Abstract
The saltmarsh sparrow Ammospiza caudacuta and 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 interbreeding and current backcrossing result in substantial genomic introgression in both directions, although few hybrids are currently produced in most locations. However, because both species are nonterritorial 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 postcopulatory prezygotic 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 postcopulatory 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 postzygotic barrier, as sperm from hybrids was unexceptional.
Highlights
Allopatric populations may evolve divergent phenotypes due to differing ecological pressures and sexual selection, potentially resulting in pre-and postzygotic reproductive isolation upon secondary contact (Coyne & Orr, 2004)
Determining which phenotypes have diverged and how diverged phenotypes contribute to reproductive isolation upon secondary contact is an integral part of understanding speciation (Shaw & Mullen, 2011), for example, complementing the understanding of how genomic architecture impacts speciation (Campbell et al, 2018) and how selection on one isolating mechanism impacts the evolution of other reproductive barriers (Lorch & Servedio, 2007)
Adaptation to divergent ecological pressures appears to have played a key role in speciation between the sister species saltmarsh sparrow (Ammospiza caudacuta) and Nelson's sparrow (A. nelsoni) (e.g., Walsh et al, 2018, 2019; Walsh, Olsen et al, 2016)
Summary
Allopatric populations may evolve divergent phenotypes due to differing ecological pressures and sexual selection, potentially resulting in pre-and postzygotic reproductive isolation upon secondary contact (Coyne & Orr, 2004). Anecdotal evidence suggests that saltmarsh sparrow males preferentially follow and copulate with conspecific females (Greenlaw, 1993) Due to these phenotypic differences, offspring production occurs primarily within species at coastal sites throughout the hybrid zone, with more conspecific offspring produced than would be expected via random mating, given the proportion of available partners of each species (Maxwell et al, 2021; Walsh, Maxwell et al, 2018). Given that hybrid males produce offspring in this hybrid zone (Maxwell et al, 2021; Walsh, Maxwell et al, 2018), we expected to find some normal sperm cells among hybrid males
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