Independently Evolving Species in Asexual Bdelloid Rotifers

Abstract

Asexuals are an important test case for theories of why species exist. If asexual clades displayed the same pattern of discrete variation as sexual clades, this would challenge the traditional view that sex is necessary for diversification into species. However, critical evidence has been lacking: all putative examples have involved organisms with recent or ongoing histories of recombination and have relied on visual interpretation of patterns of genetic and phenotypic variation rather than on formal tests of alternative evolutionary scenarios. Here we show that a classic asexual clade, the bdelloid rotifers, has diversified into distinct evolutionary species. Intensive sampling of the genus Rotaria reveals the presence of well-separated genetic clusters indicative of independent evolution. Moreover, combined genetic and morphological analyses reveal divergent selection in feeding morphology, indicative of niche divergence. Some of the morphologically coherent groups experiencing divergent selection contain several genetic clusters, in common with findings of cryptic species in sexual organisms. Our results show that the main causes of speciation in sexual organisms, population isolation and divergent selection, have the same qualitative effects in an asexual clade. The study also demonstrates how combined molecular and morphological analyses can shed new light on the evolutionary nature of species.

Speciation and DNA barcodes: testing the effects of dispersal on the formation of discrete sequence clusters

“…Waiting times between diversification events are modelled using a stochastic branching rate model, equivalent to a Yule model with branching rate l D ( Yule 1924;Nee et al 1992) but with an additional scaling parameter, p D , which allows for smooth changes in per lineage branching rate over time, as might be expected under background extinction models or if species samples are incomplete (Barraclough & Nee 2001). Waiting times between coalescent events within species are modelled using a separate coalescent process for each species (Hudson 1991;Wakeley 2006), with branching rate, l C , but again modified by including a scaling parameter, p C , that relaxes the strict assumption of neutral coalescence and constant population size by allowing smooth changes in branching rate over time, as might arise if population size has increased or declined through time or if there have been recent selective sweeps (Pons et al 2006;Fontaneto et al 2007). Therefore, branches crossing the threshold define k genetic clusters each obeying an independent coalescent process but with branching rate, l C , and scaling parameter, p C , assumed to be constant across clusters.…”

“…The recently developed mixed Yule coalescent (MYC) model (Pons et al 2006;Fontaneto et al 2007) was employed to delimit genetic clusters and to compare the strength of clustering between beetle groups as well as in the simulated gene trees produced under different levels of migration.…”

Functional Divergence of Former Alleles in an Ancient Asexual Invertebrate

“…The process of abandoning sexual reproduction and meiosis, and the resulting sequence homogenization of homologous chromosomes, is similar to genome duplication, which is a major evolutionary force (28,29) that results in orthologous genes evolving relatively independently. Similarly, apomixis could drive evolutionary change by allowing former alleles to diversify in function and may partly explain how bdelloid rotifers have, without genetic exchange, diversified into almost 400 taxonomic species (30,31 Wen-Yee Choi, 1,2 Antonio J. Giraldez, 1,3 * Alexander F. Schier 1 * MicroRNAs (miRNAs) repress hundreds of target messenger RNAs (mRNAs), but the physiological roles of specific miRNA-mRNA interactions remain largely elusive. We report that zebrafish microRNA-430 (miR-430) dampens and balances the expression of the transforming growth factor-b (TGF-b) Nodal agonist squint and the TGF-b Nodal antagonist lefty.…”

Demarcation of bacterial ecotypes from DNA sequence data: A comparative analysis of four algorithms

“…The GMYC algorithm was originally designed to delineate species from sequences for sexually reproducing organisms such as insects [31]. It was later shown to be applicable to asexually reproducing organisms such as bdelloid rotifers [32] and more recently to bacterial sequence data (Barraclough et al, 2009). GMYC assumes a Yule model of speciation followed by a neutral coalescent model within species, where drift is the only process yielding coalescence.…”