Multi-Locus Estimates of Population Structure and Migration in a Fence Lizard Hybrid Zone

Adam D Leaché,Simon Joly

doi:10.1371/journal.pone.0025827

Adam D Leaché, Simon Joly

Open Access

https://doi.org/10.1371/journal.pone.0025827

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Abstract

A hybrid zone between two species of lizards in the genus Sceloporus (S. cowlesi and S. tristichus) on the Mogollon Rim in Arizona provides a unique opportunity to study the processes of lineage divergence and merging. This hybrid zone involves complex interactions between 2 morphologically and ecologically divergent subspecies, 3 chromosomal groups, and 4 mitochondrial DNA (mtDNA) clades. The spatial patterns of divergence between morphology, chromosomes and mtDNA are discordant, and determining which of these character types (if any) reflects the underlying population-level lineages that are of interest has remained impeded by character conflict. The focus of this study is to estimate the number of populations interacting in the hybrid zone using multi-locus nuclear data, and to then estimate the migration rates and divergence time between the inferred populations. Multi-locus estimates of population structure and gene flow were obtained from 12 anonymous nuclear loci sequenced for 93 specimens of Sceloporus. Population structure estimates support two populations, and this result is robust to changes to the prior probability distribution used in the Bayesian analysis and the use of spatially-explicit or non-spatial models. A coalescent analysis of population divergence suggests that gene flow is high between the two populations, and that the timing of divergence is restricted to the Pleistocene. The hybrid zone is more accurately described as involving two populations belonging to S. tristichus, and the presence of S. cowlesi mtDNA haplotypes in the hybrid zone is an anomaly resulting from mitochondrial introgression.

Highlights

Studies of lineage divergence and the subsequent processes that occur when lineages reconnect and merge are greatly facilitated by investigations of naturally occurring hybrid zones
Multi-locus nuclear data Population structure and gene flow were estimated from 12 anonymous nuclear loci collected for 93 specimens from throughout the hybrid zone (Supporting Information Table S1)
The population sample from the hybrid zone is weakly differentiated at the 12 nuclear loci, and these low levels of differentiation are typical for slowly evolving nuclear loci and recently diverged lineages

Summary

Introduction

Studies of lineage divergence and the subsequent processes that occur when lineages reconnect and merge are greatly facilitated by investigations of naturally occurring hybrid zones. A fundamental question to address in this hybrid zone is whether multi-locus nuclear data corroborate the two distinctive morphological forms, the three chromosomal polymorphisms, or the four mtDNA clades. Coalescent analyses of the multi-locus data are used to estimate migration rates and divergence times between the inferred populations.

Results

Conclusion