The role of sexual isolation during rapid ecological divergence: evidence for a new dimension of isolation in Rhagoletis pomonella

Abstract

The pace of divergence and likelihood of complete speciation may depend how and when different types of reproductive barriers evolve. After initial reproductive barriers evolve, questions remain about how subsequently evolving barriers may facilitate additional divergence and potential speciation. We tested for the presence of sexual isolation (reduced mating between populations due to divergent mating preferences and traits) in Rhagoletis pomonella flies, a model system for incipient ecological speciation. We measured the strength of sexual isolation between two very recently diverged (~170 years) sympatric populations, adapted to different host fruits. We found that sexual isolation was significantly stronger than expectations of random mating. Thus, sexual isolation may play an important role in reducing gene flow allowed by earlier-acting ecological barriers. We also found that sexual isolation was markedly asymmetric between the sexes of each population. Lastly, we tested how warmer temperatures predicted under climate change could alter sexual isolation and found that mating interactions were sensitive to temperature experienced during development. Our findings provide a window into the early divergence process and the role of sexual isolation after initial ecological divergence, in addition to examining multiple factors that could shape the likelihood of further divergence.

Simulated climate warming causes asymmetric responses in insect life‐history timing potentially disrupting a classic ecological speciation system

“…In addition to shifting median eclosion earlier, the warm treatment also caused 59% and 85% increases in the variance of eclosion times in apple and hawthorn flies, respectively (Fligner-Killeen test for homogeneity of variances in non-normal data, both host associations: Χ 2 > 69, df = 1, p < 2.2 × 10 −16 ; Figure 4b,c). The pronounced bimodality in the eclosion curves is likely reflective of the complex mix of categorical and continuous variation in gene-by-environment interaction-driven diapause phenotypes known to exist in this system (Calvert et al, 2022;Dambroski & Feder, 2007), and the same pattern occurred in the next year when the same treatments were used for a follow-up genomic response experiment (Lackey & Powell, 2019).…”

“…Because temporal isolation is an early-acting form for premating isolation, it can have disproportionate effects on total RI compared to lateracting barriers (Ramsey et al, 2003). In this system, temporal isolation acts alongside divergence in olfactory host plant preference (Dambroski et al, 2005) to drive total prezygotic reproductive isolation of ~93% (Lackey et al, 2023). Cases of incomplete ecological speciation may exist in a fragile equilibrium between migration and divergent selection (Nosil et al, 2009).…”