Widespread generalist clones are associated with range and niche expansion in allopolyploids of Pacific Northwest Hawthorns (Crataegus L.)

Abstract

Range and niche expansion are commonly associated with transitions to asexuality, polyploidy and hybridity (allopolyploidy) in plants. The ability of asexual polyploids to colonize novel habitats may be due to widespread generalist clones, multiple ecologically specialized clones, or may be a neutral by-product of multiple, independent origins of asexual polyploids throughout the range. We have quantified niche size and divergence for hawthorns of the Pacific Northwest using data from herbarium vouchers with known cytotypes. We find that all polyploid niches diverge from that of the diploid range, and allopolyploids have the broadest niches. Allotetraploids have the largest niche and the widest geographic distribution. We then assessed the genetic mechanism of range expansion by surveying the ecological and geographic distribution of genotypes within each cytotype from sites in which fine-scale habitat assessments were completed. We find no isolation by either geographic or ecological distance in allopolyploids, suggesting high dispersal and colonization ability. In contrast, autotriploids and diploids show patterns of isolation by geographic distance. We also compared the geographic and ecological distributions of clonal genotypes with those of randomly drawn sites of the most widespread cytotype. We found that most clones are geographically widespread and occur in a variety of habitats. We interpret these findings to suggest that patterns of range and niche expansion in Pacific Northwest Hawthorns may stem from these widespread, ecologically generalist clones of hybrid origin.

Determinants of geographic range size in plants

“…Allopolyploids that form via hybridization between two species often exhibit asexual reproduction, and are thereby triply poised to have broader environmental tolerances via reproductive assurance, greater adaptive potential due to mutations, and greater adaptive potential due to the joining of two parental genomes. Further, allopolyploids that reproduce asexually via apomictic seeds represent fixed heterozygotes, and may either behave as general-purpose genotypes with broad phenotypic plasticity and environmental tolerance, or specialized lineages that collectively confer broad tolerance to the polyploid taxon as a whole (Coughlan et al, 2017). Second, larger genome and cell sizes associated with polyploidy may lead to instantaneous shifts in physiological tolerances and trait values (Levin, 2002).…”

“…In the Alyssum montanum species complex, allopolyploids had higher rates of climatic niche evolution and expanded into unique climates not occupied by their diploid ancestors, while autopolyploids had slower rates of niche evolution and their niches more closely resembled those of their diploid progenitors (Arrigo et al, 2016). In hawthorns (genus Crataegus), allopolyploids had larger geographic ranges and ecological niches than diploids or autopolyploids, and this difference in range size was related to increased resource allocation to dispersal traits such as larger quantities of fruit pulp and smaller seed size trading off with reduced allocation to competitive traits (Coughlan et al, 2017(Coughlan et al, , 2014. Further, many hybrid clones occurred in several habitat types, supporting the hypothesis that polyploidization results in generalpurpose genotypes (Coughlan et al, 2017).…”

“…In hawthorns (genus Crataegus), allopolyploids had larger geographic ranges and ecological niches than diploids or autopolyploids, and this difference in range size was related to increased resource allocation to dispersal traits such as larger quantities of fruit pulp and smaller seed size trading off with reduced allocation to competitive traits (Coughlan et al, 2017(Coughlan et al, , 2014. Further, many hybrid clones occurred in several habitat types, supporting the hypothesis that polyploidization results in generalpurpose genotypes (Coughlan et al, 2017). Transplant experiments of diploid and polyploid cytotypes (i.e.…”

Sex and Rosaceae apomicts

“…The population-level sampling in the study demonstrated a clear contrast between diploids and tetraploids with respect to the geographic extent of their ranges. Subsequently, wider geographic distributions in allopolyploids relative to diploids were found to be correlated with greater ecological amplitude in the polyploids (Lo & al., 2013;McGoey & al., 2014;Coughlan & al., 2017). Coughlan & al.…”

“…(2014) showed that these features of the polyploids also correlated with increased relative investment in fruit biomass compared to seed biomass, suggesting a contrast between features favoring dispersal (in polyploids) versus establishment (in diploids). What we now know about hybridization in these hawthorns suggests that this contrast in fruit composition in the black-fruited taxa arose from apomixis and polyploidy having been introduced by crosses with widespread eastern North American apomictic tetraploids with much larger, red fruits (Coughlan & al., 2014(Coughlan & al., , 2017.…”