Bdelloid rotifers are common fresh water invertebrates classified among 4 families, 19 genera and some 350 species. Males, hermaphrodites, and meiosis are unknown within the Class. Eggs are produced from oocytes by two mitotic divisions with no chromosome pairing and no reduction in chromosome number. Studies of four different bdelloid species, representing three different bdelloid families, have shown that individual genomes of bdelloid rotifers contain highly diverged gene copies of each of four genes examined: hsp82, tbp, tpi, and RNApolIII. This unusual finding could result from a long standing absence of meiosis during which former alleles have greatly diverged, so that nuclei no longer contain closely homologous chromosome sets. Alternatively, it could mean that despite morphological, cytogenetic and molecular observations to the contrary, bdelloids are meiotic diploids or polyploids with an unusual predilection for gene duplication and with an occasional and unrecognized meiotic cycle that maintains homology between chromosome sets. In order to determine whether bdelloids are meiotic diploids (or polyploids), bdelloid metaphase chromosomes will be labeled by fluorescent in situ hybridization, using genomic cosmid and YAC clones as probes. Homologous chromosomes, if present, will be revealed by their homologous patterns of labeling. While the absence of homologous chromosome pairs in bdelloid rotifers would show that bdelloids are not meiotic diploids or polyploids, such a result would leave the possibility that they are female haploids, a condition unprecedented among metazoans, and that syngamy and meiosis occur in some infrequent and unrecognized sexual phase. Such unconventional sexual reproduction, as well as any other form of genetic exchange, would generate recombinant genotypes. In order to investigate this possibility, natural bdelloid populations will be screened for evidence of recombination between the mitochondrial gene COI and the nuclear genes hsp82 and tpi by determining the nucleotide sequences of these genes in numerous individuals of the bdelloids Philodina roseola and Macrotrachela quadricornifera, collected from an ancient glacial pond. In addition to providing a sensitive test for any form of recombination, the population studies will provide information on the genetic diversity of bdelloid rotifers that may be unique to ancient asexual taxa. Rigorous tests of the possibility that the Class Bdelloidea of the Phylum Rotifera has evolved apomictically, reproducing without meiosis or syngamy, will be undertaken. Demonstration that bdelloids have evolved asexually would challenge the prevailing view that sexual reproduction is essential for evolutionary success and would provide a unique system to test hypothesis that seek to account for the almost universal maintenance of sexual reproduction in animals and plants.
