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.