Article,
Selection and hybridization shaped the Africanized honey bee invasion of the Americas
bioRxiv, (2020)
DOI: 10.1101/2020.03.17.994632
Abstract
Recent biological invasions offer remarkable ’natural’ laboratories to understand the genetics and ecology of adaptation, hybridization, and range limits. One of the most impressive and well-documented invasions of the 20th century began in Brazil with the escape of introduced African honey bees (Apis mellifera scutellata) in 1957. In less than 50 years, populations of ’Africanized’ honey bees spread across much of the Americas, hybridizing with and outcompeting resident European honey bees. We use broad geographic sampling and whole genome sequencing of over 300 bees to map the distribution of African ancestry where the northern and southern invasions have presently stalled, forming replicated hybrid zones between Africanized and European bees in California and Argentina. California is much further from Brazil, yet these hybrid zones occur at very similar latitudes, consistent with the invasion having reached a climate barrier. At these range limits, we observe genome-wide clines for African ancestry, and parallel clines for wing length that span hundreds of kilometers, supporting a smooth transition from climates favoring Africanized bees to climates where they cannot survive winter. Ancestry clines at individual SNPs are consistent with a broad genetic basis for climate-associated fitness trade-offs, with a build-up of somewhat steeper clines in regions of low recombination and no large effect loci maintaining exceptionally steep ancestry transitions. Additionally, we find no substantial reductions in genetic diversity associated with rapid expansions nor complete dropout of African ancestry at any individual loci, which suggests that the competitive fitness advantage of African ancestry at lower latitudes has a broad genetic basis and that Africanized bees maintained large population sizes during their invasion. To test for parallel selection across continents, we develop a null model that accounts for drift in ancestry frequencies during the rapid expansion. We identify several peaks within a larger genomic region where selection has pushed African ancestry to high frequency hundreds of kilometers past the present cline centers in both North and South America and that may underlie high-fitness African traits driving the invasion.
