Adaptive differentiation in life history traits despite propensity for
hybridization in two sister salamanders
Abstract
Hybridization can have a profound negative effect on population fitness
when species exhibit divergence in adaptive traits. The Streamside
salamander, Ambystoma barbouri, and the Smallmouth salamander, A.
texanum, are closely related species differentiated primarily by
breeding habitat and reproduction-related traits, but previous work
suggests patterns of hybridization and introgression between them. Here
we investigate whether 1) hybridization occurs between the two species
in laboratory settings, 2) their divergent reproduction-related traits
are retained under a common treatment, and 3) hybrid offspring exhibit
similar fitness as their non-hybrid counterparts. We bred conspecific
and heterospecific pairs of A. barbouri and A. texanum and raised hybrid
and non-hybrid offspring to metamorphosis. Reproduction-related traits
such as oviposition location and clutch size remained well
differentiated between the two species. Heterospecific pairs hybridized
readily, at similar rates to conspecific pairs. The hybrid larvae
generally exhibited a maternal effect and, less frequently, intermediate
phenotypes with respect to measured traits. Hybrid larvae did not
exhibit reduced fitness as measured by survival to metamorphosis. Our
results suggest that traits differentiating A. barbouri and A. texanum
are likely genetically determined despite the lack of reproductive
isolation between them. This suggests that the generally parapatric
distribution of the two species, often paired with abrupt transition in
traits, might be driven by selection on these traits in local habitats.
Further, the maternal effect and consequent lack of intermediate
phenotypes observed for several traits likely reduces the negative
effect of hybridization in locally adapted populations.