Differing patterns of clonal propagation
For species that commonly reproduce clonally, there are two proposed strategies that can explain geographic distributions of individual clonal genotypes. The “frozen niche” strategy is characterized by multiple genotypes that are each adapted to a specific geographically localized environment (Róis et al., 2015; Vrijenhoek 1979). Conversely, the “general purpose genotype” strategy is characterized by a few genotypes that are widely distributed across a range of environments (Baker, 1965; Bricker, Calladine, Virnstein, & Waycott, 2018; Coughlan, Han, Stefanović, & Dickinson, 2017). The occurrence of interspecific hybridization could be an important factor in determining clonal propagation strategy. Hybrids, which are often obligately asexual, are a common model system to investigate patterns of clonal distribution; however, opportunities to investigate differential clonal reproduction strategies between a hybrid and its parental species are rare as the parental species are often obligately sexual (Coughlan, Han, Stefanović, & Dickinson, 2017). With our collection of clones from both the hybrid coarse-textured P. vaginatum and one of its parent species, fine-textured P. vaginatum, we were able to explicitly test for and confirm differences in the geographic and environmental ranges of clonal genotypes.
Hybridization is expected to yield genotypes with increased phenotypic plasticity, as they can access alleles from both parental species (Shimizu‐Inatsugi et al., 2017). Since high plasticity genotypes should have increased ability to survive in multiple environments, hybrid genotypes may thus be more likely to follow the “general-purpose genotypes” strategy than either parental species (Baker, 1965; Coughlan, Han, Stefanović, & Dickinson, 2017; Lynch, 1984). Our findings are consistent with this prediction in that the hybrid coarse-textured ecotypes follow a “general-purpose genotype” pattern while fine-textured plants follow a “frozen niche” pattern. This indicates that hybridization may have induced a transition in clonal propagation strategy in P. vaginatum. Furthermore, within the hybrid coarse-textured genotypes, allotriploidy may provide an additional boost to plasticity relative to diploids as the two most common and widespread genotypes are both triploids (Fig. 1). This is consistent with the assumption that allopolyploids have access to even more potentially adaptive alleles than diploid hybrids.
Differences in the rate of sexual reproduction may also influence clonal propagation strategy. Recombination during sexual reproduction can produce novel clonally propagating genotypes. Clones of the most fit genotype in a given location will then dominate it, resulting in a “frozen-niche” pattern (Vrijenhoek & Parker, 2009). Consistent with this proposed strategy, fine-textured P. vaginatum genotypes are known to be capable of sexual reproduction in addition to vegetative clonal propagation (Duncan & Carrow 2000). Sexual reproduction of coarse-textured ecotypes has not been tested; however, the low number of unique genotypes indicate that it is likely rare ­­(although the presence of admixed individuals suggests that it is possible) (Fig. 3, Fig. S3, Fig. S4). This lack of post-hybridization recombination could make it difficult for coarse-textured plants to specialize in a given location. Additionally, clonal propagation can serve to maintain plasticity by preventing the loss of heterozygosity in hybrids. Both of these factors could explain the “general-purpose genotype” pattern seen in coarse-textured clones.