Sperm length divergence as a potential pre-zygotic barrier in a passerine hybrid zone

The saltmarsh sparrow Ammospiza caudacuta and the Nelson’s sparrow A.
nelsoni differ in ecological niche, mating behavior, and plumage, but they
hybridize where their breeding distributions overlap. In this advanced
hybrid zone, past inter-breeding and current back-crossing result in
substantial genomic introgression in both directions, although few hybrids
are currently produced in most locations. However, because both species
are non-territorial and have only brief male-female interactions, it is
difficult to determine to what extent assortative mating explains the low
frequency of hybrid offspring. Since females often copulate with multiple
males, a role of sperm as a post-copulatory pre-zygotic barrier appears
plausible. Here we show that sperm length differs between the two species
in the hybrid zone, with low among-male variation consistent with strong
post-copulatory sexual selection on sperm cells. We hypothesize that
divergence in sperm length may constitute a reproductive barrier between
species, as sperm length co-evolves with the size of specialized female
sperm storage tubules. Sperm does not appear to act as a post-zygotic
barrier, as sperm from hybrids was unexceptional.