Geographical distance and barriers explain population genetic patterns in an endangered island perennial.

Elisabete F Dias,M Moura,Luís Silva,H Schaefer

doi:10.1093/aobpla/plw072

Abstract

Island plants are frequently used as model systems in evolutionary biology to understand factors that might explain genetic diversity and population differentiation levels. Theory suggests that island plants should have lower levels of genetic diversity than their continental relatives, but this hypothesis has been rejected in several recent studies. In the Azores, the population level genetic diversity is generally low. However, like in most island systems, there are high levels of genetic differentiation between different islands. The Azores lettuce, Lactuca watsoniana, is an endangered Asteraceae with small population sizes. Therefore, we expect to find a lower level of genetic diversity than in the other more common endemic Asteraceae. The intra- and interpopulation genetic structure and diversity of L. watsoniana was assessed using eight newly developed microsatellite markers. We included 135 individuals, from all 13 known populations in the study. Because our microsatellite results suggested that the species is tetraploid, we analysed the microsatellite data (i) in codominant format using PolySat (Principal Coordinate Analysis, PCoA) and SPAgedi (genetic diversity indexes) and (ii) in dominant format using Arlequin (AMOVA) and STRUCTURE (Bayesian genetic cluster analysis). A total of 129 alleles were found for all L. watsoniana populations. In contrast to our expectations, we found a high level of intrapopulation genetic diversity (total heterozigosity = 0.85; total multilocus average proportion of private alleles per population = 26.5%, Fis = -0.19). Our results show the existence of five well-defined genetic groups, one for each of the three islands São Miguel, Terceira and Faial, plus two groups for the East and West side of Pico Island (Fst = 0.45). The study revealed the existence of high levels of genetic diversity, which should be interpreted taking into consideration the ploidy level of this rare taxon.

Highlights

Oceanic islands have been important study systems for ecologists, evolutionary and conservation biologists and are widely recognized as natural laboratories, for studying evolution due to their discrete geographical nature and diversity of species and habitats (Emerson 2002)
A total of 129 alleles were found for L. watsoniana, with a total genetic diversity (HT) of 0.85, an overall excess of heterozygotes (Multilocus Fis 1⁄4 À0.1912, P < 0.001; Average Multilocus Fi values 1⁄4 À0.074, P < 0.001), and with a total multilocus average proportion of private alleles of 26.5 %
The genetic variability found in the L. watsoniana populations was comparably high to that found for other Azorean herbaceous endemic taxa (Dias et al 2014; Silva et al 2015), and we found no evidence for inbreeding, despite the population decline that most likely took place in the past five centuries

Summary

Introduction

Oceanic islands have been important study systems for ecologists, evolutionary and conservation biologists and are widely recognized as natural laboratories, for studying evolution due to their discrete geographical nature and diversity of species and habitats (Emerson 2002). Oceanic islands generally have lower overall species numbers per unit area (Whittaker and FernandezPalacios 2007) but show higher percentages of endemism than mainland areas (Kier et al 2009).

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