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.