Reconstructing hotspots of genetic diversity from glacial refugia and subsequent dispersal in Italian common toads (Bufo bufo)

Daniele Canestrelli,Roberta Bisconti,Alice Pezzarossa,Andrea Chiocchio,Iñigo Martínez-Solano,Wouter de Vries,Jan. W. Arntzen,Luigi Maiorano

doi:10.1038/s41598-020-79046-y

Abstract

Genetic diversity feeds the evolutionary process and allows populations to adapt to environmental changes. However, we still lack a thorough understanding of why hotspots of genetic diversity are so 'hot'. Here, we analysed the relative contribution of bioclimatic stability and genetic admixture between divergent lineages in shaping spatial patterns of genetic diversity in the common toad Bufo bufo along the Italian peninsula. We combined population genetic, phylogeographic and species distribution modelling (SDM) approaches to map ancestral areas, glacial refugia, and secondary contact zones. We consistently identified three phylogeographic lineages, distributed in northern, central and southern Italy. These lineages expanded from their ancestral areas and established secondary contact zones, before the last interglacial. SDM identified widespread glacial refugia in peninsular Italy, sometimes located under the present-day sea-level. Generalized linear models indicated genetic admixture as the only significant predictor of the levels of population genetic diversity. Our results show that glacial refugia contributed to preserving both levels and patterns of genetic diversity across glacial-interglacial cycles, but not to their formation, and highlight a general principle emerging in Mediterranean species: higher levels of genetic diversity mark populations with substantial contributions from multiple genetic lineages, irrespective of the location of glacial refugia.

Highlights

Genetic diversity feeds the evolutionary process and allows populations to adapt to environmental changes
Substantial evidence has been gathered in favour of each scenario (e.g.20–22) with different levels of genetic diversity that can be explained for different populations by two factors: distance from putative refugia and extent of admixture[23,24]
In the combined mitochondrial DNA (mtDNA) dataset (1239 bp), we found 83 different haplotypes defined by 160 variable positions

Summary

Introduction

Genetic diversity feeds the evolutionary process and allows populations to adapt to environmental changes. A major research arena has involved the identification of hotspots of intraspecific genetic diversity, that is, geographic regions harbouring exceptionally high d iversity[11,12] These hotspots are increasingly recognised as key targets in conservation biology[11, 13,14], and their correct identification is an important step in designing effective strategies for the long-term persistence of populations in the face of global change[15]. Hotspots may result from secondary contact and admixture between intraspecific lineages, differentiated within sub-refugia during periods of unfavourable climatic conditions[17] Under the latter scenario, hotspots of genetic diversity would be melting-pots[18,19]. The relative contribution of the two factors to the formation of spatial patterns of genetic variation, and of hotspots, remains poorly explored (but s ee[11,18,19,20,21,22])

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Conclusion