Asymmetric acoustic signal recognition led to asymmetric genetic introgression between two parapatric rhacophorid treefrogs

Correct discrimination between courtship signals could help maintain
genetic integrity between closely related species. However, asymmetric
usage of signals might cause asymmetric magnitude of gene flow across the
contact zone. Buergeria japonica and B. otai are sibling species with
parapatric distribution pattern in Taiwan, forming two narrow contact
zones in eastern and western sides of the island. Both species use a
shared acoustic signal of calls (Type 1a/1b), whereas B. otai presents
another unique call type (Type 2) which never appears in B. japonica.
Combining behavioral experiments with genome-wide RAD-seq analyses, we aim
to test whether the ability of signal recognition influences genetic
introgression across their species boundary. The playback experiments
showed that the western population of B. otai has evolved a stronger level
of reproductive character displacement by showing the inclusive usage of
their unique Type 2 signal. In contrast, the eastern population used both
unique and shared signals and has a stronger preference for the latter.
Consistent with behavioral difference, genetic introgression across the
contact zone was detected only in the eastern boundary but not in the
western one. Furthermore, the gene flow in this contact zone tends to be
unidirectional from B. japonica toward B. otai. Our results support the
prediction that a more specialized signal user might have a higher
probability to maintain their genetic integrity compared to a generalized
signal user.