Ploidy Mosaicism and Molecular Differentiation among Populations

of the South American twist-necked Platemys platycephala

Emily R. Darr

Ryan M. Huebinger

Dr. John W. Bickham

Department of Wildlife and Fisheries Sciences

Texas A&M University

College Station, TX 77843

Diploid-triploid mosaicism or chimerism is a rare birth defect in man and other animals and is found as a natural condition in only one sexually reproducing species, the turtle Platemys platycephala.  Within this species geographic variation exists for ploidy mosaicism.  Turtles from Bolivia are normal diploids with no evidence of mosaicism.  Animals from Suriname are highly variable, including, diploids, triploids, and diploid-triploid (and rarely triploid-tetraploid) mosaics.  Animals from French Guyana show a higher proportion of triploidy than those from Suriname, being mostly triploid or triploid-tetraploid mosaics, and some individuals with small diploid cell populations. Genetic divergence among populations of P. platycephala was investigated by sequencing part of the cytochrome b gene of the mitochondrial DNA.  This was done with Polymerase Chain Reaction (PCR), and sequencing with an internal primer.  The resulting sequences show a relatively high level of divergence among all three populations, each of which is monophyletic.  The French Guiana population is the most divergent while the Suriname and Bolivian populations share a more recent common ancestor.  There is no evidence of differentiation among diploids and triploid or mosaic turtles from Suriname.  In addition, the sequences show a high frequency of heteroplasmy, or nuclear inserts, of the mitochondrial DNA.  We compared the frequencies of specimens with mixed-based sequences obtained from DNA extracted directly from whole-genomic DNA versus purified mitochondrial DNA isolated on cesium chloride in order to differentiate between nuclear inserts or mtDNA heteroplasmy. 

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