Patterns of introgression vary within an avian hybrid zone

Abstract

Background Exploring hybrid zone dynamics at different spatial scales allows for better understanding of local factors that influence hybrid zone structure. In this study, we tested hypotheses about drivers of introgression at two spatial scales within the Saltmarsh Sparrow (Ammospiza caudacuta) and Nelson’s Sparrow (A. nelsoni) hybrid zone. Specifically, we evaluated the influence of neutral demographic processes (relative species abundance), natural selection (exogenous environmental factors and genetic incompatibilities), and sexual selection (assortative mating) in this mosaic hybrid zone. By intensively sampling adults (n = 218) and chicks (n = 326) at two geographically proximate locations in the center of the hybrid zone, we determined patterns of introgression on a fine scale across sites of differing habitat. We made broadscale comparisons of patterns from the center with those of prior studies in the southern edge of the hybrid zone. Results A panel of fixed SNPs (135) identified from ddRAD sequencing was used to calculate a hybrid index and determine genotypic composition/admixture level of the populations. Another panel of polymorphic SNPs (589) was used to assign paternity and reconstruct mating pairs to test for sexual selection. On a broad-scale, patterns of introgression were not explained by random mating within marshes. We found high rates of back-crossing and similarly low rates of recent-generation (F1/F2) hybrids in the center and south of the zone. Offspring genotypic proportions did not meet those expected from random mating within the parental genotypic distribution. Additionally, we observed half as many F1/F2 hybrid female adults than nestlings, while respective male groups showed no difference, in support of Haldane’s Rule. The observed proportion of interspecific mating was lower than expected when accounting for mate availability, indicating assortative mating was limiting widespread hybridization. On a fine spatial scale, we found variation in the relative influence of neutral and selective forces between inland and coastal habitats, with the smaller, inland marsh influenced primarily by neutral demographic processes, and the expansive, coastal marsh experiencing higher selective pressures in the form of natural (exogenous and endogenous) and sexual selection. Conclusions Multiple drivers of introgression, including neutral and selective pressures (exogenous, endogenous, and sexual selection), are structuring this hybrid zone, and their relative influence is site and context-dependent.

An evidence-based overview of hybridization in tinamous

“…For instance, Apolinario and Silveira (2019) compared the plumage patterns of several museum specimens to identify hybrids between the Plush-crested Jay (Cyanocorax chrysops) and the White-naped Jay (C. cynanopogon) in Brazil. The third criterion includes genetic analyses that confirm the hybrid origin of an individual (Knowlton et al 2014;Toews et al 2020) or document recent gene flow between interbreeding species (Coster et al 2018;Maxwell et al 2021).…”

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

“…Sparrows were sampled during the 2016 breeding season (May-August), as part of a larger study (Maxwell et al, 2021) (Maxwell et al, 2021).…”

“…We followed the Peterson et al (2012) protocol, as described in Maxwell et al (2021). Thirty allopatric individuals of each parental species were also included as reference for building a hybrid index (Maxwell et al, 2021) Illumina HiSeq 2,500 lanes and one HiSeq 2,500 rapid run lanes of (100 bp reads).…”

“…The two species hybridize along a narrow stretch of coast in North America, with high genetic admixture despite the rarity of intermediates equivalent to F1 hybrids (Maxwell et al, 2021;Walsh et al, 2015). Within the hybrid zone, several genes involved in osmotic balance show particularly high introgression, which is hypothesized to be adaptive in the context of the mosaic of habitats, with varying salinity, found within the hybrid zone .…”

“…A broad range of hybrid sperm phenotypes is observed in other hybridizing passerine sister (or near-sister) species pairs, ranging from an absence of normal sperm cells (Ålund et al, 2013) to apparently normal morphology and swimming performance (Albrecht et al, 2019;Cramer et al, 2015). Given that hybrid males produce offspring in this hybrid zone (Maxwell et al, 2021;Walsh, Maxwell et al, 2018), we expected to find some normal sperm cells among hybrid males.…”