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A review of hybridization in North American hawthorns - Another look at "The Crataegus problem"
Abstract
The study of nearly 400 syntype, other type, and probable type elements of Crataegus, in the context of over 4000 other Missouri specimens examined for the Flora of Missouri Project, provided a unique opportunity to look at putative hybridization in the Missouri Crataegus flora. These specimens collectively represent essentially all the taxonomic variation observed to date for the state. Each type specimen was studied and then categorized as being either: (a) a primary species (orthospecies); (b) a synonym of an orthospecies; or (c) representing a wide hybrid (nothospecies). These nothospecies, all putative, constitute the totality of alleged hybrids for Missouri. Computation of hybrid frequency yielded the conclusion that wide hybrids occur between about 15% of the possible interserial combinations, and field and herbarium studies show that populations of these hybrids are usually small and non-persistent. Working with weaker data roughly similar results were generated for the whole of the Crataegus-rich northeastern quadrant of the United States (and adjacent Canada). Interestingly, the very small surviving, or even extinct, nature of the populations of these hybrids, together with their high reliability of correct identification, indicate that known hybrids do not contribute significantly to "The Crataegus Problem" routinely touted by North American authors.
... Поширені в Європі C. rhipidohylla, C. monogyna та C. laevigata часто гібридизують між собою, що затрудняє ідентифікацію таксонів глоду. Значні проблеми створює наявність різних концептуальних підходів до визначення виду, особливо в такому складному роді як Crataegus, якому властиві апоміксис, поліплоїдія і гібридизація (Camp, 1942a;Palmer, 1943Palmer, , 1946Клоков, 1954;Krushke, 1965;Завадский, 1967;Циновскис, 1971;Phipps, Muniyamma, 1980;Phipps, 1988aPhipps, , 1997Phipps, , 2005Campbell, Dickinson, 1990;Dickinson, 1998Dickinson, , 2002Macklin, 2001;Dickinson et al., 2007;Цвелев, 2001;Ена, 2012 та ін.). ...
... Унаслідок цього кількість північноамериканських видів глоду стали оцінювати чотиризначним числом. Згодом це явище отримало назву "Crataegus problem" (Eggleston, 1910;Palmer, 1932b;Camp, 1942b;Dickinson, 2003;Phipps, 2005). Декілька поколінь вчених доклали зусиль, щоб розібратися з таким складним родом як Crataegus, який вважали за "справжнє відьомське вариво" (Camp, 1942b;Phipps, 1983a). ...
... Її укладено на основі узагальнення даних багатьох авторів (Phipps et al., , 2003b(Phipps et al., , 2005Talent, 2001;Talent, Dickinson, 2005;Dönmez, 2007;Lo et al., 2007Lo et al., , 2009aPhipps, O`Kennon, 2007) Три секції Mespilus, Brevispinae і Henryanae відокремлені від основних клад А (Eurasiana-Americana), В (Americana), С (Crataegus), які можна прирівняти до підродів роду Crataegus. Секції Parvifoliae, Microcarpae і Cordatae формують окрему кладу D, що поєднує види давнього гібридогенного походження. ...
The results of the investigation of hawthorn species introduced by authors in Artemivsk Experimental Station of Nursery Cultivation have been given, and works on Crataegus in Ukraine have been reviewed. Genus Crataegus system has been considered, a new combination C. submollis var. arnoldiana has been proposed. Eco-biological characterization of hawthorn species under condition South-East of Ukraine has been given. Species and cultivars promising for growing as ornamental, fruit and medicinal plants have been described.
... However, gene flow and reproduction in this genus have been difficult to study, and the relationship between hybridization and polyploidy remains unclear (Talent and Dickinson 2005). There is some evidence that the morphologically identifiable hybrids (''wide hybrids'') of North American Crataegus have historically been largely ephemeral (Phipps 2005). The loss of these taxa is undoubtedly due in large part to changes in land use, but it has also been suggested that the hybrids that briefly flourished after land clearing and abandonment in the 19th and early 20th centuries have disappeared because of low fertility (Phipps 2005). ...
... There is some evidence that the morphologically identifiable hybrids (''wide hybrids'') of North American Crataegus have historically been largely ephemeral (Phipps 2005). The loss of these taxa is undoubtedly due in large part to changes in land use, but it has also been suggested that the hybrids that briefly flourished after land clearing and abandonment in the 19th and early 20th centuries have disappeared because of low fertility (Phipps 2005). Longley (1924) found numerous triploids among the distinctive forms that had been collected from the wild and were then in cultivation, and the morphology of at least some triploids suggests allopolyploidy (Dickinson 1983;Dickinson and Phipps 1986). ...
... why Crataegus species found growing together do not (apparently) hybridize (Palmer 1932(Palmer , 1943Campbell et al. 1991;Phipps 2005). Our results suggest at least three more reasons. ...
Unlike their diploid relatives, some triploid and tetraploid Crataegus frequently produce unreduced megagametophytes. In all cases, pollination is required for successful seed set, but in polyploids, endosperm formation can involve fertilization by either one or both sperm. Apomixis, in which the egg develops parthenogenetically, is widely documented in polyploid Crataegus, and as in many other groups with gametophytic apomeiosis, fertilization of unreduced eggs can also occur. Reciprocal pollinations were made between diploids, triploids, and tetraploids belonging to five taxonomic series in the genus to evaluate opportunities for gene flow between ploidy levels. The ploidy levels of embryo and endosperm in mature seeds, estimated from flow-cytometric DNA measurements, indicate the meiotic or apomeiotic origin of the megagametophyte and whether fertilization has occurred. These experiments demonstrated that although some tetraploids maintain near-obligate apomixis when supplied with pollen from diploids, others produced seeds containing embryos ranging from diploid to hexaploid. Allotriploid embryos were produced when a diploid was provided with pollen from tetraploids. A triploid produced tetraploid embryos when pollinated by a diploid and pentaploid embryos when pollinated by a tetraploid. Gametophytic apomixis in Crataegus thus can be facultative or near-obligate and may be implicated in the formation of interserial hybrids.
... A critical component of this approach is recognition of the likelihood that hybridization does occur in Crataegus. In a recent review of this question, Phipps (2005) has argued that in Missouri fewer than 15% (20% for the northeastern United States) of all possible cases of interserial hybridization have occurred. Moreover, ''. . . the numbers of hybrid individuals on the ground appear to be low at best and perhaps, more commonly at this time, non-existent'' (Phipps 2005). ...
... In a recent review of this question, Phipps (2005) has argued that in Missouri fewer than 15% (20% for the northeastern United States) of all possible cases of interserial hybridization have occurred. Moreover, ''. . . the numbers of hybrid individuals on the ground appear to be low at best and perhaps, more commonly at this time, non-existent'' (Phipps 2005). Phipps goes on to suggest that frequent, implicitly interserial, hybridization is thus not the root cause of taxonomic confusion in North American hawthorns. ...
... Phipps goes on to suggest that frequent, implicitly interserial, hybridization is thus not the root cause of taxonomic confusion in North American hawthorns. In doing so, however, he accepts the possibility that hybridization between similar entities may not have been detected (Phipps 2005). Elsewhere we have documented not only evidence for wide hybridization Talent and Dickinson 2007a;Lo et al. 2008c), but also for just the kind of reticulation referred to by Phipps, in series Douglasianae here (Fig. 9) and elsewhere Lo et al. 2008a). ...
Black-fruited hawthorns in North America comprise two taxonomic groups within the genus Crataegus, section Brevispinae and section Douglasianae. The first of these has recently been shown to be monospecific, consisting of the blueberry haw, Crataegusbrachyacantha Sarg. & Engelm., of Louisiana and Texas. Crataegus section Douglasianae, however, comprises several taxa in a single clade that is not closely related to section Brevispinae, and that is now one of the best-studied groups of hawthorns at least in North America. Most taxa in the group are found in, or west of, the Rocky Mountains. They include diploids, triploids, and tetraploids that can be ascribed to four or more species that differ in distribution and ecology, thorn morphology, leaf shape, and floral architecture. Diploids are self-incompatible, whereas polyploidy is associated with pseudogamous, gametophytic apomixis and self compatibility. Molecular data suggest that polyploids have arisen repeatedly, both within and from crosses between ploidy levels. We suggest that Crataegus section Douglasianae represents at least two agamic complexes that may serve as models for understanding the biology of, the relationships within, and the appropriate taxonomic treatment of other such groups that may make up much of the rest of the genus.
... With the current infrageneric classification, only the large section Crataegus has representatives in both the old and new worlds (i.e. it is predominantly Eurasian, but includes the eastern North American monotypic series Apiifoliae (Loudon) Rehder). Twenty-nine series are North American, 16 of which probably belong in the large and taxonomically difficult section Coccineae Loudon (Phipps et al. 1990) and of these, three have been suggested as possibly being of hybrid origin (Phipps 2005). ...
... Data analyses were carried out using the R Package v. 4.0 (Casgrain and Legendre 2004), and Fig. 3 was plotted using S-Plus 6.2 (Insightful Corporation 2003) and Feigen 1978, Dickinson and Phipps 1986, Smith and Phipps 1988, Wells and Phipps 1989Dickinson et al. 1996, Purich 2005. Although it is argued that hybridization is generally infrequent (Phipps 2005), these and other experiments (Talent and Dickinson 2007c) as well as those of horticulturalists, indicate inter-species compatibility of the diploid species and between some diploids and tetraploids when the female parent is diploid. Phipps (2005) suggests that in Crataegus section Coccineae Loud. ...
... Although it is argued that hybridization is generally infrequent (Phipps 2005), these and other experiments (Talent and Dickinson 2007c) as well as those of horticulturalists, indicate inter-species compatibility of the diploid species and between some diploids and tetraploids when the female parent is diploid. Phipps (2005) suggests that in Crataegus section Coccineae Loud. as many as three series may have originated by inter-series hybridization. ...
The relationship between polyploidy and breeding system is of critical importance for understanding evolution and improving
the taxonomy of large Rosaceous genera. Reviewing the data available for the family and for tribe Pyreae (formerly subfamily
Maloideae) in particular, it appears that hybridization, pseudogamous gametophytic apomixis, polyploidy, and self-compatibility
are closely linked. Studies of the evolutionary significance of any one or two of these factors need to consider the others
as well. Taxonomic decisions likewise need to be informed by knowledge of how these factors affect patterns of phenetic and
genetic variation.
... Nevertheless, comparisons of microsatellites, and of nuclear and chloroplast loci (Lo, Stefanović & Dickinson, 2009b;Lo, Stefanović & Dickinson, 2010), and ribosomal DNA (ITS2) copy number variation correlated with differences in ploidy level (Zarrei, Stefanović & Dickinson, 2014), strongly suggest that hybridization between infrageneric groups (subgenera, sections, series) has played an important role in the diversification of Crataegus. Prior to these data becoming available, however, with one exception (Phipps, 1988) hybridization was not seen to be a factor in Crataegus diversification (Haines, 2011;Phipps, 2005), and several new species were described in North America during the period 1980-2007 with their possible hybrid origin being either ignored or explicitly rejected. Subsequently, however, taxonomic and floristic works on Crataegus have reversed this trend (Kurtto, Sennikov & Lampinen, 2013;Lance, 2014;Phipps, 2013;Phipps, 2015). ...
... Homoploid hybrids between diploid hawthorns are uncommon, and are best known for involving C. monogyna Jacq. and other species with which it is sympatric either in its native range or where it has been introduced (Christensen, 1992;Christensen et al., 2014;Phipps, 2005;Phipps, 2015). Only three taxa in the sample studied by Lo et al. (2009a) exhibited incongruence between trees built using chloroplast loci and ones built from nuclear ones, and none of these (putative trans-Atlantic paleohybrids) is included in our samples here. ...
Background
Hawthorn species ( Crataegus L.; Rosaceae tribe Maleae) form a well-defined clade comprising five subgeneric groups readily distinguished using either molecular or morphological data. While multiple subsidiary groups (taxonomic sections, series) are recognized within some subgenera, the number of and relationships among species in these groups are subject to disagreement. Gametophytic apomixis and polyploidy are prevalent in the genus, and disagreement concerns whether and how apomictic genotypes should be recognized taxonomically. Recent studies suggest that many polyploids arise from hybridization between members of different infrageneric groups.
Methods
We used target capture and high throughput sequencing to obtain nucleotide sequences for 257 nuclear loci and nearly complete chloroplast genomes from a sample of hawthorns representing all five currently recognized subgenera. Our sample is structured to include two examples of intersubgeneric hybrids and their putative diploid and tetraploid parents. We queried the alignment of nuclear loci directly for evidence of hybridization, and compared individual gene trees with each other, and with both the maximum likelihood plastome tree and the nuclear concatenated and multilocus coalescent-based trees. Tree comparisons provided a promising, if challenging (because of the number of comparisons involved) method for visualizing variation in tree topology. We found it useful to deploy comparisons based not only on tree-tree distances but also on a metric of tree-tree concordance that uses extrinsic information about the relatedness of the terminals in comparing tree topologies.
Results
We obtained well-supported phylogenies from plastome sequences and from a minimum of 244 low copy-number nuclear loci. These are consistent with a previous morphology-based subgeneric classification of the genus. Despite the high heterogeneity of individual gene trees, we corroborate earlier evidence for the importance of hybridization in the evolution of Crataegus . Hybridization between subgenus Americanae and subgenus Sanguineae was documented for the origin of Sanguineae tetraploids, but not for a tetraploid Americanae species. This is also the first application of target capture probes designed with apple genome sequence. We successfully assembled 95% of 257 loci in Crataegus , indicating their potential utility across the genera of the apple tribe.
... In this way, our results will help us to evaluate the way in which taxonomists have minimized the role of hybridization in Crataegus (e.g. Phipps, 2005) in describing new taxa for the genus from the Pacific Northwest. ...
... Our interpretation of the results presented here (and earlier, regarding the origins of polyploid C. suksdorfii; Lo et al., 2009bLo et al., , 2010 runs counter to recent suggestions that intersectional hybridization should be rare in Crataegus (Phipps, 2005). This latter conclusion was based on qualitative analyses of morphological variation in the well-collected Crataegus flora of Missouri. ...
Background and aims:
The taxonomic complexity of Crataegus (hawthorn; Rosaceae, Maleae), especially in North America, has been attributed by some to hybridization in combination with gametophytic apomixis and polyploidization, whereas others have considered the roles of hybridization and apomixis to be minimal. Study of the chemical composition and therapeutic value of hawthorn extracts requires reproducible differentiation of entities that may be difficult to distinguish by morphology alone. This study sought to address this by using the nuclear ribosomal spacer region ITS2 as a supplementary DNA barcode; however, a lack of success prompted an investigation to discover why this locus gave unsatisfactory results.
Methods:
ITS2 was extensively cloned so as to document inter- and intraindividual variation in this locus, using hawthorns of western North America where the genus Crataegus is represented by only two widely divergent groups, the red-fruited section Coccineae and the black-fruited section Douglasia. Additional sequence data from selected loci on the plastid genome were obtained to enhance further the interpretation of the ITS2 results.
Key results:
In the ITS2 gene tree, ribotypes from western North American hawthorns are found in two clades. Ribotypes from diploid members of section Douglasia occur in one clade (with representatives of the east-Asian section Sanguineae). The other clade comprises those from diploid and polyploid members of section Coccineae. Both clades contribute ribotypes to polyploid Douglasia. Data from four plastid-derived intergenic spacers demonstrate the maternal parentage of these allopolyploids.
Conclusions:
Repeated hybridization between species of section Douglasia and western North American members of section Coccineae involving the fertilization of unreduced female gametes explains the observed distribution of ribotypes and accounts for the phenetic intermediacy of many members of section Douglasia.
... all the Missouri taxa with types outside the state. A relatively large proportion of these Missouri names have still not been accounted for (either as accepted names or synonyms), even in treatments in the more detailed taxonomic literature, for example, Kruschke (1955Kruschke ( , 1965 for Wisconsin and Phipps and Muniyamma (1980) for Ontario. Thus, to develop an understanding of the taxonomy and nomenclature of Crataegus in Missouri and surrounding states, it has become necessary to undertake the typification of names based on Missouri types. ...
... Although apomixis and polyploidy are frequent in North American Crataegus, this has not yet been shown to account for the taxonomic complexity encountered. A percentage of Missouri species (about 20%) are, however, putative hybrids (Phipps, 2004), and to this extent hybridization is a factor in taxonomic complexity. However, all the putative hybrids, taxonomically difficult though they and their synonymous entities may be to deal with, are either very rare (e.g., C. ×vailiae Britton) or, more usually, apparently extinct (e.g., C. ×latebrosa Sarg.). ...
We lectotypify or neotypify 134 names (131 specific and 3 infraspecific ones) of Crataegus from the Missouri flora.
... Unique bands were observed for eight Crataegus genotypes (sizes 150 to 1100 bp). Seven out of these genotypes (7,9,30,38,41, 46 and 47) had only a single unique band while the genotype number 8 had two unique bands. There were three unique bands for the outgroup Pyrus sample. ...
... The taxonomic treatment of species of this genus has varied widely, and the authors of various floristic treatments have misidentified species that occur in regions cov ered by their work [27]. The genus has vexed so many taxonomists that early experts on the group termed the situation as "the Crataegus problem" [28][29][30]. Intrageneric classification of the maloid genera has been studied in detail by Phipps [6] and Phipps et al. [3]. A comprehensive monographic study of the Asi atic and European Crataegus species was carried out by Christensen [7]. ...
This study aimed at the indentification of the species and genotypes of the genus Crataegus in Syria and determination of the genetic relationships among them based on the analysis of genomic and chloroplast DNA (cpDNA) using ISSRs and CAPS techniques. Morphological characterization carried out on 49 Crataegus samples collected from different geographical regions of Syria revealed four Crataegus species: C. monogyna, C. sinaica, C. aronia and C. azarolus. In the dendrogram constructed for those samples based on ISSRs (20 primers), all samples that belong to C. monogyna were clustered in one cluster. Samples of the other three species were overlaped in another cluster. Two samples of these were the most distant from all other samples in the dendrogram and were suggested to represent hybrid species or subspecies. When CAPS technique was applied on four Crataegus samples that represent the four suggested species using 22 cpDNA regions and 90 endonucleases, no polymorphism was detected neither in amplification products sizes nor in restriction profiles. The inability of detection of variation in cpDNA among species suggested can be attributed to the low level of evolution of the cpDNA in the genus, and to the possibility that some of these species are either subspecies or hybrids since the cpDNA is inherited through one parent only.
... It is likely that species previously isolated during interglacial phases (if they were successful in maintaining refugial populations) came into contact and hybridized. There is abundant evidence of the contemporary potential for hybridization between Crataegus species within clade B (Phipps, 2005;Talent and Dickinson, 2007), as well as between North American species in clades A and B on the one hand, and between North American and introduced Eurasian species from clade C on the other (Love and Feigen, 1978;Wells and Phipps, 1989). Follow-ing deglaciation, as vegetation zones moved north, radiation into new habitats would have taken place, allowing successful genotypes (i.e., species of polyploid/hybrid origin) to establish themselves. ...
... Follow-ing deglaciation, as vegetation zones moved north, radiation into new habitats would have taken place, allowing successful genotypes (i.e., species of polyploid/hybrid origin) to establish themselves. It is perhaps during this phase that the phenological isolation seen between many North American Crataegus species (Phipps and Muniyamma, 1980;Campbell et al., 1991;Phipps, 2005) became established. ...
Phylogeographic relationships were constructed for 72 Old and New World Crataegus species using combinations of four chloroplast and up to five nuclear regions. Maximum parsimony, maximum likelihood, and Bayesian results yield consistent relationships among major lineages. The close associations of the East Asian and western North American species point toward ancient trans-Beringian migrations. Relationships among eastern North American species are poorly resolved and few groups are identified that are congruent with existing classifications. Scant variation and short internal branches among these species suggest rapid divergence associated with polyploidy and hybridization. Incongruence between the chloroplast and nuclear data, and morphology suggest hybrid origins of three species from an extinct European lineage (the male parent) and three different North American female parents. Europe and eastern North America are suggested as the most recent common areas for Crataegus; at least four dispersal events are inferred to explain the present distribution of the genus.
... При этом отмечается, что морфологическая изменчивость создает сложности для внутривидовой идентификации боярышников (34). Современные методы молекулярно-генетического анализа позволяют определить гибридные экземпляры (33,35,36). Так, A.A. Dönmez (33), проводя филогенетический анализ рода Crataegus, указывал, что в Турции распространена гибридизация между видами Crataegus. ...
... При этом отмечается, что морфологическая изменчивость создает сложности для внутривидовой идентификации боярышников (34). Современные методы молекулярно-генетического анализа позволяют определить гибридные экземпляры (33,35,36). Так, A.A. Dönmez (33), проводя филогенетический анализ рода Crataegus, указывал, что в Турции распространена гибридизация между видами Crataegus. ...
... Evans and Campbell (2002) and Ma and Lu (2016) emphasized that the classification of existing hawthorn varieties is historically complex, and its classification and evolution are still not well understood. Taxonomic problems and the description of many new species are the result of hybridization between species (Phipps, 2005). Problems in Crataegus arise because of several factors. ...
This research was carried out in Idlib Governorate, located in northwestern Syria, between 2021 and 2022. The types of wild and cultivated hawthorn spread in the study area were identified and described. The results showed that there were significant differences between the types in most of the studied leaf characteristics (leaf length and width, lobulation depth, leaf area, and leaf wet and dry weight), as well as in flower characteristics (floral cluster length, number of flowers per cluster, and flower petiole length). The types varied in the characteristics of the fruit, whether quantitative (fruit length and width, fruit petiole length, and fruit size weight) or qualitative characteristics (color of the fruit) and in the seed characteristics (length, width, weight, and size of the seed). The correlation analysis table showed a high positive linear correlation between leaf area and leaf wet weight, between fruit size and fruit weight, and between seed size and seed diameter and weight. The results of the cluster analysis showed that the studied types are grouped into two main clusters. The first cluster includes 3 subgroups, while the second cluster includes one subgroup. Principal components analysis (PCA) indicated that there were three main factors responsible for 100% of the variances between the types. The factor PC1 was responsible for 69.3%, the variables combined by PC2 were responsible for 17.3% of the variances, while the PC3 accounted for 13.4%. Finally, the study showed the spread of the Azarolus types mainly in the northwestern region of Syria, and to a lesser extent Monogyna, as well as the great relationship between the wild type Aronia and the cultivated types, Yellow Azarolus and Red Azarolus.
... The apomixes, interspecific hybridization, and polyploidy widely exist in the Crataegus genus, leading to high genetic differences in morphological characteristics of Crataegus and increasing difficulties in its classification (Dönmez 2008). Flowering or fruiting periods could only reflect unique characteristics of some Crataegus, which makes their classification more complicated (Christensen 1992;Phipps 2005). PCA analyses of the phenotypic diversity of C. pentagyna Waldst. ...
According to the phenotypic cluster analysis, Cratae-gus accessions were clustered into five main clades. The genetic diversity and population structure of seven Crataegus species were analyzed by thirteen reported and five newly designed SCoT markers. In total, eighteen SCoT markers produced 148 polymor-phic bands with 94.87% polymorphic loci. Thirty-six Crataegus accessions were divided into five clades in the phylogenetic tree constructed by SCoT markers. C. bretschneideri accessions were clustered into a single clade which had a closer relationship with C. pinnatifida Bunge accessions. Population structure analyses indicated that there was a mixed gene pool in C. bretschneideri accessions. These findings revealed that C. bretschneideri may be of hybrid origin. Newly designed SCoT markers are practical for genetic diversity analyses of genus Crataegus.
... f.) Medik.) from the southeastern USA were hybrids derived from European and North American ancestors (Lo et al. 2009). Thus, Phipps (2005) posited that hybridization is a potential explanatory factor for speciation in Crataegus. C. bretschneideri, C. maximowiczii, and partial C. pinnatifida originated in Northeast China. ...
Crataegus bretschneideri C. K. Schneid. is one of the species cultivated in China. Due to its unclear taxonomic classification status, the conservation and utilization of this germplasm resource have been limited. In this study, we analyzed the chloroplast genomes and nuclear sequences to reveal the taxonomic relationships among C. bretschneideri and related species. We assembled the chloroplast genomes of C. bretschneider and related species and varieties, including C. maximowiczii C. K. Schneid., C. maximowiczii var. ninganensis S. Q. Nie & B. J. Jen., C. pinnatifida Bunge, and C. pinnatifida var. major N. E. Br. The lengths of the chloroplast genomes ranged from 159,644 bp (C. bretschneideri) to 159,947 bp (C. pinnatifida var. major). The five Crataegus chloroplast genomes had similar features and possessed 86 to 88 protein-coding genes, 37 tRNA genes, and eight rRNA genes which were arranged in the same order. Eight mutation hotspot regions, including matk, psaB, accD, petA, clpP, trnD-GUC, psbH-petB, and trnN-GUU-trnR-ACG could be used as potential molecular markers for further studies of Crataegus genetic diversity. Phylogenetic analyses based on 17 chloroplast genomes of Crataegus and Amelanchier indicated that C. bretschneideri was related to C. maximowiczii and C. maximowiczii var. ninganensis. However, the phylogenetic trees constructed by nuclear sequences of 36 Crataegus accessions reflected a closer relationship between C. bretschneideri and C. pinnatifida. Furthermore, divergence time estimation suggested that C. bretschneideri and C. maximowiczii diverged in the late Miocene and that speciation of C. pinnatifida occurred during the middle to late Miocene. These findings revealed that C. bretschneideri is an independent species and may be of hybrid origin.
... Унаслідок цього кількість північноамериканських видів глоду стали оцінювати чотиризначним числом. Згодом це явище отримало назву "Crataegus problem" (Eggleston, 1910;Palmer, 1932;Camp, 1942;Dickinson, 2003;Phipps, 2005). Декілька поколінь учених доклали зусиль, щоб розібратися з таким складним родом як Crataegus, який вважали за "справжнє відьомське вариво" (Camp, 1942;Phipps, 1983). ...
The history of the formation of the collection of rare fruit and ornamental plants in the Agronomic Experimental Station of the National University of Life and Environmental Sciences of Ukraine is described. Now the collection includes 284 species of trees and shrubs, which belong to 52 genera and 16 families. There are a total of 1183 accessions, including 552 cultivars and selects. Accession number, species and cultivar names, and as well as the origin of samples are specified. Numerous new binominal combinations are proposed in genera Aria, Hedlundia, and Micromeles (Rosaceae).
... These have led to high genetic differences in morphological characteristics (Dönmez, 2008) and increased di culties in classifying Crataegus species. Flowering or fruiting periods could only re ect unique characteristics of some Crataegus, which makes their identi cation and classi cation more complicated (Christensen, 1992, Phipps, 2005. PCA of the phenotypic diversity of C. pentagyna Waldst. ...
Crataegus bretschnesideri C. K. Schneid. originated in Northeast China, it has excellent characteristics of cold resistance and early maturity. However, the taxonomic status of C. bretschnesideri has been controversial due to the intra-specific hybridization and apomixes. Comparative assessment of phenotypic and molecular evaluations used to explore genetic diversity of C. bretschneideri and relative species. Qualitative and quantitative traits of C. bretschneideri and relative species were evaluated. Genetic diversity and population structure of several Crataegus species were analyzed by thirteen reported and five newly designed SCoT markers. Leaf traits of Crataegus had a higher Shannon-wiener index than fruit and flower traits; Fruit contained diverse traits among seven Crataegus species by principal component analysis. SCoT markers amplified 148 polymorphic bands with 94.87% polymorphic loci. Crataegus individuals clustered into five clades, and C. bretschneideri had a closer relationship with C. pinnatifida Bge. and C. hupehensis Sarg.. Population structure suggested that investigated the mixed gene pool appeared in C. bretschneideri . There had variation among Crataegus species, and part of the C. bretschneideri individuals may be a hybrid origin compared to the phenotypic and molecular evaluations. Newly designed SCoT markers were practical for Crataegus diversity analyses.
... On the one hand, models by means of which hybridization might be inferred from phenetic data were developed (Phipps, 1984) but have not been pursued except as noted. On the other hand, however, hard evidence of hybridization involving apomictic polyploids was lacking, so that it remained easy to dismiss it as infrequent (Phipps, 2005;Haines, 2011). New species from hitherto poorly explored areas of western North America were described without reference to the possibility they could be either apomictic or of hybrid origin (Phipps, 1990;Phipps & O'Kennon, 1998. ...
This paper updates earlier reviews of polyploidy and reproductive biology in the Rosaceae, and does so with a focus on hybridization in relation to polyploidy and (facultative) gametophytic apomixis. Taking data mainly from tribe Maleae, it also seeks to point out evidence for a potential role for fertilization of infrequent unreduced gametes in diploid-diploid crosses in producing autopolyploids. Apomixis may originate in these autopolyploids, and spread as they cross with diploids and other polyploids. © International Association for Plant Taxonomy (IAPT) 2018, all rights reserved.
... Dönmez (2008) suggested that the taxonomy of Crataegus is problematic owing to biological, historical, and many other factors. In particular, the taxonomic problems and the description of a large number of new species are the result of intergeneric hybridization (Phipps, 2005). To these factors are added introgression, extensive polyploidy, and gametophytic apomixis, which may occur and blur the boundaries between species (Christensen, 1992). ...
The Chinese hawthorn (Crataegus pinnatifida Bge. var. major N.E.Br.) is uniquely originated in northern China. The ecological and horticultural importance of Chinese hawthorn is considerable and some varieties are valued for their fruit or medicine extracts. Its taxonomy and phylogeny remain poorly understood. Apart from general plant morphological traits, pollen is an important trait for the classification of plants and their evolutionary origin. However, few studies have investigated the pollen of Chinese hawthorn. Here, an analysis of plant and pollen morphological characteristics was conducted in 57 cultivars from the Shenyang region. Thirty plant morphological characters and nine pollen grain characters were investigated. The plant morphological analysis revealed that the coefficient of variation for 13 traits was >20%, which indicates a high degree of variability. We also found that the pollen grains varied greatly in size, shape (from prolate to perprolate), and exine pattern (striate-perforate predominantly). The number of apertures was typically three. Based on these findings, we suggest that pollen morphology associated with plant morphological traits can be used for classification and phylogenetic analysis of Chinese hawthorn cultivars. In sum, our results provide new insights and constitute a scientific basis for future studies on the classification and evolution of Chinese hawthorn.
... [20À23] Taxonomic problems in the genus Crataegus and the description of a large number of new species and nothospecies during the last century by Old World botanists are the result of several factors. These factors are hybridisation, [24] introgression, polyploidy and probably apomixis, which may occur and cause difficulties in identifying species. [3,4,25] Grant [26] explains the correlation between polyploidy and various factors, including climate, latitude, altitude, type of habitat, life form, breeding system, hybridity, cell size, chromosome size, chromosome structure, sex chromosome mechanism and genotype. ...
Hawthorn (Crataegus spp.) is an important forest fruit species in Iran. Genetic variability among some genotypes of hawthorn was investigated using morphological traits and random amplified polymorphic DNA (RAPD) marker. The collected genotypes belonged to four species of Crataegus genus. High variability among genotypes was found for most of the traits. Results from the principal component analysis (PCA) indicated that 85.05% of the observed variability was accounted by the first five components. The first two components explained about 55.24% of the total achieved variability. In PCA, fruit weight, fruit length, fruit diameter, fruit moisture, fruit dry matter, leaf length, leaf area, leaf width, number of leaves per node, seed weight and seed length were predominant in the first two components, indicating that they were useful for the assessment of hawthorn germplasm characterization. A total of 58 polymorphic bands were produced with 10 RAPD primers. The bands' sizes ranged from 180 to 2700 bp. The number of the observed polymorphic bands for each primer ranged from 4 to 8, with an average of 5.8 alleles per locus. The polymorphism information content was observed to be the highest (0.79) in the Oligo_32 locus, whereas the Oligo_339 locus had the lowest value of 0.64, with an average of 0.72, among the RAPD primers. The Jaccard's genetic similarity coefficient ranged from 0.12 to 0.95 among the genotypes. At a similarity coefficient of 0.46, the unweighted pair group method with arithmetic mean (UPGMA) cluster analysis divided the genotypes into three major groups.
... Natural hybridization occurs fairly commonly among and within genera of the subtribe Pyrinae (formerly the Maloideae, Rosaceae), which contains many economically important fruits, such as apple, pawpaw, pear and loquat [4]. Intergeneric hybridization has been observed in 16 genera of Pyrinae [5], while intrageneric hybridization is expected to be even more frequent and has been found in many genera of Pyrinae, including Amelanchier [6], Crataegus [7][8][9], and Sorbus [10,11]. The prevalence of natural hybridization among and within the genera of Pyrinae indicates that hybridization may play important roles in the evolution of Pyrinae [4], and provides enormous opportunities to breed new cultivars of fruits. ...
Background
Interspecific hybridization has long been recognized as a pivotal process in plant evolution and speciation. It occurs fairly common in the genera of the subtribe Pyrinae. In Eriobotrya, a small tree genus of Pyrinae, E. prinoides var. daduheensis has been recognized as either a variety of E. prinoides, a natural hybrid between E. prinoides and E. japonica, or a variety of E. japonica. However, to date, there has been no convincing evidence on its status.ResultsFour nuclear genes and two chloroplast regions were sequenced in 89 individuals of these three Eriobotrya taxa from two locations where they coexist. A few fixed nucleotide substitutions or gaps were found in each of the investigated nuclear and chloroplast loci between E. japonica and E. prinoides. Of the 35 individuals of E. prinoides var. daduheensis, 33 showed nucleotide additivity of E. japonica and E. prinoides in at least one nuclear gene, and 10 of them harboured nucleotide additivity at all the four nuclear genes. Most haplotypes of E. prinoides var. daduheensis were also shared with those of E. japonica and E. prinoides. In the two chloroplast regions, 28 and 7 individuals were identical with E. japonica and E. prinoides, respectively.Conclusions
Our study provides compelling evidence for a hybrid status for E. prinoides var. daduheensis. Most hybrid individuals are later-generation hybrids. Both E. japonica and E. prinoides can serve as female parent. Differential adaptation might maintain the species boundary of E. prinoides and E. japonica in the face of hybridization and potential introgression.
... Species naturally occur as small trees and shrubs, mainly diploid (x = 17) ( Evans and Campbell, 2002). The systematic of the genus based on morphological (Phipps, 2005), cytological (Dickinson and Talent, 2007) and reproductive traits (Christensen, 1992;Evans and Campbell, 2002) is still unclear owing to interspecific hybridization, polyploidy and apomixis. Studies of phylogenic relationships among species based on molecular markers are currently under progress (Albarouki and Peterson, 2007;Evans and Campbell, 2002;Fineschi et al., 2005;Lo et al., 2007;2009). ...
• The genetic diversity of nine wild Tunisian Crataegus azarolus var. aronia L. populations from different bioclimates was assessed using RAPD markers.
• Eight selected primers generated a total of 105 bands, 81 of which were polymorphic. Shannon’s index (H′) ranged from 0.222 to 0.278 according to a population with an average of 0.245. The genetic variation within the species (H
SP = 0.423) was relatively low. A high differentiation (G
ST = 0.421) among populations coupled with a low level of gene flow (N
m = 0.472) were observed. The analysis of molecular variance (AMOVA) revealed also significant differentiation among populations (ΦST = 0.371), even at a low scale space. The majority of variation occurred within populations (63.31%). The Mantel test performed on genetic (ΦST) and geographic distance matrices among population pairs did not reveal an isolation by distance.
• Interpretation of Neighbour-joining tree based on Nei’s and Li’s genetic distance among individuals showed distinct population groupings. The UPGMA dendrogram based on ΦST values revealed two population sub-clusters, each including populations from different bioclimates and/or geographic regions.
• The low level of genetic diversity and the high genetic structure of populations resulted from genetic drift caused both by habitat fragmentation and the low size of populations.
• The high differentiation among populations and the similar low level of diversity within populations suggest that in situ conservation should interest all populations. The ex situ conservation should be based on the collection of seeds rather within than among populations because of the maximum of variation was revealed within populations.
... However, it is a difficult task to separate accessions into species, due to individual variations of hawthorn plants and the high number of species. Hybridization (Phipps 2005),4 UPGMA dendrogram of RAPD bands generated by 19 arbitrary oligonucleotide primers for Crataegus accessions sampled from Hatay, Turkey. Numbers in nodes represent bootstrap values in percentage and only higher than 50% are presented polyploidy, and apomixis (Christensen 1992; Lippert 1995) contributed to the number of new species and, therefore, taxonomic complexity. ...
The genus Crataegus known as hawthorns, is the largest genus among the Maloideae, which comprises 265 species. Turkey is one of the genetic centers of Crataegus and there are more than 20 species found in Turkey. The fruits of Crataegus are used as food and have high flavonoid, vitamin C, glycoside, athocyanidin, saponin, tannin, and antioxidant levels. In this study, we attempted to characterize 15 Crataegus accessions sampled from Hatay, located in Eastern Mediterranean region of Turkey. The accessions belonged to several species; C. aronia (L.) DC. var. aronia, C. aronia var. dentata
Browicz, C. aronia var. minuta Browicz, C. monogyna Jacq. subsp. azarella (Griseb.) Franco, and
C. orientalis Pall. ex M. Bieb. var. orientalis. Fruit characteristics of the accessions exhibited considerable
variation. The multivariate, principle component and cluster analyses indicated that the accessions
belonged to three groups: (1) C. aronia var. arona accessions; (2) C. aronia var. dentata accessions;
and, (3) C. monogyna subsp. azarella and C. orientalis var. orientalis accessions. The principle component
analysis results also revealed that the first three components explained 46, 21, and 14% of the
variation, comprising a total of 81%. The fruit length and width, leaf area, and soluble solids contents were
highly correlated characteristics for the first three components. The 19 RAPD primers generated a total
of 107 bands, where 76 of these were polymorphic. The molecular data analyses by principle coordinate
and clustering showed similar results to those of pomological characteristics. There were three groups,
(1) C. aronia var. arona accessions; (2) C. aronia var. dentata accession; and, (3) C. monogyna subsp.
azarella. C. orientalis var. orientalis accession grouped with C. aronia var. arona accessions.
Therefore, it can be concluded that, overall, the diversity patterns of pomological and molecular data,
generated by RAPD, for Crataegus are in good agreement and the accessions of C. aronia var.
aronia, C. aronia var. minuta, C. monogyna subsp. azarella and C. orientalis var. orientalis accessions.
... Historically it was suggested that hybridization is primarily responsible for the systematic difficulty (Eggleston 1908;Brown 1910;Standish 1916), but although hybridization is well documented, its extent is now debated (Phipps 2005). Thus, it is unclear at this stage whether a phylogenetic analysis using molecular data would be hampered by reticulate relationships. ...
Hawthorns and medlars are closely related genera in Rosaceae subfamily Maloideae, whose taxonomy remains poorly understood. Gametophytic apomixis occurs in polyploids, and diploids are sexual out-crossers, so ploidy level is of great interest, but suitable material for chromosome counts is of limited availability each year. The promise of flow cytometry is that it permits rapid measurement of nuclear DNA amounts from most tissues, and ploidy level can be inferred if climatic and taxonomic differences do not interfere. Our DNA measurements cover most of the taxonomic series in Crataegus, adding cultivated and naturalized Eurasian plants to the many wild plants collected mainly from south-central Canada and the southeastern and northwestern United States. We found that some variation in DNA amount per genome copy distinguishes certain taxa, but ploidy-level estimates are at least as clear as the published chromosome counts, especially in the most common diploid-triploid-tetraploid range, and to the single published higher (hexaploid) chromosome count, we add evidence of pentaploids. By comparing ploidy evaluations to morphology, we hypothesize that both autopolyploidy and allopolyploidy contribute to the taxonomic complexity. We compared DNA amounts in Maloideae with those in Gillenia, a likely sister genus to the subfamily, which has a smaller chromosome number.
... Taxonomic problems in the genus Crataegus and the description of a large number of new species and nothospecies during the last century by Old World botanists are the result of several factors such as hybridization (Phipps, 2005), introgression, polyploidy and probably apomixis, which may occur and confuse the lines between species (Christensen, 1992;Lippert, 1995). Within this genus, inherently variable species occur, such as C. monogyna on which different types of leaf blades can be found, a feature which is considered to be of taxonomic significance in other species (Christensen, 1992). ...
The systematics of the genus Crataegus (Rosaceae) have been considered problematic owing to the fact that hybridization, introgression, polyploidy and apomixis may occur in this genus. A study of the Crataegus species from the Arab mountains, Sweida Province, Syria, has been undertaken based on both plastid DNA sequences (trnL-trnF, psbA-trnH) and morphological data. In the investigated region, three morphologically distinguishable Crataegus species: C. azarolus var. aronia L., C. × sinaica Boiss. ssp. sinaica and C. monogyna var. monogyna Jacq. were investigated. Crataegus azarolus can be clearly distinguished morphologically from C. monogyna by the colour, size and structure of fruits, the number of pyrenes, the flowering and ripening time, the density of thorns, the tree shape and also the leaf shape. According to our morphological data, in Syria, C. × sinaica is variable and could represent a hybrid of C. azarolus × C. monogyna; the cpDNA sequence analysis showed sequences corresponding to C. monogyna as the plausible mother of the hybrid. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153, 255–263.
... A similar model has been tested and is well accepted for some other plant groups (Hörandl 2006), but in Crataegus we have not been able to find diploid–diploid hybrids among the numerous sexual diploid and apomictic triploid and tetraploid North American species (Talent and Dickinson 2005; Talent and Dickinson 2007c). Thus, the hybridization that is evident to taxonomists (Camp 1942b; Eggleston 1910; Palmer 1932 Palmer , 1943 Phipps 2005) might not result in frequent or persistent diploid hybrids. Camp reasoned that diploid hybrids give rise to triploids because " in other groups such hybrids are notorious for the production of at least some unreduced gametes. ...
Gametophytic apomixis, asexual reproduction involving megagametophytes, occurs in many flowering-plant families and as several variant mechanisms. Developmental destabilization of sexual reproduction as a result of hybridization and/or polyploidy appears to be a general trigger for its evolution, but the evidence is complicated by ploidy-level changes and hybridization occurring with facultative apomixis. The repeated origins of polyploid apomictic complexes in the palaeopolyploid Maloid Rosaceae suggest a new model of evolutionary transitions that may have wider applicability. Two conjectures are fundamental to this model: (1) that as previously suggested by Rutishauser, like many sexual flowering plants the polyploid apomicts require maternal-paternal balance in the second fertilization event that gives rise to the endosperm, and (2) that the observed variation in endosperm ploidy levels relates less to flexibility late in development than to the known variation in developmental origin of the megagametophyte between mechanisms loosely categorized as diplospory and apospory. The model suggests explanations for the relative frequencies of apospory and diplospory, and for the wide but incomplete associations of apospory with a pollination requirement (pseudogamy) and of diplospory with autonomous development of the endosperm. It is suggested that pollination from other taxa may provide some adaptive advantage to pseudogamous apospory.
Field Manual of Michigan Flora is the most up-to-date guide available for all seed plants growing wild in Michigan. Significantly expanding and updating the three-volume Michigan Flora, the book incorporates the discoveries of numerous additional species, recent systematic research, and a vast trove of new information on the shifting distributions of Michigan species. It presents concise identification keys, information about habitats, and completely updated distribution maps for all the seed plants, native or naturalized, that have been recorded from the state, fully treating over 2,700 species. All non-native species are included with notes on their first discovery in the state and comments on invasive tendencies. Rare native species that appear to be declining or to have shrinking ranges are also noted. This book is an essential reference for anyone interested in appreciating Michigan's natural heritage and understanding our ever-changing environment.
Crataegus spes-aestatum J. B. Phipps is described as a new species from Missouri and Illinois. It is critically compared to C. dawsoniana Sargent, to which it keys out in the article where that species was published, and which is here lectotypified. Also, lectotypes for the following new combinations or status from the Missouri Crataegus flora are designated: C. collina Chapman var. hirtiflora (Sargent) J. B. Phipps, C. crus-galli L. var. regalis (Beadle) J. B. Phipps, C. pruinosa (H. L. Wendland) K. Koch var. magnifolia (Sargent) J. B. Phipps, C. reverchonii Sargent var. palmeri (Sargent) J. B. Phipps, C. viridis L. var. glabriuscula (Sargent) J. B. Phipps, and C. viridis L. var. nitens (Sargent) J. B. Phipps. In addition, lectotypes are designated for the varietal basionyms C. hirtiflora Sargent, C. regalis Beadle, C. magnifolia Sargent, C. palmeri Sargent, and C. glabriuscula Sargent, as well as for several of the species names involved, C. collina Chapman, Mespilus pruinosa H. L. Wendland, and C. reverchonii Sargent. Also, a lectotype and epitype for C. berberifolia Torrey & A. Gray, a species with a Missouri variety, are indicated.
Mespilus and Crataegus are sister genera in Rosaceae tribe Pyreae. Mespilus has been seen to comprise not only the medlar, Mespilus germanica, of western Eurasia but also the Arkansas, U.S.A. endemic, Mespilus canescens. Crataegus, on the other hand, consists of 140–200 species found throughout the northern hemisphere. Diagnoses of these two genera rely on morphological features of leaves, flowers and fruits. However, character states supposed to be diagnostic of Mespilus occur in species of Crataegus. We used two nuclear (ribosomal ITS and LEAFY intron2) and four intergenic chloroplast DNA regions (trnS-trnG, psbA-trnH, trnH-rpl2, and rpl20-rps12) to estimate the phylogeny of Mespilus and Crataegus. Maximum parsimony, maximum likelihood, and Bayesian analyses all corroborate the sister group relationship between Crataegus and Mespilus, and Crataegus brachyacantha sister to the rest of Crataegus. However, incongruence between chloroplast and nuclear data supports the hypothesis of a hybrid origin for Mespilus canescens, with Crataegus brachyacantha or its ancestor as the maternal parent. Accordingly, we (1) restrict Crataegus section Brevispinae to Crataegus brachyacantha (2) distinguish the Arkansas endemic as a nothospecies; (3) describe a new section and a new nothosection within Crataegus to contain the former species of Mespilus and Crataemespilus; and (4) make two new combinations under Crataegus.
Polyploidy in C. crus-galli is accompanied by self-compatibility. The weediness of the component taxa of C. crus-galli s.l., in general, and their tendency toward uniparental reproduction, in particular, may explain the homogeneity of their topodemes. -from Authors
The number of genera recognized in subfam. Maloideae by different authors has varied greatly; a historical summary is presented here. We have evaluated generic limits based on our own observations of a suite of morphological characters for about 200 species chosen to represent the taxonomic and geographic diversity of the approximately 940 species of Maloideae. A narrow circumscription of genera is generally adopted, with 28 genera recognized. Hesperomeles is considered distinct from Osteomeles; Eriolobus and Docyniopsis from Malus; and Aria, Chamaemespilus, Cormus, and Torminalis from Sorbus. Micromeles is included within Aria and Stranvaesia and Aronia within Photinia. The genera are either relatively small or relatively large: 19 genera have 11 or fewer species, seven between 26 and 97 species, and only two contain more than 100 species. The traditional division of Maloideae into two tribes is unwarranted and does not reflect relationships of the genera. The extensive hybridization between genera and subgeneric groups seems to reflect weak overall barriers to hybridization in the subfamily rather than indicate evolutionary relationships. Diagnoses are presented for each genus as well as a table comparing morphological characters for all genera. The following new combinations are made: Chamaemespilus alpina, Photinia floribunda, P. melanocarpa, P. pyrifolia, Malus subg. Sorbomalus, Torminalis clusii, and T. orientalis.
In the preparation of a revised treatment of the genus Crataegus in the northeastern United States and adjacent Canada, to be published soon, it has been found necessary to propose a new specific name to replace one invalidated by an earlier homonym and to change the status of a number of names from species to varieties or forms. And in treating the species under a broader concept than that of the original authors, many names are placed in synonymy. This more conservative treatment of the genus seems to be amply justified by extensive field studies, the examination of a very large amount of material in herbaria and in private collections, and by observations extending over a number of years of the hundreds of living plants in the Arnold Arboretum, most of which were grown from seeds taken from type lrees or from type localities. As a result of these studies it is hoped that progress has been made in the understanding of many of the species and of their limits and relationships. But much more work must be done before anything like an adequate or final treatment of this difficult genus can be undertaken. At present many conclusions can only be tentative and based upon the best judgment of the investigator. It must be recognized that there is no real uniformity of opinion among taxonomists on what constitutes a species or on how specific lines can be drawn between very similar and closely related plants or animals with variable and intergrading characters. And it would seem that there must always be a border-line or zone along which such agreement is impossible and within which there will continue to be differences of opinion.. Under an evolutionary concept of nature it can readily be understood that various groups of organisms may stand at different stages in the process of differentiation, and that some forms may have become widely separated from their nearest allies while others are only beginning to diverge in different directions. It is, therefore, not at all surprising to find that authors and reviewers are often in disagreement as to whether certain plants should be regarded as distinct species, as varieties, or as forms, or should be merged with other forms in the broader concept of a single variable species. As everyone who has followed recent taxonomic literature knows, many botanical names have undergone changes in status, and it is not uncommon for authors to change their opinions in subsequent publications even about the validity or standing of their own proposed names. The logical significance of this situation is that classification into species in some sections at least of many of the larger plant genera is largely artificial and a matter of convenience. Crataegus, particularly in the parts of North America where it is most abundantly developed, is a conspicuous example of a comparatively young a~.d expanding genus in a state of instability and in process of evolution. Most of the species are small trees or arborescent shrubs growing on the borders of woods, in glades, or on rocky hills, or springing up in naturally or artificially cleared areas where competition with other plants is not too keen. They are essentially "opportunists, ~' and under primitive conditions when most of eastern North America was covered with a dense unbroken forest, the space available to them was very limited and there was little chance for the different species to come in contact with each other. Such conditions were not generally favorable for hybridization or for
The structure of dichotomous keys is discussed, and some general theorems are proved concerning the number and arrangement of the taxa in any given key. The possibility is considered that the various questions in a key may occasionally be answered wrongly, the key then leading to a wrong determination. It is shown that if this possibility exists, then the probability of a right determination is almost always improved by changing the structure of the key to make it as ‘short’as possible.
The possibility is also considered of adding alternative identification paths through the key (‘reticulations’), starting at points where a particularly difficult question is asked. It is shown that this practice usually improves the key in respect of the determination of certain taxa, but may often diminish the overall probability of correct determination averaged over all taxa.
Differences in the numbers of stamens and styles per flower are conspicuous features of variation in North American hawthorns
(Crataegus L.). Variation in stamen number between individuals is discontinuous, with modes of approximately 20 and 10 (or fewer). In
North American black-fruited sectionDouglasii
Loudon the 10-stamen morphotype is exclusively polyploid, whereas the 20-stamen morphotype comprises both diploids and polyploids.
Polyploidy is associated with apospory, self-fertility, and varying degrees of pollen sterility. Variation in stamen number
may also be correlated with variation in distribution, phenology, leaf shape, and other features of floral morphology, leading
to recognition of taxa at the specific or infraspecific level. Comparable variation in stamen number is virtually unknown
in Eurasian hawthorns, as in the majority of flowering plants. In sectionDouglasii stamen number morphotypes have been recognized as either varieties or species; although correlations between stamen number
and other features are not as straightforward as was previously surmised, the higher rank appears to be appropriate. These
data on breeding system and morphological variation may be explainable in terms of hypotheses linking gametophytic apomixis,
polyploidization, and optimal strategies for pollen dispersal.
The Crataegus problem
- Camp W.H.