High and distinct range-edge genetic diversity despite local bottlenecks.

Jorge Assis,Myriam Valero,Nelson Castilho Coelho,Ester Alvares Serrão,Dan Reed,Filipe Alberto,Pete Raimondi,Sharyn Jane Goldstien

doi:10.1371/journal.pone.0068646

Abstract

The genetic consequences of living on the edge of distributional ranges have been the subject of a largely unresolved debate. Populations occurring along persistent low latitude ranges (rear-edge) are expected to retain high and unique genetic diversity. In contrast, currently less favourable environmental conditions limiting population size at such range-edges may have caused genetic erosion that prevails over past historical effects, with potential consequences on reducing future adaptive capacity. The present study provides an empirical test of whether population declines towards a peripheral range might be reflected on decreasing diversity and increasing population isolation and differentiation. We compare population genetic differentiation and diversity with trends in abundance along a latitudinal gradient towards the peripheral distribution range of Saccorhiza polyschides , a large brown seaweed that is the main structural species of kelp forests in SW Europe. Signatures of recent bottleneck events were also evaluated to determine whether the recently recorded distributional shifts had a negative influence on effective population size. Our findings show decreasing population density and increasing spatial fragmentation and local extinctions towards the southern edge. Genetic data revealed two well supported groups with a central contact zone. As predicted, higher differentiation and signs of bottlenecks were found at the southern edge region. However, a decrease in genetic diversity associated with this pattern was not verified. Surprisingly, genetic diversity increased towards the edge despite bottlenecks and much lower densities, suggesting that extinctions and recolonizations have not strongly reduced diversity or that diversity might have been even higher there in the past, a process of shifting genetic baselines.

Highlights

Understanding the processes shaping genetic diversity of range-edge populations is an important current challenge, where rich former glacial refugia populations with high conservation value have become isolated in decreasing suitable habitat islands [1]
Areas where past history created higher genetic diversity due to long term persistence of populations exposed to climatic refugia, or gene flow from differentiated populations [7,19,20] are expected to be lost by current bottlenecks, regional diversity might retain a diverse signature [5]
Along the Portuguese coast, presence-absence records of S. polyschides were performed on 48 visited cells (Table S1)

Summary

Introduction

Understanding the processes shaping genetic diversity of range-edge populations is an important current challenge, where rich former glacial refugia populations with high conservation value have become isolated in decreasing suitable habitat islands [1]. Empirical data for populations at distributional edges do not all support the same general geographic pattern These can vary from diverse persistent populations where habitat has remained favourable over the long term [2] to margins with small and low density populations where genetic diversity may be lower and clonal reproduction and inbreeding may prevail [3,4]. Such populations might represent the last refugia of threatened distinct genetic diversity [5,6]. Areas where past history created higher genetic diversity due to long term persistence of populations exposed to climatic refugia, or gene flow from differentiated populations [7,19,20] are expected to be lost by current bottlenecks, regional diversity might retain a diverse signature [5]

Methods

Results

Conclusion