Surviving in mountain climate refugia: new insights from the genetic diversity and structure of the relict shrub Myrtus nivellei (Myrtaceae) in the Sahara Desert.

Nadir Alvarez,Jérémy Migliore,Marianick Juin,Bruno Fady,Alex Baumel,Frédéric Médail,Nathalie Duong,Anne Roig

doi:10.1371/journal.pone.0073795

Abstract

The identification of past glacial refugia has become a key topic for conservation under environmental change, since they contribute importantly to shaping current patterns of biodiversity. However, little attention has been paid so far to interglacial refugia despite their key role for the survival of relict species currently occurring in climate refugia. Here, we focus on the genetic consequences of range contraction on the relict populations of the evergreen shrub Myrtus nivellei, endemic in the Saharan mountains since at least the end of the last Green Sahara period, around 5.5 ka B.P. Multilocus genotypes (nuclear microsatellites and AFLP) were obtained from 215 individuals collected from 23 wadis (temporary rivers) in the three main mountain ranges in southern Algeria (the Hoggar, Tassili n’Ajjer and Tassili n’Immidir ranges). Identical genotypes were found in several plants growing far apart within the same wadis, a pattern taken as evidence of clonality. Multivariate analyses and Bayesian clustering revealed that genetic diversity was mainly structured among the mountain ranges, while low isolation by distance was observed within each mountain range. The range contraction induced by the last episode of aridification has likely increased the genetic isolation of the populations of M. nivellei, without greatly affecting the genetic diversity of the species as a whole. The pattern of genetic diversity observed here suggests that high connectivity may have prevailed during humid periods, which is consistent with recent paleoenvironmental reconstructions.

Highlights

During millennia under various successive periods of climatic changes, some areas have facilitated the survival of biota and species; these refugia have attracted increasing attention in biodiversity studies [1,2,3]
We examine the genetic structure of the endemic Nivelle myrtle (Myrtus nivellei Batt. et Trab.), the only species growing in the central Saharan mountains, among the 5650 Myrtaceae species described in the world [40]
As fossil and molecular data combined with paleoecological reconstructions (Figure 1) support the occurrence of a strong range contraction during the late Pleistocene, our study aims at better understanding to what extent the levels of genetic diversity of the relict M. nivellei have been affected by past climatic changes

Summary

Introduction

During millennia under various successive periods of climatic changes, some areas have facilitated the survival of biota and species; these refugia have attracted increasing attention in biodiversity studies [1,2,3]. The question of how individual species persist when they are confined to climate refugia remains open [3]. In this context, extant interglacial refugia provide opportunities to solve this question, since they can be studied directly in situ using a wide range of evolutionary ecology methods, whereas the exact locations and areas occupied in the past by glacial refugia often are uncertain [10,11]. We have focused our study on the largest desert worldwide, the Sahara desert (ca. 9 million km2), which acts either as a barrier or a corridor between tropical Africa and temperate regions [20]

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Results

Conclusion