Diploid-triploid mosaicism and tissue ploidy diversity within Platemys platycephala from Suriname.

The twist-necked turtle, Platemys platycephala, is 1 of only 2 known species to possess sexual reproduction and diploid-triploid mosaicism. Previous studies have shown that mosaics occur in Suriname and French Guiana but only diploids are known from Bolivia and Brazil. In this paper, the frequency of ploidy mosaicism was studied in a large sample of P. platycephala from Suriname to more fully explore the diversity of ploidy levels within and among individuals. Flow-cytometric analysis of blood revealed a wide diversity of conditions including diploids, diploid-triploid mosaics, triploids, and triploid-tetraploid mosaics. The largest frequency class was 100% diploid, and the second largest was 100% triploid. However, mosaic individuals were observed from the entire spectrum of mixtures ranging from nearly all-diploid to nearly all-triploid and 2 individuals were triploid-tetraploid mosaics. It appears likely that diploids, triploids and mosaics do not represent distinct biotypes, but simply different conditions within a spectrum of possible ploidy mixtures. Studies of multiple tissues from 5 individuals showed blood alone is a good indicator of ploidy, but subtle differences were found among tissues for some individuals, and some individuals that were all-diploid or all-triploid in blood were found to be mosaic in other tissues. Triploidy was statistically associated with males, and we hypothesize that genome size plays a role in sex determination in this species.

Centriolar length variability in testicular cells from side-necked turtles.

Silver nitrate staining of surface spread testicular material from side-necked turtles revealed centrioles associated with the nuclei of Sertoli cells, spermatogonia, primary spermatocytes, and spermatids. Sertoli cells possessed minute centrioles comparable to those described in somatic tissues of other organisms. The other three cell types contained greatly enlarged centrioles, with those of the primary spermatocytes and spermatids exhibiting maximum lengths of 4-5 microns. During spermatogenesis, the centriolar pair apparently replicated only at prophase I, as the developing spermatids each possessed only one centriole.