Relative resource allocation to dispersal and competition demonstrates the putative role of hybridity in geographical parthenogenesis
Asexual organisms frequently have larger ranges than their sexual progenitors, a phenomenon referred to as geographical parthenogenesis (GP). In plants, GP is associated not only with asexuality (apomixis), but also with polyploidization and hybridity (allopolyploidy). Dispersal is thought to play a role in range-size differences in other taxa, but has not been directly related to GP. Here, we compare resource allocation to dispersal-related traits in sexual diploids, asexual autopolyploids and asexual allopolyploids, and relate these differences to patterns of GP.
The Pacific Northwest, North America.
We created distribution maps for all cytotypes known in Crataegus series Douglasianae using herbarium records. To quantify dispersal ability, we collected fruit samples from sexual diploids, apomictic allopolyploids and apomictic autopolyploids across their ranges, and used the masses of each fruit component as a proportion of the total fruit mass to gauge relative investment in dispersal and competitive ability.
The largest ranges belong to apomictic allopolyploids, whereas apomictic autotriploids and sexual diploids have the smallest ranges. Compared to sexual diploids and apomictic autotriploids, the allotetraploids exhibit a more dispersal-orientated strategy, with proportionally heavier pyrenes and more fruit pulp, but proportionally lighter seeds. Allotriploid taxa, which arose via back-crosses between sexual diploids and asexual allotetraploids, exhibit an intermediate range size as well as intermediate investment in dispersal.
In Crataegus series Douglasianae, GP is associated only with allopolyploids, highlighting the potential role of hybridization in range expansion. The data suggest that allopolyploids are associated with increased resource allocation to dispersal-related traits, whereas sexual diploid C. suksdorfii and asexual autotriploid C. gaylussacia exhibit a more competition-orientated strategy. These findings are consistent with the hypothesis that hybridization contributes to patterns of GP in asexual allopolyploids, potentially by increasing their dispersal ability.