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Molecular contributions to species descriptions of dicyemid mesozoans Eshragh, Roya Marie


Dicyemids are enigmatic parasites found only within the excretory systems of benthic cephalopods. Over the past century, dicyemids have been considered to be either complex protozoa, “mesozoa” that are ambiguously intermediate between protozoa and metazoa, or reduced metazoans. The phylogenetic position and overall diversity of dicyemids is poorly understood. Current species identification criteria are unconvincing because they are based solely on morphological traits. I set out to test current morphological species concepts with DNA barcodes from dicyemids collected from Pacific Northwest cephalopods. Variation within sequences of the small subunit (18S) rRNA gene was explored because this marker (1) is known to be fast-evolving in parasitic eukaryotes, (2) is one of the few molecular markers to have been previously sequenced in some dicyemids, and (3) has been used successfully as a barcode in other groups of parasites. Three host species of cephalopods were collected in this study, each hosting multiple historical morphospecies of dicyemid parasites. Thirty-four individual dicyemids encompassing eight morphospecies were isolated and their 18S rDNA sequenced. Molecular phylogenetic analyses of these data were incongruent with current morphology-based species concepts. The 18S rDNA sequences suggest that each host species of cephalopod harbors only one species of dicyemid with a great deal of morphological variation. However, the 18S rDNA sequences should eventually be tested with other rapidly evolving molecular markers. Attempts were made to sequence the mitochondrial cytochrome oxidase I (COI) gene, the mitochondrial 16S rRNA gene, and both Internal Transcribed Spacers (ITS) of the nuclear rRNA operon. With so little of the dicyemid genome known, I was unable to establish reliable primer pairs for these genes within the time constraints of my MSc thesis. Nonetheless, this study has shown that DNA barcoding is a powerful tool for the delimitation of dicyemid species. Understanding the diversity of parasite species is particularly problematic because they tend to be devoid of consistent (informative) morphological traits while simultaneously rich in morphological variation associated with developmental stages and environmental conditions. The addition of DNA barcodes to dicyemid diversity will simplify and improve species boundaries in a lineage that is difficult to define in every aspect.

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