Rock outcrop orchids reveal the genetic connectivity and diversity of inselbergs of northeastern Brazil

Fábio Pinheiro,Fábio De Barros,Michael F Fay,David Draper,Clarisse Palma-Silva,Leonardo P Félix,Salvatore Cozzolino

doi:10.1186/1471-2148-14-49

Abstract

BackgroundBecause of their fragmented nature, inselberg species are interesting biological models for studying the genetic consequences of disjoint populations. Inselbergs are commonly compared with oceanic islands, as most of them display a marked ecological isolation from the surrounding area. The isolation of these rock outcrops is reflected in the high number of recorded endemic species and the strong floristic differences between individual inselbergs and adjacent habitats. We examined the genetic connectivity of orchids Epidendrum cinnabarinum and E. secundum adapted to Neotropical inselbergs of northeastern Brazil. Our goals were to identify major genetic divergences or disjunctions across the range of the species and to investigate potential demographic and evolutionary mechanisms leading to lineage divergence in Neotropical mountain ecosystems.ResultsBased on plastid markers, high genetic differentiation was found for E. cinnabarinum (FST = 0.644) and E. secundum (FST = 0.636). Haplotypes were not geographically structured in either taxon, suggesting that restricted gene flow and genetic drift may be significant factors influencing the diversification of these inselberg populations. Moreover, strong differentiation was found between populations over short spatial scales, indicating substantial periods of isolation among populations. For E. secundum, nuclear markers indicated higher gene flow by pollen than by seeds.ConclusionsThe comparative approach adopted in this study contributed to the elucidation of patterns in both species. Our results confirm the ancient and highly isolated nature of inselberg populations. Both species showed similar patterns of genetic diversity and structure, highlighting the importance of seed-restricted gene flow and genetic drift as drivers of plant diversification in terrestrial islands such as inselbergs.

Highlights

Because of their fragmented nature, inselberg species are interesting biological models for studying the genetic consequences of disjoint populations
The following specific questions were addressed: (1) What is the current extent of genetic structure and the degree of isolation among disjoint populations on inselbergs? (2) Were the current range distributions of both species stable during Late Quaternary climatic oscillations? and (3) Are there phylogeographic breaks separating populations from different biomes (Caatinga and Brazilian Atlantic Forest (BAF)) and ecoregions (Chapada Diamantina and Planalto da Borborema)? We considered the phylogeographical and genetic structure of E. cinnabarinum and E. secundum in light of results based on climate modeling, paleovegetation reconstructions and the island-like nature of populations distributed on inselbergs
The population genetic structure recovered for both E. cinnabarinum and E. secundum reflects the isolated nature of their inselberg habitats

Summary

Introduction

Because of their fragmented nature, inselberg species are interesting biological models for studying the genetic consequences of disjoint populations. Inselbergs are isolated rock outcrops typically harboring rupicolous vegetation and embedded within a landscape composed of contrasting plant communities Because of their disconnected geographic nature, inselbergs are frequently compared with oceanic islands, since most of them display a marked ecological isolation from the surrounding area [1]. Phylogeographic studies of inselberg-adapted species have provided insights into evolutionary processes leading to diversification of lineages and species [7,8,9,10,11] Such studies have uncovered strong phylogeographic structure, high population differentiation and extensive genetic diversity levels, supporting the view of inselbergs as centers of species diversity and endemism. Gene flow is constrained, and genetic drift is expected to be the predominant force governing allele frequencies [8,16]

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