Mito‐nuclear discordance at a mimicry color transition zone in bumble bee Bombus melanopygus

Abstract

As hybrid zones exhibit selective patterns of gene flow between otherwise distinct lineages, they can be especially valuable for informing processes of microevolution and speciation. The bumble bee, Bombus melanopygus, displays two distinct color forms generated by Müllerian mimicry: a northern “Rocky Mountain'’ color form with ferruginous mid‐abdominal segments ( B. m. melanopygus) and a southern “Pacific'’ form with black mid‐abdominal segments ( B. m. edwardsii). These morphs meet in a mimetic transition zone in northern California and southern Oregon that is more narrow and transitions further west than comimetic bumble bee species. To understand the historical formation of this mimicry zone, we assessed color distribution data for B. melanopygus from the last 100 years. We then examined gene flow among the color forms in the transition zone by comparing sequences from mitochondrial COI barcode sequences, color‐controlling loci, and the rest of the nuclear genome. These data support two geographically distinct mitochondrial haplogroups aligned to the ancestrally ferruginous and black forms that meet within the color transition zone. This clustering is also supported by the nuclear genome, which, while showing strong admixture across individuals, distinguishes individuals most by their mitochondrial haplotype, followed by geography. These data suggest the two lineages most likely were historically isolated, acquired fixed color differences, and then came into secondary contact with ongoing gene flow. The transition zone, however, exhibits asymmetries: mitochondrial haplotypes transition further south than color pattern, and both transition over shorter distances in the south. This system thus demonstrates alternative patterns of gene flow that occur in contact zones, presenting another example of mito‐nuclear discordance. Discordant gene flow is inferred to most likely be driven by a combination of mimetic selection, dominance effects, and assortative mating.

Abstract

This study examines the shift in color and genotype (mitochondrial and nuclear) frequencies along a narrow transition zone between mimetic color variants of bumble bee Bombus melanopygus. The data support this being a secondary contact zone for two historically distinct populations with different colors, but alternative patterns of gene flow have resulted in different distributions of color alleles from mitochondrial markers. This discordance may be driven by a combination of mimetic selection and asymmetric premating bias, a partial barrier which would place this species along the speciation continuum.