Geographic variation in hybridization across a reinforcement contact zone of chorus frogs (Pseudacris)

Abstract

Reinforcement contact zones, which are secondary contact zones where species are diverging in reproductive behaviors due to selection against hybridization, represent natural laboratories for studying speciation‐in‐action. Here, we examined replicate localities across the entire reinforcement contact zone between North American chorus frogs Pseudacris feriarum and P. nigrita to investigate geographic variation in hybridization frequencies and to assess whether reinforcement may have contributed to increased genetic divergence within species. Previous work indicated these species have undergone reproductive character displacement (RCD) in male acoustic signals and female preferences due to reinforcement. We also examined acoustic signal variation across the contact zone to assess whether signal characteristics reliably predict hybrid index and to elucidate whether the degree of RCD predicts hybridization rate. Using microsatellites, mitochondrial sequences, and acoustic signal information from >1,000 individuals across >50 localities and ten sympatric focal regions, we demonstrate: (1) hybridization occurs and (2) varies substantially across the geographic range of the contact zone, (3) hybridization is asymmetric and in the direction predicted from observed patterns of asymmetric RCD, (4) in one species, genetic distance is higher between conspecific localities where one or both have been reinforced than between nonreinforced localities, after controlling for geographic distance, (5) acoustic signal characters strongly predict hybrid index, and (6) the degree of RCD does not strongly predict admixture levels. By showing that hybridization occurs in all sympatric localities, this study provides the fifth and final line of evidence that reproductive character displacement is due to reinforcement in the chorus frog contact zone. Furthermore, this work suggests that the dual action of cascade reinforcement and partial geographic isolation is promoting genetic diversification within one of the reinforced species.

Fine‐scale geographic patterns of gene flow and reproductive character displacement inDrosophila subquinariaandDrosophila recens

“…The signature of admixture between species has been tested in some other systems undergoing reinforcement, and results vary in the magnitude and direction of introgression (e.g., Burri et al., ; Kulathinal, Stevison, & Noor, ; Lemmon & Juenger, ; Roda, Mendes, Hahn, & Hopkins, ; Turissini & Matute, ). A similar pattern to our system is seen in Phlox plants, where introgression between species occurs in sympatric but not in allopatric populations and the direction of introgression is asymmetric (Roda et al., ).…”

Do phylogeny and habitat influence admixture among four North American chickadee (family: Paridae) species?

“…Our study emphasizes how variable zones of admixture and hybridization are (Gill 2004, Lemmon and Juenger 2017); both in terms of the rate of admixture, and the geographic distribution of admixture. Admixture varied across habitats, and although the highest rates of admixture were observed in areas where species pairs occupied more similar habitats, habitat similarity was not a strong predictor of hybridization, as has been shown in other studies (Taylor et al 2014, Bell and Irian 2019).…”

“…Rates of admixture and hybridization are influenced through a variety of factors including prezygotic and postzygotic reproductive isolation (Borge et al 2005, Lemmon and Lemmon 2010, Moran et al 2019; for example, reductions in hybrid fitness appear to have a greater effect on reproductive isolation than assortative mating (Irwin 2020). Importantly, rates of admixture and hybridization can vary substantially across geographic space (Bell 1996, Lemmon andJuenger 2017), and therefore studies that examine admixture and hybridization across geographic space can provide greater insights into the processes facilitating admixture and hybridization between species pairs. Hybridization is common in natural populations.…”

Patterns, Mechanisms and Genetics of Speciation in Reptiles and Amphibians

“…There is also evidence for speciation driven by reinforcement on the call variation in Australian treefrogs ( Litoria ; [26]). Other important systems in which calls are important to reproductive isolation include North American spadefoot toads ( Spea ; [192,193]) and chorus frogs ( Pseudacris ; [194,195]). An unresolved challenge for studies of frog speciation is to determine whether call variation is the initial cause of lineage splitting or merely helps distinct lineages remain distinct (especially given that many frog species appear to arise in allopatry, where call differences are expected to be irrelevant to speciation).…”

“…An unresolved challenge for studies of frog speciation is to determine whether call variation is the initial cause of lineage splitting or merely helps distinct lineages remain distinct (especially given that many frog species appear to arise in allopatry, where call differences are expected to be irrelevant to speciation). Interestingly, an important cause of reproductive isolation among populations in several systems may be call divergence between conspecific populations where some populations are sympatric with heterospecifics ([26,193,195]).…”