Subgenome composition of hybrids and allopolyploids drives genetic differentiation between groups
Interspecific hybridization and gene flow can introduce alleles into a population, which can potentially result in adaptation to novel environmental conditions. Previous work identified apparent admixture between salt-tolerant P. vaginatum and non-tolerant P. distichum , which could produce genotypes adapted to intermediate salinity levels (Eudy, Bahri, Harrison, Raymer, & Devos, 2017). We found that the two ecotypes of P. vaginatum and P. distichum formed distinct genetic groups, with no signatures of gene flow between P. vaginatum and P. distichum (Fig. 3, Fig. S3), although we did identify limited gene flow between the coarse- and fine-textured ecotypes of P. vaginatum (Fig. S4). It is possible that ploidy differences between P. distichum (5x and 6x) and P. vaginatum (2x and 3x) represent a sufficient reproductive barrier to prevent gene flow.
While we did not find evidence for interspecific hybridization in the wild between P. distichum and P. vaginatum , both the coarse-textured and P. distichum groups appear to have been formed from interspecific hybridization between fine-textured P. vaginatum and other Paspalum species. Interestingly, coarse-textured P. vaginatum and P. distichum do not appear to share the same non-P. vaginatum subgenome (Fig. S5). This difference in subgenome composition is thus likely the result of hybridization between P. vaginatum and at least two differentPaspalum species.
Niche differentiation between these two hybrid groups could be due to the genomic contribution of these non-P. vaginatum parents. To our knowledge, no Paspalum species is as salt-tolerant asP. vaginatum ; therefore, it is likely that the non-P. vaginatum parental species of both groups are not halophytic. Despite this, coarse-textured P. vaginatum largely maintained its high salt tolerance while P. distichum shows adaptations for freshwater environments. Hypotheses for why this might have occurred include lower expression of the P. vaginatum subgenome inP. distichum relative to coarse-textured P. vaginatum , differences in salt tolerance between the non-P. vaginatumparents, and post-hybridization selection for freshwater adaptations inP. distichum .