The Barcode Blog

A mostly scientific blog about short DNA sequences for species identification and discovery. I encourage your commentary. -- Mark Stoeckle

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100 million years without sex: COI clustering in bdelloid rotifers challenges theories of how species are formed and maintained

Philodina roseola, 400 um, Meselson LaboratoryWhy are there species? The usual answer is sex: reproductive isolation maintains differences between species and reproductive mixing maintains similarity within species.  According to recent work with bdelloid (the “b” is silent) rotifers, a group of microscopic invertebrates thought to have adopted asexuality 100 million years ago, sex is not necessary! In September 2005 Hydrobiologia 546:29, researchers at the University of Arizona analyzed mitochondrial COI of 102 females of 21 morphologically defined species of bdelloid rotifers, including many sympatric morphospecies. Contrary to predictions of evolutionary theory for asexual organisms, Birky et al show that these are 21 independently evolving clades, with small differences within and large differences among lineages, the same patterning seen in COI analyses of sexual reproducing species. Also contrary to predictions, the Ka/Ks ratio  (expressed/silent mutations) indicates that COI is subject to strong selection. [In asexual organisms, there is less need for sampling multiple genes because the entire genome is a single linkage unit. Thus genetic differences in COI are expected to reflect evolutionary history of the organism, i.e. the “gene tree” is expected to be the same as the “species tree.”] (For fun see Birky lab bdelloid video!)

Fontaneto et al PLoS Biol April 2007In April 2007 PloS Biology researchers from University of Milan, Italy, Imperial College London, University of Cambridge, and Royal Botanic Gardens, Kew analyze morphometric and molecular data of a comprehensive international sampling of Rotaria sp. bdelloid rotifers. All 9 morphologically defined taxonomic species form monophyletic clades in genetic analysis. Multiple clusters in several morphospecies show distinct morphometric measurements of mouth parts, suggesting these represent cryptic species with ecological specialization. Fontaneto et al observe “bdelloids display the same qualitative pattern of genetic and morphological clusters, indicative of diversification into independently evolving and distinct entities, as found in sexual clades” and conclude “this refutes the idea that sex is necessary for diversification into evolutionary species.”  

In these studies, COI sequences accurately identify bdelloid rotifer species, further demonstrating the robustness of DNA barcoding. What is scientifically exciting is how broad application of standardized, minimalist genetic analysis (aka DNA barcoding), combined with traditional morphologic and ecologic study, is providing radical new insights into biology.

This entry was posted on Friday, May 4th, 2007 at 10:44 am and is filed under General. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.

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About this site

This web site is an outgrowth of the Taxonomy, DNA, and Barcode of Life meeting held at Banbury Center, Cold Spring Harbor Laboratory, September 9-12, 2003. It is designed and managed by Mark Stoeckle, Perrin Meyer, and Jason Yung at the Program for the Human Environment (PHE) at The Rockefeller University.

About the Program for the Human Environment

The involvement of the Program for the Human Environment in DNA barcoding dates to Jesse Ausubel's attendance in February 2002 at a conference in Nova Scotia organized by the Canadian Center for Marine Biodiversity. At the conference, Paul Hebert presented for the first time his concept of large-scale DNA barcoding for species identification. Impressed by the potential for this technology to address difficult challenges in the Census of Marine Life, Jesse agreed with Paul on encouraging a conference to explore the contribution taxonomy and DNA could make to the Census as well as other large-scale terrestrial efforts. In his capacity as a Program Director of the Sloan Foundation, Jesse turned to the Banbury Conference Center of Cold Spring Harbor Laboratory, whose leader Jan Witkowski prepared a strong proposal to explore both the scientific reliability of barcoding and the processes that might bring it to broad application. Concurrently, PHE researcher Mark Stoeckle began to work with the Hebert lab on analytic studies of barcoding in birds. Our involvement in barcoding now takes 3 forms: assisting the organizational development of the Consortium for the Barcode of Life and the Barcode of Life Initiative; contributing to the scientific development of the field, especially by studies in birds, and contributing to public understanding of the science and technology of barcoding and its applications through improved visualization techniques and preparation of brochures and other broadly accessible means, including this website. While the Sloan Foundation continues to support CBOL through a grant to the Smithsonian Institution, it does not provide financial support for barcoding research itself or support to the PHE for its research in this field.