Abstract
Understanding the process of speciation requires understanding how gene flow influences divergence. Recent analyses indicate that divergence can take place despite gene flow and that the sex chromosomes can exhibit different levels of gene flow than autosomes and mitochondrial DNA. Using an eight marker dataset including autosomal, z-linked, and mitochondrial loci we tested the hypothesis that blue-footed (Sula nebouxii) and Peruvian (S. variegata) boobies diverged from their common ancestor with gene flow, paying specific attention to the differences in gene flow estimates from nuclear and mitochondrial markers. We found no gene flow at mitochondrial markers, but found evidence from the combined autosomal and z-linked dataset that blue-footed and Peruvian boobies experienced asymmetrical gene flow during or after their initial divergence, predominantly from Peruvian boobies into blue-footed boobies. This gene exchange may have occurred either sporadically between periods of allopatry, or regularly throughout the divergence process. Our results add to growing evidence that diverging species can remain distinct but exchange genes.
Highlights
Understanding speciation necessarily involves understanding the influence of gene flow on the process of divergence [1,2,3,4]
Sample collection and laboratory methods hybridization has been recorded between blue-footed and Peruvian boobies it is restricted to two islands in northern Peru and does not cause extensive introgression [34,35] to minimize the influence of ongoing hybridization on gene flow and divergence time estimates, samples for each species were assembled from areas of allopatry
Lengths of sequence blocks ranged from 114 bp for LIPO to 1184 for the concatenated mitochondrial sequence of Cytochrome b gene (Cyt b) and ND2 (Table 1)
Summary
Understanding speciation necessarily involves understanding the influence of gene flow on the process of divergence [1,2,3,4]. Until recently, quantifying gene flow and introgression during and following divergence was difficult; advances both in sequencing technology and in analytical methods based on coalescent theory are removing these barriers [5]. New analytical methods based on coalescent theory have revealed that divergence occurs with some level of gene flow more commonly than originally thought [4,5], but estimating the timing of gene flow events remains difficult [11,12,13]. To evaluate the frequency of divergence with versus without gene flow, the two scenarios must be tested explicitly in a diverse array of taxa
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