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Genetic variability in parthenogenetic and amphigonic populations of Platyarthrus aiasensis Legrand from Sicily (Isopoda, Oniscidea)


Abstract and Figures

The aim of this work was to measure the genetic diversity of the Sicilian populations of Platyarthrus aiasensis as related to their sex ratios. Platyarthrus aiasensis, a small species of terrestrial isopods, was described as a parthenogenetic population from the island of Aix (Charente Maritime, France). Parthenogenesis was also demonstrated in some populations from one of the Eolian islands. During several expeditions, from 2001 to 2003, we studied 17 Sicilian populations and two populations from the islands of Panarea (Eolian archipelago) and Marettimo (Egadi archipelago) and obtained information about their sex ratios. Subsequently, the populations were analyzed by biochemical and molecular approaches (Multi-Locus Enzyme Electrophoresis and RAPD-PCR), and genetic variability evaluated in the parthenogenetic and anphigonic populations with different percentages of males. Indices of genetic variability, including the number of alleles per locus, percentage of polymorphic loci, heterozygosity, and index of dissimilarity, were very low in natural populations comprising only females (parthenogenetic). Values were higher in populations in which the percentage of males was up to 13% (spanandric and anphigonic populations). In populations with a higher number of males, the genetic variability seems to not increase, even if the male percentage rises.
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Proceedings of the International Symposium of Terrestrial Isopod Biology – ISTIB-07
I.Biodiversity & Systematics: 59-67
Genetic variability in parthenogenetic and amphigonic populations
of Platyarthrus aiasensis Legrand from Sicily (Isopoda, Oniscidea)
Giuseppe Montesanto* – Domenico Caruso – Bianca M. Lombardo
Department of Animal Biology “Marcello La Greca”, University of Catania, Via Androne 81,
I-95124 Catania, Italy
* E-mail:
The aim of this work was to measure the genetic diversity of the Sicilian populations of Platyarthrus
aiasensis as related to their sex ratios. Platyarthrus aiasensis, a small species of terrestrial
isopods, was described as a parthenogenetic population from the island of Aix (Charente Maritime,
France). Parthenogenesis was also demonstrated in some populations from one of the Eolian
islands. During several expeditions, from 2001 to 2003, we studied 17 Sicilian populations and two
populations from the islands of Panarea (Eolian archipelago) and Marettimo (Egadi archipelago)
and obtained information about their sex ratios. Subsequently, the populations were analyzed by
biochemical and molecular approaches (Multi-Locus Enzyme Electrophoresis and RAPD-PCR),
and genetic variability evaluated in the parthenogenetic and anphigonic populations with different
percentages of males. Indices of genetic variability, including the number of alleles per locus,
percentage of polymorphic loci, heterozygosity, and index of dissimilarity, were very low in natural
populations comprising only females (parthenogenetic). Values were higher in populations in which
the percentage of males was up to 13% (spanandric and anphigonic populations). In populations
with a higher number of males, the genetic variability seems to not increase, even if the male
percentage rises.
Keywords – allozyme, parthenogenesis, Platyarthrus aiasensis, RAPD, Sicily
Platyarthrus aiasensis (Isopoda: Oniscidea)
(Fig. 1) has been described by Legrand (1954),
from specimens on the Island of Aix (Charente
Maritime, France), as a parthenogenetic sub-
species of P. schöbli. Later, one member of
our group (Caruso, 1968a) demonstrated
parthenogenesis in some populations from the
island of Panarea (Eolian islands) and
discovered and described males for the first
time. In the same paper, Caruso raised P.
aiasensis to the species rank due to the
absence of transition forms between P. schöbli
aiasensis and P. schöbli briani, which were
collected in Sicily and the Eolian islands from
the same localities, and also for its wide
geographic distribution. In fact, many other
populations were found all over the world.
The geographic distribution actually
includes the western Mediterranean countries
(islands included) (Caruso, 1968b, 1970, 1973,
1976; Caruso & Lombardo, 1976; Ferrara &
Taiti, 1978; Taiti & Ferrara, 1980; Caruso &
Lombardo, 1982), the Canary Islands (Caruso,
Figure 1. Platyarthrus aiasensis Legrand.
1970), North and Central America
(Garthwaite, 1989), and South Africa (Taiti &
Ferrara, 1980). Possibly, this species is more
widespread, and the known distribution is due
to a lack of findings or determination errors.
The cephalon and pereionites of P.
aiasensis females exhibit pronounced dorsal
ridges, which are used to identify the species
of this genus; while male specimens do not
exhibit any difference in the dorsal ridges.
60 Montesanto et al.
Proceedings of the International Symposium of Terrestrial Isopod Biology – ISTIB-07
M. Zimmer, F. Charfi-Cheikhrouha & S. Taiti (Eds)
Many species of the genus Platyarthrus
exhibit spanandry, a reduction in the number
of males, with different sex-ratios (Vandel,
1962). Numerous Platyarthrus species are
myrmecophilous; P. aiasensis has always been
found inside ant nests.
The aims of this work were to (1) calculate
the percentage of P. aiasensis males in the
Sicilian populations and in two populations
from islands around Sicily, (2) asses the
genetic diversity of the populations studied,
(3) correlate the genetic variability of the
populations with their sex-ratio, and (4)
compare the results obtained from different
markers used.
Material and methods
Approximately 4,000 Platyarthrus aiasensis
specimens belonging to 19 populations were
collected between 2001 and 2003. Seventeen
populations were collected in Sicily (Acate,
Butera, Calatabiano, Caltavuturo, Castrofilippo,
Donnafugata, Fiumefreddo, Gela, Grammichele,
Marina di Modica, Palagonia, Partanna, Riesi, San
Cusumano, Tindari, Torretta Granitola, and
Villasmundo), one on the island of Marettimo
(Egadi Archipelago), and one on the island of
Panarea (Eolian Archipelago) (Fig. 2). The sex of
each specimen was established. In order to assess
the spanandry rate for this genus, specimens from
other Platyarthrus species were collected from
Sicily: 608 P. caudatus specimens and 139 P.
hoffmanseggi specimens.
In order to determine possible differences
between populations, as well as within a single
population, at least 20 specimens were studied for
each population of P. aiasensis. The observations
were conducted with a light stereo-microscope
(Zeiss Stemi SV8) and concerned the number,
shape, and relative length of the dorsal ridges.
Drawings were made of the different ridge patterns
and transferred to a file card with the sex and
length of each specimen indicated.
The genetic composition of the populations was
studied using Multi-Locus Enzyme Electrophoresis
Figure 2. Sampling sites for Platyarthrus aiasensis:
, Acate (RG); 2
, Butera (CL); 3
, Calatabiano
(CT); 4
, Caltavuturo (PA); 5, Castrofilippo (AG); 6,
Donnafugata (RG); 7
, Fiumefreddo (CT); 8
Gela (CL); 9
, Grammichele (CT); 10
, Island of
Marettimo (TP); 11
, Marina di Modica; 12
Palagonia (CT); 13, Island of Panarea (ME); 14
Partanna (TP); 15
, Riesi (CL); 16
, San
Cusumano (SR); 17, Tindari (ME); 18, Torretta
Granitola (TP); 19
, Villasmundo (SR), and
percentage of males (white segments).;
population was analyzed by allozyme
: population was analyzed by
E.C. Enzyme Loci scored Buffer system
Reference for stain
recipe Malate dehydrogenase Mdh
, Mdh
PC Shaw & Prasad (1970) Malic enzyme Me TM Ayala et al. (1972) Glutamate-oxalacetate
transaminase Got
, Got
TCII Selander et al. (1971) Isocitrate dehydrogenase Idh TCII Ayala et al. (1972) Phosphoglucomutase Pgm TM Brewer & Sing (1970) Glucosephosphate isomerase Gpi PC Selander et al. (1971)
Table 1. Details of allozyme analysis in populations of Platyarthrus aiasensis.
PC, Phosphate-citrate pH 6.3 (Harris & Hopkinson, 1976); TM, Tris-maleate I pH 7.4 (Brewer & Sing,
1970); TCII, continuous Tris-citrate pH 8.0 (Selander et al., 1971).
Population genetics of Platyarthrus aiasensis in Sicily 61
Proceedings of the International Symposium of Terrestrial Isopod Biology – ISTIB-07
M. Zimmer, F. Charfi-Cheikhrouha & S. Taiti (Eds)
(MLEE) on starch gel according to standard
procedures. Thirteen Sicilian populations and one
population from the island of Marettimo (Fig. 2)
were assayed, and for each population sample at
least 25 specimens were examined at 8 enzyme
loci: Mdh
, Mdh
, Me, Got
, Got
, Idh, Pgm, Gpi
(Table 1). Staining for enzyme activity after
electrophoresis was performed according to Harris
& Hopkinson (1976).
Randomly amplified polymorphic DNA (RAPD)
analysis was conducted on eight Sicilian
populations and the population from the island of
Marettimo. At least 20 individuals per population,
to a total of about 200 specimens, were analyzed
(Fig. 2). Genomic DNA was isolated using the
whole animal and a modified version of the
method previously described by Bender et al.
(1983). The DNA was quantified by agarose gel
electrophoresis. The amplification reaction was
carried out using the Amersham Biosciences
RAPD Kit (no. 27-9502-01). The following
arbitrary primers were chosen from the same kit:
A 2 µL aliquot of each PCR product was run on
2.5 % (w/v) agarose gel (1 % SeaKem GTG-FMC
plus 1.5 % NuSieve GTG-FMC) with TBE buffer
containing 0.5 mg/mL (w/v) ethidium bromide
(Sambrook et al., 1989). Two molecular weight
markers (Roche 100 bp ladder, n. 14) as well as
positive (E. coli C1a DNA) and negative controls
(no DNA) were placed on each gel. The
electrophoresis was carried out at 5 V/cm for 3 h.
The experimental conditions differed slightly from
those reported in standard protocols (Williams et
al., 1990), as the method of Welsh & McClelland
(1990) was used.
Data analysis
The allelic frequency, genetic distance values, and
indices of genetic variability obtained from the
allozyme data were calculated using the BIOSYS
program (Swofford & Selander, 1989). The genetic
variability was estimated by the value of mean
expected heterozygosity (He), observed
heterozygosity (Ho), proportion of the
polymorphic loci (P), and average number of
alleles per locus (A). The equilibrium of the
Hardy–Weinberg distribution was evaluated using
with Yates correction, since low expected
frequencies were regularly found (Beck & Price,
1981), and applying Levene’s correction (1949) for
small samples (Nei, 1978).
For the RAPD data, a 1/0 matrix was
constructed for each of the eight populations,
where 1 or 0 indicate, respectively, the
presence or absence of a specific band. This
presence/absence data was then analysed to
estimate the degree of polymorphism (P) for
each band and the dissimilarity between pairs
of individuals. We calculated an index of
dissimilarity (D) according to Nei & Li (1985):
D = 1-M = 1-[2N
)], where M is the
index of similarity (matching), N
is the
number of RAPD bands common to individual
A and individual B, and N
and N
the total
number of bands for individual A and
individual B, respectively. To calculate the
values of D and create a matrix of dissimilarity
values, the FORTRAN program RAPDPLOT
(Black, 1995; Apostol et al., 1996) was used.
The matrix obtained with the average values of
D per population was used for this analysis.
Results and Discussion
Nineteen populations of P. aiasensis from
Sicily and two islands near Sicily (Marettimo
and Panarea) were studied. The isopods were
always found in ant nests of the species
Messor capitatus, Messor structor, Messor
bouvieri, Linepithema umilis, Pheidole
pallidula, Tetramorium semilaeve,
Aphaenogaster sicula, Aphaenogaster
semipolita, and Camponotus barbaricus.
Figure 3. Examples of the dorsal ridges on
cephalon and tergites I and II in Sicilian
populations of Platyarthrus aiasensis. a-c,
Castrofilippo (no males); d-f, Acate (12 % males);
g-i, Butera (34 % males).
62 Montesanto et al.
Proceedings of the International Symposium of Terrestrial Isopod Biology – ISTIB-07
M. Zimmer, F. Charfi-Cheikhrouha & S. Taiti (Eds)
The sex ratios are depicted by location in
Fig. 2. Populations without males were found
in Castrofilippo, Calatabiano, and
Fiumefreddo; strongly spanandric populations,
with 2-10 % males, were found in
Villasmundo, San Cusumano, Palagonia,
Partanna, Grammichele, and Acate; and
populations with 20-37 % males were found in
Caltavuturo, Marina di Modica, Gela, Riesi,
Butera, and Marettimo. In contrast to P.
aiasensis, populations of P. caudatus
comprised 27-30 % male specimens, and
23-39 % of the collected P. hoffmannseggi
were males.
Light-microscopic observations revealed a
substantial uniformity for the morphological
characteristics of P. aiasensis; only slight
variations were found concerning the fourth
ridge position and length on the first and
second tergite of the pereion. The fifth and
sixth ridges also showed little variation,
especially concerning the front enlargement on
the first pereionite. No notable differences
were noticed between males (if present) and
females. No morphological characteristic were
obvious to characterise a particular population;
the little variation detected was always
quantitative, and a clear discontinuity was
never found between the populations studied,
not even between those with a very high
difference in sex ratio (Fig. 3).
Electrophoretic analysis was carried out on the
allozymes of 14 populations (Fig. 2). Three of
the eight loci, namely Me, Mdh
, and Mdh
were monomorphic, sharing the same allele for
all populations; five loci were found to be
polymorphic: Got
, Got
, Phi, Pgm and Idh.
The Phi locus was the most variable with five
alleles. Three alleles were found at Got
, Idh,
and Pgm, and two alleles were found at the
locus (Tab. 2).
Values for the genetic variability of
polymorphic loci (Tab. 3), P
ranged from
12.5 % to 50% in the populations with a higher
percentage of males, such as Caltavuturo,
Marina di Modica, Riesi, Butera, Partanna,
Marettimo, Acate, and Palagonia. These
results are comparable to those obtained in
previous studies on other terrestrial isopods
species (Viglianisi et al., 1992; Lombardo et
al., 2001, Medini-Bouaziz et al., 2006;
Montesanto et al., 2007). The percent values
of polymorphic loci obtained for the
parthenogenetic populations (Calatabiano,
Fiumefreddo, Villasmundo, and San
Cusumano) became zero.
The data above and the mean number of
alleles per locus (1.1 to 1.6 for spanandric
populations) were comparable to the values
known for terrestrial isopods; the mean
number of alleles per locus turned out to be 1
Table 2. Allele frequencies of polymorphic allozyme loci in 14 populations
of Platyarthrus aiasensis.
A 1.000
B 0.661
A 1.000
B 1.000
A 0.309
B 0.180
C 1.000
D 0.983
E 0.017
A 0.971
B 1.000
C 0.038
A 1.000
B 0.075
C 0.007
as listed in Fig. 2
Population genetics of Platyarthrus aiasensis in Sicily 63
Proceedings of the International Symposium of Terrestrial Isopod Biology – ISTIB-07
M. Zimmer, F. Charfi-Cheikhrouha & S. Taiti (Eds)
for the populations made up only by females.
Parthenogenetic populations (Calatabiano,
Fiumefreddo, San Cusumano, and
Villasmundo) showed, as expected, a lack of
heterozygosity. Heterozygosity increased with
increasing male percentage. By contrast, the
polymorphic loci percentage obtained for the
population from Grammichele (12.5 %) is
similar to the values obtained for the
populations with only few males, but the mean
number of alleles per locus was comparable to
the other populations with higher numbers of
A possible explanation for these last results
may be the well-known restriction of allozyme
electrophoresis for the quantification of
genetic variability. Actually, these biochemical
markers may highlight only 1/3 of genetic
variability (Nei, 1975; Singh et al., 1976).
Each time the analysed specimens showed the
same electrophoretic pattern, it was assumed
that the same pattern was defined by the same
genotype. This assumption is not always true;
the presence of iso-electrophoretic alleles,
different alleles with the same electrophoretic
mobility, has been demonstrated previously
(Avise, 1974; Richardson et al., 1986; Hillis &
Moritz, 1990).
Finally, the results from the allozyme
electrophoresis confirmed a general trend of
increasing genetic variability indices with an
increasing proportion of males, with the
exception of some of the populations;
unisexual populations showed lower or null
values for all indices. Therefore, it is possible
to confirm parthenogenesis as the reproductive
method of these last populations, at least at the
RAPD patterns obtained with the two primers
for the populations from Calatabiano (0 %
males) and Caltavuturo (37 % males) clearly
indicate the lower variability in the unisexual
population from Calatabiano (Fig. 4).
Genomic DNA from the studied specimens
had RAPD patterns of 4 to 16 evident bands,
from 200 bp to 3,000 bp in length. The
variation considered within and among
populations was the presence or absence of a
single band. The RAPD products of a
homozygotic individual (presence/presence)
cannot be distinguished from those of a
heterozygotic one (presence/absence). We
noted 2 species-specific bands (400 bp with
primer P2 and 450 bp with primer P3) for all
specimens. In addition, we found bands
exclusive to some populations.
Table 3.
Estimation of genetic variability from allozyme data for 14 populations of Platyarthrus aiasensis.
Mean heterozygosity
Mean no. of
allele per locus
% of polymorphic
Direct count Expected
Caltavuturo 37 1.3 (± 0.2) 25.0 0.085 (± 0.062)
0.126 (± 0.082)
Riesi 36 1.1 (± 0.1) 12.5 0.057 (± 0.057)
0.076 (± 0.076)
Gela 36 1.0 (± 0.0) 0 0.000 (± 0.000)
0.000 (± 0.000)
Butera 34 1.1 (± 0.1) 12.5 0.007 (± 0.007)
0.007 (± 0.007)
Marina di Modica
18 1.6 (± 0.3) 50.0 0.081 (± 0.054)
0.132 (± 0.071)
Partanna 17 1.1 (± 0.1) 12.5 0.006 0.006)
0.018 (± 0.018)
Isl. of Marettimo
13 1.3 (± 0.3) 12.5 0.027 (± 0.027)
0.056 (± 0.056)
Acate 12 1.1 (± 0.1) 12.5 0.004 (± 0.004)
0.004 (± 0.004)
Palagonia 9 1.1 (± 0.1) 12.5 0.005 (± 0.005)
0.005 (± 0.005)
San Cusumano
6 1.0 (± 0.1) 0 0.000 (± 0.000)
0.000 (± 0.000)
Grammichele 4 1.1 (± 0.1) 12.5 0.010 0.010)
0.010 (± 0.010)
Villasmundo 2 1.0 (± 0.0) 0 0.000 (± 0.000)
0.000 (± 0.000)
Fiumefreddo 0 1.0 (± 0.0) 0 0.000 (± 0.000)
0.000 (± 0.000)
Calatabiano 0 1.0 (± 0.0) 0 0.000 (± 0.000)
0.000 (± 0.000)
loci were considered polymorphic if more than one allele was detected
unbiased estimate (Nei, 1978)
64 Montesanto et al.
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M. Zimmer, F. Charfi-Cheikhrouha & S. Taiti (Eds)
From the presence/absence matrix for the
obtained fragments, mean values for the
dissimilarity index were calculated for each
population. The dissimilarity values ranged
from a minimum of 0.035 for the Fiumefreddo
specimens to a maximum of 0.351 for the
specimens from Marina di Modica. The mean
value for the dissimilarity index was 0.253.
The variability [dissimilarity index values
(D = 1-M)] increased almost proportionally to
male percentage up to a threshold of 13 % of
males (Fig. 5). Indices remained on a stable
level of 0.326-0.351 with any other increase in
the percentage of males, indicating that 13 %
males in any population is sufficient to
guarantee the maximum genetic variability.
Moreover, a population with only females
(Calatabiano) was monitored for the presence
of males throughout one year. Samples were
collected three times in 2003 and 2004 (2003:
Figure 4. Examples of RAPD patterns obtained from two populations of Platyarthrus aiasensis using primers
P2 and P3. CLT, Calatabiano (no males); CAL, Caltavuturo (37 % males).
Figure 5. Dependence of dissimilarity index D=1-M (Nei & Li, 1985) on percent males in 9 populations of
Platyarthrus aiasensis: 2, Butera; 3, Calatabiano; 4, Caltavuturo; 7, Fiumefreddo; 8, Gela; 10, Island of
Marettimo; 11, Marina di Modica; 15, Riesi; 19,
Villasmundo; for locations, see Fig. 2.
Population genetics of Platyarthrus aiasensis in Sicily 65
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M. Zimmer, F. Charfi-Cheikhrouha & S. Taiti (Eds)
March 22; 2004: January 2 and March 10). In
each case, only females were found. RAPD
analysis was carried out for 20 specimens from
the three samplings. The results were the same
as obtained in 2003, that is, the RAPD patterns
were perfectly comparable to the patterns
previously obtained (Fig. 6). These results
demonstrate that parthenogenetic reproduction
was stable over at least two years. In addition,
they gave useful information about the
accuracy of the marker used.
We are currently screening Sicilian
populations of P. aiasensis for the bacterial
endosymbiont Wolbachia, which is known to
interfere with the host reproductive activity in
many invertebrate species (Rousset et al.,
1992; Hurst et al., 1999). In terrestrial isopods,
Wolbachia may transform genetic males into
functional females, as has been thoroughly
studied in Armadillidium vulgare (Cordaux et
al., 2004). With the feminisation of males,
symbionts may convert a host incapable of
transmitting them into a vector for offspring
infection. It is also well-known that Wolbachia
has the ability to cause parthenogenesis in
many species of insects, such as social
Hymenoptera (wasps, ants) and Collembola
(Schiltuizen et al., 1998; Frati et al., 2004). As
P. aiasensis lives with ants, it is tempting to
hypothesized that some populations were
horizontally infected by Wolbachia.
The study of specimens from populations of P.
aiasensis, collected during numerous
samplings in Sicily and small islands near
Sicily, revealed a variation in the sex ratio,
ranging from a total absence of males to a
maximum of 37 % in the population from
Caltavuturo. The specimens exhibited a great
morphological uniformity independent of sex
ratio: major differences were not seen between
the populations without males and those with
the highest male percentage.
The genetic variability indices of eight loci
demonstrated, with the exception of some
populations, a correlation with the percentage
of males in the population. Thus, in unisexual
or spanandric populations such indices were
very low, supporting the hypothesis that the
reproduction in unisexual populations is
through parthenogenesis.
RAPD patterns revealed a high similarity
among individuals of unisexual or strongly
spanandric populations. The dissimilarity
calculated in the various populations increased
proportional to the relative numbers of males,
at least up to a certain value, beyond which it
remained stable. The maximum variability
value was reached with a male percentage of
13 %, providing a possible explanation for the
widespread spanandry of this species.
The two different types of genetic markers
used yielded congruent results, even though
the RAPD profile uniformity for the
Figure 6. RAPD patterns of Platyarthrus aiasensis individuals from Calatabiano (no males) on two
occasions in 2003 and 2004.
66 Montesanto et al.
Proceedings of the International Symposium of Terrestrial Isopod Biology – ISTIB-07
M. Zimmer, F. Charfi-Cheikhrouha & S. Taiti (Eds)
population from Calatabiano certainly has a
different meaning from that of the minimum
values of the variability parameters calculated
from allozyme data. The lack of
heterozygosity and the low mean number of
alleles per locus in the population from Gela
contrasted the high male percentage, while the
dissimilarity index obtained through RAPD is
perfectly congruent with the male percentage.
The latter methodology, therefore, appears to
be more suitable to detect genetic variability.
The sampling in Calatabiano over the
course of one year confirmed parthenogenetic
reproduction in this population, and verifies
the adequacy of the technique.
The possibility of male-feminising
Wolbachia that stem from the ant hosts of the
studied isopod species is currently under
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... Isopods associated with RWAs had on average a much stronger female-biased sex ratio than isopods living with L. flavus, but the sex ratio of some RWA-associated populations deviated from this pattern. Interestingly, populations of the ant-associated congeneric species P. aiasensis ranged from 63% females to complete parthenogenesis (Montesanto, Caruso, & Lombardo, 2008). The female-inducing bacteria Wolbachia is widespread in isopods (Bouchon, Rigaud, & Juchault, 1998) and might cause the observed variation in sex ratio. ...
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Many symbionts live in association with different species. It can be expected that these distinct hosts might have a different effect on key life history traits of the associated symbionts. Here, we compared the key trait body size of the obligatorily ant-associated isopod Platyarthrus hoffmannseggii collected in nests of two types of sympatric ant hosts. This isopod species showed surprisingly large differences in body size depending on type of host ant, with the head width of females and males associated with organic mound building red wood ants being respectively 1.30 and 1.17 times larger than isopods sympatrically living in earth nests of Lasius flavus. There was also a higher proportion of females in many red wood ant nests, but this pattern was not consistent across all nests. Genetic analyses and aggression trials did not reveal cryptic groups specialized to different hosts. Therefore we argue that the isopods exhibit size plasticity because of different host nest conditions. Absence of host aggression and optimal abiotic conditions in red wood ant nests might promote a larger isopod body size. Overall, this study shows that the association of a symbiont with different hosts might induce phenotypic plasticity in a symbiont key trait.
... fi gures 1?5 in Montesanto et al., 2011; fi gures 6, 7, and 9 in Montesanto et al., 2012; fi gure 2 in Agodi et al., 2015) and other fi gures, such as maps, dendrograms, graphs and diagrams (e.g. Medini-Bouaziz et al., 2006; Montesanto et al., 2007; 2008; Messina et al., 2011; Lupett i et al., 2013). ...
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In a recent paper (2015) I proposed a method to draw accurate line drawings for taxonomic studies, using taxa from Oniscidea. To complement that work, this short communication proposes a free-hand way to quickly draw areas with small setae or hairs, such as in penicils of mouth parts. Th is method enhances the previous drawing procedure, it takes a brief practice time, and allows a bett er quality of scientific illustrations.
... are aware of only a few long term studies where populations were frequently sampled to obtain changes of sex ratio over time (e.g Paris and Pitelka 1962, Sorensen and Burkett 1977). It is therefore important to excercise caution when comparing data for different populations even for the same species, as variations both in space and time may occur. Montesanto et al. (2008) reported a special case for Platyarthrus aiasensis Legrand, 1954 comparing 19 populations in and around Sicily. The populations differed in male ratio from 0 (parthenogenetic ) to 0.37 of males. Deviation from the expected 0.5:0.5 ratio may be due to behavioral differences between the sexes especially during reproductive period as propo ...
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Introduced species dominate the terrestrial isopod fauna in most inland habitats of North America, including urban landscapes. These non-native species are often very abundant and thus potentially play a significant role in detritus processing. We monitored isopod assemblages in an urban forest for a year to examine the relationship between surface activity and abiotic environmental factors, and to analyze reproductive characteristics that might contribute to their successful establishment. Using pitfall trap samples we recorded five species, two of which, Trachelipus rathkii and Cylisticus convexus, were highly abundant. We determined size, sex and reproductive state of each individual. Surface activity of both species reflected variability in abiotic stress factors for isopods, such as soil moisture and soil temperature. Early spring the main trigger was soil temperature while later in the season increasing temperature and decreasing soil moisture jointly affected population dynamics. Activity significantly correlated with soil moisture. The temporal pattern of sex ratios supported the secondary sex ratio hypothesis. Males dominated the samples on the onset of the mating season in search of females. The pattern was reversed as females searched for suitable microsites for their offspring. Size independent fecundity decreased as conditions became more stressful late in the season.
... Trichoniscus pygmaeus or Armadillidium nasatum (Vandel 1925), the male/female ratio does not vary greatly from 1:1; in other species, such as Philoscia muscorum, it is about 1:11 and in Trichoniscus pusillus (Frankel et al. 1981) and Ocelloscia floridana (Johnson 1986) it is less than 1:200. In other species, such as Platyarthrus aiasensis, the sex ratio of the population differs in relation to the geographic area of its provenance ranging from a male/female ratio of 1:1 to total lack of males, with obvious recourse to parthenogenesis (Caruso 1968; Montesanto et al. 2008). The female-biased sex ratio is often due to Wolbachia, an endocellular α-proteobacteria that causes feminization of genetic males in many species of Oniscidea (Martin et al. 1973; Bouchon et al. 1998; Rigaud et al. 1999), while in Cylisticus convexus (Moret at al. 2001) and in Porcellio dilatatus (Legrand et al. 1978; Rousset et al. 1992) it induces cytoplasmic incompatibility, an effect commonly found in insects. ...
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In females of Isopoda Oniscidea, the genital system displays a remarkable variability of its morphological and functional organization as possible adaptation to different strategies of sperm storage. In all the oniscidean species, the sperm received from the male during mating are temporarily stored in the bursa copulatrix, a chitinous pouch of the oviduct. In 27 of the 32 species we studied, the sperm that remain in the bursa copulatrix after the fertilization of eggs of the first oviposition are transferred into the seminal receptacle, where they can be stored for a variable time. In these species, the seminal receptacle is a small kidney-shaped cup, localized at the insertion of the oviduct into the ovary. Only in two species of the genus Trichoniscus, in which the oviduct is very short, is the bursa copulatrix modified to form a large lateral diverticle.Instead, in three species of the family Halophilosciidae, Halophiloscia couchii, Halophiloscia hirsuta and Stenophiloscia glarearum, the ovary is consistently shorter, while the seminal receptacle is greater; after mating, both seminal receptacle and ovary appear completely filled with sperm. Finally, in two species of the family Tylidae, Tylos europaeus and Helleria brevicornis, the female genital system lacks specialized structures for sperm storage, and every oviposition requires a mating for the eggs fertilization.The authors present some hypotheses to explain the variability of the female genital system morphology and the sperm storage strategies.
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Intimate associations between different species drive community composition across ecosystems. Understanding the ecological and evolutionary drivers of these symbiotic associations is challenging because their structure eventually determines stability and resilience of the entire species network. Here, we compiled a detailed database on naturally occurring ant–symbiont networks in Europe to identify factors that affect symbiont network topology. These networks host an unrivalled diversity of macrosymbiotic associations, spanning the entire mutualism–antagonism continuum, including: (1) myrmecophiles – commensalistic and parasitic arthropods; (2) trophobionts – mutualistic aphids, scale insects, planthoppers and caterpillars; (3) social parasites – parasitic ant species; (4) parasitic helminths; and (5) parasitic fungi. We dissected network topology to investigate what determines host specificity, symbiont species richness, and the capacity of different symbiont types to switch hosts. We found 722 macrosymbionts (multicellular symbionts) associated with European ants. Symbiont type explained host specificity and the average relatedness of the host species. Social parasites were associated with few hosts that were phylogenetically highly related, whereas the other symbiont types interacted with a larger number of hosts across a wider taxonomic distribution. The hosts of trophobionts were the least phylogenetically related across all symbiont types. Colony size, host range and habitat type predicted total symbiont richness: ant hosts with larger colony size, a larger distribution range or with a wider habitat range contained more symbiont species. However, we found that different sets of host factors affected diversity in the different types of symbionts. Ecological factors, such as colony size, host range and niche width predominantly determined myrmecophile species richness, whereas host phylogeny was the most important predictor of mutualistic trophobiont, social parasite and parasitic helminth species richness. Lastly, we found that hosts with a common biogeographic history support a more similar community of symbionts. Phylogenetically related hosts also shared more trophobionts, social parasites and helminths, but not myrmecophiles. Taken together, these results suggest that ecological and evolutionary processes structure host specificity and symbiont richness in large-scale ant–symbiont networks, but these drivers may shift in importance depending on the type of symbiosis. Our findings highlight the potential of well-characterized bipartite networks composed of different types of symbioses to identify candidate processes driving community composition.
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Nowadays only digital figures are accepted by the most important journals of taxonomy. These may be produced by scanning conventional drawings, made with high precision technical ink-pens, which normally use capillary cartridge and various line widths. Digital drawing techniques that use vector graphics, have already been described in literature to support scientists in drawing figures and plates for scientific illustrations; these techniques use many different software and hardware devices. The present work gives step-by-step instructions on how to make accurate line drawings with a new procedure that uses bitmap graphics with the GNU Image Manipulation Program (GIMP). This method is noteworthy: it is very accurate, producing detailed lines at the highest resolution; the raster lines appear as realistic ink-made drawings; it is faster than the traditional way of making illustrations; everyone can use this simple technique ; this method is completely free as it does not use expensive and licensed software and it can be used with different operating systems. The method has been developed drawing figures of terrestrial isopods and some examples are here given.
BIOSYS-1 is a FORTRAN IV program designed to aid biochemical population geneticists and systematists in the analysis of electrophoretically detectable allelic variation. It can be used to compute allele frequencies and genetic variability measures, to test for deviation of genotype frequencies from Hardy-Weinberg expectations, to calculate F-statistics, to perform heterogeneity chi-square analysis, to calculate a variety of similarity and distance coefficients, and to construct dendrograms using cluster analysis and Wagner procedures. The program, documentation, and test data are available from the authors.