Genetic analysis of Marsh Wrens across a broad transition zone in southern Saskatchewan reveals deep divergence and little hybridization between two cultural phenotypes

We here explore the relationship between biological and cultural evolution in the Marsh Wren (Cistothorus palustris), which across Canada and the United States can be easily differentiated into two geographic groups based on diagnostic western and eastern song types. The wren songs are culturally transmitted, as young males raised in the laboratory readily learn songs of either type. We find that, in cattail- and bulrush-dominated marshes across a >400-km transition zone in southern Saskatchewan, males of both cultural traditions occur together and defend territories against each other; pure western and pure eastern singers are abundant, and relatively few males have learned songs of both phenotypes. To determine the genetic ancestry of singing males in these two cultural traditions, we generated reduced-representation genomic sequence data for birds from across this contact zone. We find exceptionally high genome-wide divergence between western and eastern singing Marsh Wrens from outside the contact zone (FST = 0.61), potentially the highest between any lineages that form a hybrid zone in the North American Great Plains. In the heart of the contact zone, genetic data indicate an overall low rate of hybrid ancestry, and ancestry is tightly coupled with male song type, suggesting that this cultural trait could play a role in maintaining a strong but imperfect degree of assortative mating. All nine of the mixed singers (males with both western and eastern songs) that we sequenced were of predominantly eastern ancestry, with six of them having 100% eastern genotype. These mismatched individuals could play an outsized role in driving the ongoing generation of hybrid offspring and the introgression of eastern alleles into western populations. Taken together, these genomic and vocal datasets reveal that the western and eastern Marsh Wren song phenotypes represent deeply divergent parapatric lineages despite breeding side-by-side in the same marshes. Although all males are capable of learning the songs of the other genotype, they rarely do, and two cultural traditions representing two genetic ancestries appear to coexist stably in the Great Plains of southern Saskatchewan.