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.
Highlights
Recent research supports the idea that sexual selection promotes species divergence
Previous work has shown that P. a. acuticauda and P. a. hecki show significant differences in total sperm length, but that wild‐caught and captive‐bred individuals do not differ in sperm morphology (Rowe, Griffith, et al, 2015), and it the case that in the closely related zebra finch that has been more extensively sampled, there is no difference in sperm morphology between wild and captive birds (Immler, Griffith, Zann, & Birkhead, 2012)
Our results demonstrate that the number of sperm that reach the ova dif‐ fer according to experimental pair type in the long‐tailed finch
Summary
Recent research supports the idea that sexual selection promotes species divergence (reviewed in Kraaijeveld, Kraaijeveld‐Smit, & Maan, 2011, Ritchie, 2007). The study of speciation has neglected re‐ productive barriers acting between copulation and the formation of the zygote (i.e., postmating, prezygotic barriers, PMPZ; Coyne & Orr, 2004; Howard et al, 2009) This is true for inter‐ nally fertilizing species and is likely attributable to the cryptic and complex nature of ejaculate‐female and sperm‐egg interactions in such taxa (Howard et al, 2009; Pitnick, Wolfner, & Suarez, 2009). We tested for differences in the total number of sperm reaching the egg at the time of fertilization among different experimen‐ tal crosses, including intra‐ and inter‐subspecific and backcross pairs, to examine the potential contribution of ejaculate‐fe‐ male/sperm‐egg interactions in reproductive isolation between these subspecies
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