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. Recent studies suggest that many polyploids arise from hybridization between members of different infrageneric groups. Gametophytic apomixis and polyploidy are prevalent in the genus, and disagreement concerns whether and how apomictic genotypes should be recognized taxonomically. 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. Rosaceae tribe Maleae) form a well-defined clade comprising five subgeneric groups readily distinguished using either molecular or morphological data. 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. Crataegus encompasses considerable variation in its thorns, leaves, flowers, and fruits, such that there is a well-developed infrageneric classification (Table 1 Loudon, 1838 Palmer, 1925 Phipps, 2015 Schneider, 1906 Ufimov & Dickinson, 2020) that is supported by DNA sequence data (Fig. (Li et al., 2012 Liu et al., 2020), Crataegus diverged from unarmed, berry-fruited Amelanchier, Malacomeles, and Peraphyllum by the acquisition of thorns and polypyrenous drupes whose pyrenes enclose a single seed (Campbell et al., 2007 Lo & Donoghue, 2012 Potter et al., 2007 Xiang et al., 2017 Zhang et al., 2017). Greater climate niche breadth in allopolyploids compared to diploids similarly may be more due to parental traits than to effects of genome duplication per se. Comparing sections Douglasianae and Salignae suggests that geographic parthenogenesis (larger range sizes in apomicts, compared to sexually reproducing taxa) may have less to do with adaptation than it does with reproductive assurance in the pseudogamously apomictic and self-compatible hybrids. Americanae with much wider distributions. Their morphologies, geographic ranges, and niche characteristics resemble those of their diploid, sexual parent or are intermediate between them and those of their other parent, one or both of two partially sympatric tetraploid apomicts in red-fruited C. They are presumptively self-fertile polyploids with pseudogamous gametophytic apomixis. Sanguineae (sections Douglasianae, Salignae) have previously been identified as hybrids. Members of two black-fruited taxonomic sections of Crataegus subg. Voucher specimens provided morphometric data that were analyzed using uni- and multivariate methods. Flow cytometry documented ploidy level and breeding system. Species distribution data from herbarium specimens and online databases were analyzed in order to compare ranges and climate niches described by bioclimatic variables. We have documented range, breeding system, morphology, leaf vascular architecture, and niche breadth in these hawthorns, for which phylogenetic relationships and ploidy levels are known. We compare biogeographic and morphological parameters of two agamic complexes of western North American hawthorns so as to evaluate possible explanations of the differences in range between sexually reproducing taxa and their apomictic sister taxa. Labels are placed approximately in the center of the geographic distribution of the corresponding group. Support values are > 95% for all nodes except the one supporting C. The RAxML tree was inferred from a complete plastome alignment for a sample of 14 diploid accessions representing all of the infrageneric groups shown here (Table 1 Liston et al., in prep.), rooted using the apple plastome (Velasco et al., 2010), and collapsed as described in the text to show just the subgenera and the three sections within C. Superimposed on the map is the RAxML tree for Crataegus subgenera: C, C. North polar projection of a tectonic plate reconstruction for 37 Ma produced using the service at (Hay et al., 1999).
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