Abstract

Angraecum is the largest genus of subtribe Angraecinae (Orchidaceae) with about 221 species. Madagascar is the center of the diversity for the genus with ca. 142 species, of which 90% are endemic. The great morphological diversity associated with species diversification in the genus on the island of Madagascar offers valuable insights for macroevolutionary studies. Phylogenies of the Angraecinae have been published but a lack of taxon and character sampling and their limited taxonomic resolution limit their uses for macroevolutionary studies. We present a new phylogeny of Angraecum based on chloroplast sequence data (matk, rps16, trnL), nuclear ribosomal (ITS2) and 39 morphological characters from 194 Angraecinae species of which 69 were newly sampled. Using this phylogeny, we evaluated the monophyly of the sections of Angraecum as defined by Garay and investigated the patterns of species diversification within the genus. We used maximum parsimony and bayesian analyses to generate phylogenetic trees and dated divergence times of the phylogeny. We analyzed diversification patterns within Angraecinae and Angraecum with an emphasis on four floral characters (flower color, flower size, labellum position, spur length) using macroevolutionary models to evaluate which characters or character states are associated with speciation rates, and inferred ancestral states of these characters. The phylogenetic analysis showed the polyphyly of Angraecum sensu lato and of all Angraecum sections except sect. Hadrangis, and that morphology can be consistent with the phylogeny. It appeared that the characters (flower color, flower size, spur length) formerly used by many authors to delineate Angraecum groups were insufficient to do so. However, the newly described character, position of the labellum (uppermost and lowermost), was the main character delimiting clades within a monophyletic Angraecum sensu stricto. This character also appeared to be associated with speciation rates in Angraecum. The macroevolutionary model-based phylogeny failed to detect shifts in diversification that could be associated directly with morphological diversification. Diversification in Angraecum resulted from gradual species accumulation through time rather than from rapid radiation, a diversification pattern often encountered in tropical rain forests.

Highlights

  • Madagascar is known for its rich biodiversity [1] and is a focus for evolutionary biologists who study the causes of species diversification

  • Of the 69 newly sampled specimens that we sequenced, all were fully amplified with matK for a total length of 942 base pairs; 66 with rps16 (1192 bp) except Angraecum sterophyllum, A. rhynchoglossum and Lemurella papillosa which failed to amplify; and 65 with trnL (1553 bp) except A. pseudofilicornu, A. rhynchoglossum, Oeoniella polystachys and Oeonia rosea

  • When combined with GenBank sequences data, each individual matrix was composed of 190 taxa for matK, 144 for rps16, 170 for trnL and 88 for ITS2

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Summary

Introduction

Madagascar is known for its rich biodiversity [1] and is a focus for evolutionary biologists who study the causes of species diversification. Many studies have been conducted using the high endemicity and diversity found in this isolated island as a model of diversification processes in various taxa [2,3,4,5,6,7,8,9,10]. Most of these studies came to the conclusion that species diversification resulted from rapid radiation. Understanding the diversification processes of a group of organisms could be useful for biodiversity conservation [22], especially for hotspots like Madagascar [23] where priority should be given to the most valuable species due to a lack of conservation funds

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