Morphometric data of two tree frog species and their hybrids from a hybrid zone in Poland

Under incomplete reproductive isolation, secondary contact of diverged
allopatric lineages may form hybrid zones that allow to study recombinants
over several generations as excellent systems of genomic interactions,
resulting from the evolutionary forces, acting on certain genes and
phenotypes. Hybrid phenotypes are expected to either exhibit intermediacy
or, alternatively, transgressive traits, which exceed the extremes of
their parents due to epistasis and segregation of complementary alleles.
While transgressive morphotypes have been examined in fish, reptiles,
birds, and mammals, studies in amphibians are rare. Here, we associate
microsatellite-based genotypes and morphometrics-based morphotypes of two
European tree frog species of the Hyla arborea group,
sampled across a hybrid zone in Poland, to understand whether the
genetically differentiated parental species also differ in morphology
between each other and their hybrids, and whether secondary contact leads
to the evolution of intermediate or transgressive morphotypes. Using
univariate approaches, explorative multivariate methods (Principal
Component Analyses) as well as techniques with prior grouping
(Discriminant Function Analyses), we find that morphotypes of both
parental species and hybrids differ from each other. Importantly, hybrid
morphotypes are neither intermediate nor transgressive but found to be
more similar to H. orientalis than to H.
arborea. Our study presents one of the rare datasets, in which genotypes
along with morphotypes examined the occurrence or absence of transgressive
morphologies in wild amphibians.