The effects of heterospecific mating frequency on the strength of cryptic reproductive barriers

Abstract

Heterospecific mating frequency is critical to hybrid zone dynamics and can directly impact the strength of reproductive barriers and patterns of introgression. The effectiveness of post‐mating prezygotic (PMPZ) reproductive barriers, which include reduced fecundity via heterospecific matings and conspecific sperm precedence, may depend on the number, identity and order of mates. Studies of PMPZ barriers suggest that they may be important in many systems, but whether these barriers are effective at realistic heterospecific mating frequencies has not been tested. Here, we evaluate the strength of cryptic reproductive isolation in two leaf beetles (Chrysochus auratus and C. cobaltinus) in the context of a range of heterospecific mating frequencies observed in natural populations. We found both species benefited from multiple matings, but the benefits were greater in C. cobaltinus and extended to heterospecific matings. We found that PMPZ barriers greatly limited hybrid production by C. auratus females with moderate heterospecific mating frequencies, but that their effectiveness diminished at higher heterospecific mating frequencies. In contrast, there was no evidence for PMPZ barriers in C. cobaltinus females at any heterospecific mating frequency. We show that integrating realistic estimates of cryptic isolation with information on relative abundance and heterospecific mating frequency in the field substantially improves our understanding of the strong directional bias in F1 production previously documented in the Chrysochus hybrid zone. Our results demonstrate that heterospecific mating frequency is critical to understanding the impact of cryptic post‐copulatory barriers on hybrid zone structure and dynamics, and that future studies of such barriers should incorporate field‐relevant heterospecific mating frequencies.

Sperm length divergence as a potential prezygotic barrier in a passerine hybrid zone

“…However, without linkage among loci causing postzygotic isolation and phenotypic divergences, reinforcement acting on either plumage or sperm could actually slow the evolution of a reproductive barrier based on the other trait via reinforcement, because the presence of one barrier reduces the selective pressure promoting the other (Lorch & Servedio, 2007; Marshall et al., 2002). Understanding phenotypic and genetic correlations among different phenotypes relevant for reproductive isolation remains an important challenge in studying hybrid zones and may assist with understanding how reproductive barriers accumulate over evolutionary time, and how each individual reproductive barrier contributes to overall reproductive isolation (e.g., Larson et al., 2019; Mendelson et al., 2007).…”