Oocyte nuclear DNA content and GC proportion in rotifers of the anciently asexual Class Bdelloidea

Abstract

The approximately 360 species of the Class Bdelloidea (Phylum Rotifera) constitute the largest, most diverse and oldest eukaryotic group for which obligate asexuality has morphological, cytological and molecular support. DNA sequence studies indicate that the class constitutes a single clonal lineage, 50-100 million years old. Here we report cytofluorometric measurements of oocyte nuclear DNA content and GC proportion for six bdelloid species, representing the three major families of Bdelloidea. We find wide differences between species in nuclear DNA content (0.5-2.4 pg) and proportion of GC (37-57%). The proportion of GC in the nuclear DNA of all six species is greater than that in their coding regions examined to date, and tends to be greater in species with greater nuclear DNA content. Interspecific differences in nuclear DNA content do not correlate with chromosome numbers, which range from 10 to 13, and are therefore not due to differences in ploidy. In addition, since bdelloids seemingly lack functional reverse transcriptase genes, the observed interspecific differences in nuclear DNA content and GC proportion do not appear to have resulted from retrotransposon activity. The values of oocyte nuclear DNA content obtained by cytofluorometric measurement are close to the values of total genome size we previously obtained from measurements of DNA hybridization intensity, indicating that bdelloid genomes do not undergo significant somatic chromatin diminution and, unlike the genomes of sexually reproducing diploids or polyploids, do not consist of two or more nearly identical haplotypes.