taxon that today is called E. oryzicola has
only recently evolved from a wild taxon that seems no longer to exist or has not yet been identied. In
the absence of information on this original species, it is impossible to distinguish between a long-existing
taxon (‘var. oryzicola’) and a recently evolved rice mimic (‘var. infestans’).
unknown species of
Echinochloa (diploid)
E. oryzicola
(tetraploid)
E. crus-galli
(hexaploid)
E. crus-galli
var. crus-galli
E. crus-galli
var. oryzoides
(a recently
evolved rice
mimic)
. . .
(several more
described
varieties)
‘var. oryzicola’
(unknown:
undetected?
extinct?)
‘var. infestans’
(a recently
evolved
rice mimic)
E. oryzicola
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 23
studies covering a wider range of species are available without, however, linking genet-
ics with morphology; see, e.g., Aoki and Yamaguchi (2008).
So far, Echinochloa has not benetted from the recent revival of interest in botani-
cal monographs, which has primarily been kindled by biodiversity and conservation
concerns, especially in the species-rich tropics, rather than by hopes of improving the
means to control economically damaging weeds (Grace et al. 2021). However, we
believe that a worldwide monograph based on the integration of dierent scientic
expertise including specimen-based taxonomy, genomics and phylogenetics (Muñoz-
Rodríguez et al. 2019) is a prerequisite if we are ever to understand the complex tax-
onomy and evolutionary history and taxonomy of this genus. Once the evolutionary
history is better grasped, it will become easier for weed scientists as well as the authors
of regional oras to tackle the topics of interest.
e expression ‘evolutionary history’ here refers to more than ‘ancestry of a species’
as routinely used by biologists when describing ‘natural’ events. It also involves human
history and the role humans have played, consciously or not, in the origin and evolu-
tion of plant species (Russell 2003, 2011). As for Echinochloa, a good understanding
of what took place in Southeast Asia is essential in order to properly grasp the nature
and signicance of the diversity of forms displayed by the genus’ representatives in
Southwestern Europe. In the latter geographical area, the species under considera-
tion include a single introduced American species (E. muricata), the pantropical weed
E. colona (native to the Old World, possibly including parts of Mediterranean Europe)
and the complex of E. crus-galli and E. oryzicola, originally from Eurasia. Two addi-
tional cultivated taxa with non-shattering spikelets (E. esculenta and E. frumentacea)
have both originated in Asia.
Echinochloa muricata exhibits a high degree of variation. Although within North
America the distribution of the dierent forms has been altered as the result of human
activities, such as land reclamation (Dore and McNeill 1980), the morphologic and
genetic make-up of the species has most likely not strongly been aected by anthropo-
genic factors. e same can probably be said of polymorphic E. crus-galli in Europe.
In Southeast Asia, the story is dierent and more complex. e result of a hybridiza-
tion event between tetraploid E. oryzicola and an unknown diploid species, hexaploid
E. crus-galli arose around 3.3 million years ago (Ye et al. 2014). Echinochloa oryzicola
and E. crus-galli thus share a number of features, but the latter shows a wider range
of morphological variation and ecological tolerances, which may be attributed to the
added set of chromosomes (Yabuno 1966). Over time, the natural range of E. crus-galli
has extended from East Asia to Western Europe, while the natural range of E. oryzicola
apparently remained restricted to Southeast Asia.
Circa 10 millennia ago, Echinochloa spp., along with other wetland grasses such as
rice (Oryza spp.), was gathered and processed for human consumption in China (Yang
et al. 2015). Echinochloa fell out of favour when rice gradually evolved into a better-
yielding crop (Chang 2000). It persisted, however, as a noxious weed and adapted in
response to human activities such as the creation of rice paddies, hand-weeding and
the timing of rice harvesting. Over time, E. crus-galli developed into a multitude of
physiologically, morphologically and genetically dierent forms, displaying varied life
Ivan Hoste & Filip Verloove / PhytoKeys 197: 1–31 (2022)
24
histories in a range of habitat types (Yabuno 1966; Barrett and Wilson 1981; Yamasue
et al. 1989; Fukao et al. 2003). As a result of unintentional selection, Echinochloa in
rice paddies developed similarities with rice. Along with other features such as a more
erect habit, rice mimics are characterised by green seedlings and a green culm base,
having lost the anthocyanin pigmentation that is typical for E. crus-galli var. crus-galli.
Judging from the end result, it seems logical to conclude that E. oryzicola followed
a parallel line of evolution. However, this raises a question. Regarding E. crus-galli,
both the ancestor (var. crus-galli) and the evolved rice mimic (var. oryzoides) have been
identied, yet in the case of E. oryzicola we apparently only know the mimic. Barrett
(1983) distinguished between the strategies of the general purpose genotypes (such
as var. crus-galli) and those of specialised biotic ecotypes (such as the rice mimic var.
oryzoides). As for the specialised biotic ecotype E. oryzicola, no morphologically dis-
tinct generalist ancestor has been described. Barrett’s nomenclature reects the shared
morphological features of the rice mimics that occur in wetland rice paddies. His
E. crus-galli var. oryzicola included both an early-owering hexaploid (E. oryzoides
(Ard.) Fritsch = E. crus-galli var. oryzoides) and a later-owering tetraploid (E. phyl-
lopogon (Stapf) Koss = E. oryzicola). Moreover, Yabuno (1966) described a rice mimic
of E. crus-galli in upland rice elds; it shares the stier plant habit with the mimics
from wetland rice paddies.
Echinochloa crus-galli is usually autogamous. When unconsciously transported
around the world with rice seed, the introduction of morphologically dierent forms
may, therefore, result in the establishment of seemingly quite distinct taxa (Barrett and
Seaman 1980). is, in turn, may lure botanists into describing new species based on
material that only poorly represents the range of variation in the area of origin, as illus-
trated by Panicum oryzoides Ard. (syn. E. crus-galli var. oryzoides; Crespo et al. 2020b)
and P. erectum Pollacci (‘E. hispidula’; Ardenghi et al. 2015), both based on materials
collected in Italy.
Yamaguchi et al. (2005) stressed the poverty of sequence variations within a com-
plex species such as E. crus-galli, despite the fact that the species shows a high mor-
phological diversity, including domesticated forms, non-shattering weedy forms and
shattering forms that mimic rice plants. e features of rice mimics, such as green
culm base and seedlings, may be of limited value to taxonomists. In rice paddies, plants
with red- or purple-tinged seedlings might again re-emerge now that herbicides have
replaced hand-weeding. In a genus in which over the last 10 millennia, signicant pre-
existing morphological variation has been greatly increased due to close association
with agriculture, accepting each seemingly well-dened form as a separate taxon may
not lead to a satisfactory classication. For weed scientists, less-visible features related
to ecological requirements, variation of the life cycle and development of resistance
against herbicides may prove more relevant than morphological dierences that once
originated in a dierent co-evolutionary setting.
From this short detour into the evolutionary history of Echinochloa in Southeast
Asia, one can conclude only that the study of the taxonomy of this genus in Europe
requires a broader geographical scope. is should be coupled with the consideration
of some questions that so far have been insuciently addressed. e morphological
Taxonomy of the weed species of the genus Echinochloa in Southwestern Europe 25
and genetic variation of E. crus-galli var. crus-galli within its extensive Old World na-
tive range is poorly documented, as are the interactions (occasional cross-pollination
of usually autogamous plants) between populations of var. crus-galli and those of the
derived rice mimics. As for E. oryzicola, in the absence of information about its non-
mimic ancestor, its evolutionary history is quite obscure. Identifying the unknown
diploid parent species that, together with tetraploid E. oryzicola, gave rise to E. crus-
galli would help better understand the species complex of E. crus-galli and E. oryzicola,
including ‘E. glabrescens’.
Embedding these questions in a larger project of a world monograph of Echino-
chloa, the outcome of the collaboration of experts in the elds of taxonomy, genomics
and phylogenetics, would enhance our understanding of the anities between weedy
and non-weedy taxa, and between Old and New World species. Moreover, such a pro-
ject could generate a great deal of knowledge about the evolutionary history of a group
of plants that has undergone profound changes resulting from its interactions with
humans in the course of the past millennia.
Acknowledgements
We thank Benny De Cauwer (Ghent University) for sharing unpublished data on
E. muricata with us, and Manuel-Benito Crespo Villalba (Depto. Ciencias Ambien-
tales y Recursos Naturales [Botánica] Universidad de Alicante) for making available
the not yet published manuscript on Echinochloa for Flora Iberica and his comments
on the early versions of the Ms. Illustrations 1–4, 9 and 12 courtesy Waarnemingen.
be, Stichting Observation International and the local partners. Alexander Sukhorukov
(Moscow State University) provided information on relevant Russian literature.
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