Barcoding Gastrotrichs: Species of Tetranchyroderma as a Test Case
A. Lateral view of a specimen of Tetranchyroderma. B. Closeup of pentancres. C. Closeup of tetrancres.
A Barcoding Test: Taxonomic misidentification plagues virtually every group of meiofauna due to the paucity of alpha taxonomists and the lack of data on morphological variation. In some cases, morphometric characters may separate out potential sibling species without the need for molecular tools. In other cases, molecular barcodes may be an effective way to identify cryptic species. To date, barcoding has been used to identify populations of potential sibling species of gastrotrichs, but this technique has not been used extensively. We propose to test the use of barcodes on species of Tetranchyroderma, the most speciose, abundant, and geographically widespread genus of macrodasyidan gastrotrich. Most morphospecies are easily identified by the ornamentation of their cuticle, providing a unique opportunity to correlate taxonomic characters with individual DNA tags. We focus on species of Tetranchyroderma for two practical reasons: 1) Species are readily identifiable with an up-to-date dichotomous key and 2) are often the most abundant marine gastrotrichs. We propose to use the mitochondrial cytochrome oxidase subunit I (coxI) gene for two reasons: 1) coxI has been used successfully for numerous meiofauna and 2) it allows us to contribute to the Consortium for the Barcode of Life Project, which was the basis for Dr. Hochberg’s trip to Panama. While the 648 base-pair “Folmer region” of coxI is the default barcode region for most animals, we keep in mind that it has not been used for species of Tetranchyroderma. We will only consider another candidate gene (e.g. SSU, ITS1+2) if coxI proves ineffective, i.e., 1) it cannot be amplified with current primers or new primers, and 2) sequence divergence estimated with a model of evolution (e.g., K2-P) cannot differentiate intra- and interspecific variation (among > 10 species from multiple locales) and a NJ tree does not display specific clustering. The PI has used these methods to successfully differentiate among cryptic species of microinvertebrates.
Species Concepts: Embedded within our barcode analysis is the assumption that the units identified by DNA tags are congruent with the morphological species concept, which is the basis for current gastrotrich taxonomy. In the proposed study, we will define molecular operational taxonomic units (MOTUs) by producing a NJ tree based on K2P distances among individuals. Gastrotrich morphology will be mapped onto the NJ tree of coxI divergences to reveal haplotype clusters that covary with morphological (and potentially ecological) characteristics. We hypothesize that MOTUs will be concordant with morphospecies, i.e., mtDNA and morphological monophyly will be supported. In the absence of mtDNA monophyly, we will describe these species with extra attention to morphometrics (used successfully to identify potential sibling species) and external morphology visualized with SEM.
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