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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Single locus mtDNA resolves freshwater snail species

Freshwater snails are intermediate hosts for schistosomiasis and flukes, trematode parasites that infect approximately 10% of world’s human population. Freshwater snails are also indicator species for water quality. Snail identification is essential for reducing disease burden and monitoring water quality.

Researchers at the University of Frankfurt (November 2006 BMC Evol Biol 6:100) compared efficacy of morphologic and DNA-based taxonomy in freshwater snails in the genus Radix. Regarding Radix species in northwestern Europe, “species determination by shell morphology is difficult [and] unreliable…intraspecific variability of the putatively distinctive anatomical measurements largely overlaps among species” and identifications are “further complicated by recent nomenclatorial revisions”. 

In their report, Pfenninger, Cordellier and Streit analyze morphology, mitochondrial COI and nuclear ITS-1 sequences, and describe breeding experiments with Radix snails collected at 60 sites throughout Europe. Using mtCOI sequences they found five MOTU (molecular operational taxonomic units), defined as “terminal clades with bootstrap support of 90% or more”. Populations of these MOTU overlapped broadly in geographic range and none corresponded to described species. Nuclear ITS sequences analyzed in a subset of specimens produced MOTU congruent with those generated by mtCOI.

ALL crosses between individuals from the same MOTU population were viable, whereas NONE of crosses between individuals from different MOTU produced eggs. In morphometric analysis, Radix MOTU overlapped as shown at left, and in rearing experiments, shell shape changed in 4 of 5 populations, demonstrating phenotypic plasticity of putative morphologic characters. In northwestern European Radix snails, DNA trumps morphology.

This work follows what might be a “best practices” pathway for single-locus mtDNA species discovery, aka DNA barcoding applied to species discovery:

1. COI sequence clusters (MOTU), found in analyzing  multiple specimens from geographically widespread locations, are proposed as putative species.

2. COI clustering is congruent with nuclear sequence data.

3. COI clusters show corresponding biological differences, such as morphologic characters, behavioral differences, or breeding incompatibility.

In some cases a virus or bacteria is recognized to be the causative agent even though not all of Koch’s postulates have been fulfilled. In a similar way in some cases it might be desirable to recognize mtDNA clusters as representing species without fulfilling all of the above criteria.

This entry was posted on Wednesday, December 6th, 2006 at 10:54 pm 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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Contact: mark.stoeckle@rockefeller.edu

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.