Differential sperm‐egg interactions in experimental pairings between two subspecies and their hybrids in a passerine bird

Abstract

Speciation research has largely overlooked reproductive barriers acting between copulation and the formation of the zygote (i.e., postmating, prezygotic [PMPZ] barriers), especially in internally fertilizing vertebrates. Nonetheless, it is becoming clear that PMPZ reproductive barriers can play a role in the formation and maintenance of species boundaries. We investigated sperm‐egg interactions in the recently diverged subspecies pairs of the long‐tailed finch, Poephila acuticauda acuticauda and P. a. hecki, to explore potential PMPZ barriers. Specifically, we compared the number of sperm reaching the perivitelline layer (PVL) of the ova, and hence the site of fertilization, in both intra‐ and inter‐subspecies pairings and pairings of F1 hybrid adults with one parental subspecies. Although we found no difference in PVL sperm number among intra‐ and inter‐subspecific pairs, a significantly lower number of sperm reached the site of fertilization in a backcross pairing. As low numbers of PVL sperm appear to be associated with low fertilization success in birds, our findings offer insight into the potential role of postcopulatory processes in limiting gene flow between the subspecies and may help explain the relatively narrow hybrid zone that exists in the wild in this species. Though further work is needed to gain a comprehensive understanding of the morphological, physiological, and molecular mechanisms underlying our results, our study supports the role of PMPZ reproductive barriers in avian speciation, even in recently diverged taxa, that may not yet be fully genetically incompatible.

Evaluating evidence of mitonuclear incompatibilities with the sex chromosomes in an avian hybrid zone

“…2015; Hurley et al. 2018). We focus on the most basic manifestation of Haldane's rule in birds: that hybrid females should suffer the most in terms of sterility, rarity, or absence compared to hybrid males (Haldane 1922), evaluating evidence for hybrid female scarcity.…”

“…2015), and sperm‐egg interactions (Hurley et al. 2018), and the potential hybrid zone contributions to reproductive isolation of each of these traits remains to be studied.…”

“…2015; Hurley et al. 2018; Table S1). This tissue‐based approach to mitochondrial sequencing minimizes the potential influence of NUMT (nuclear copies of mitochondrial genes) interference common in DNA extracted from avian blood (Sorenson and Quinn 1998).…”

“…Indeed, F 1 hybrid males have significant reductions in the number of sperm that reach the ova in backcross pairings – a clear intrinsic fitness cost to hybridization between subspecies (Hurley et al. 2018).…”

Geography is essential for reproductive isolation between florally diversified morning glory species from Amazon canga savannahs

“…Events happening after pollen grain deposition at a flower stigma surface constitute postmating or else known as postpollination isolation barriers in plants, which comprise pollen-pistil incompatibilities and congeneric pollen disadvantages 9,13 . Postmating barriers underpinned by gametic competition are documented in birds 14 , fish 15 and insects 16 . Reduced hybrid viability and infertility are two major phenotypic manifestations of postzygotic isolation both in plants and animals 7–9,17 .…”