Molecular Genetic Studies of Bdelloid Rotifers

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; R o o t E n t r y F =8 w @ C o m p O b j b W o r d D o c u m e n t w O b j e c t P o o l =8 w =8 w 4 @ ? @ A B C D E F G H F Microsoft Word 6.0 Document MSWordDoc Word.Document.6 ; 9514279 Meselson Bdelloid rotifers are common fresh water invertebrates classified among 4 families, 18 genera, and more than 350 species. Males, hermaphrodites, and meiosis are unknown and eggs are produced from oocytes by mitosis. The Bdelloidea constitute the only higher level metazoan taxon that appears to be entirely parthenogenetic, making them the most favorable group in which to test the possibility that extensive evolution can occur without sexual reproduction and genetic recombination. The approach being taken to determine if bdelloids evolved asexually is based on the expectation that a diploid line that lost sexual reproduction and genetic recombination millions of years ago will accumulate a large amount of neutral heterozygosity, greatly reducing the homology of chromosome pairs. Such heterozygosity will be heritable, so that the ancient origin of its accumulation will be manifested in the phylogeny of heterozygous nucleotide sites among species of the asexual group. Tests for hyper heterozygosity are being conducted by determining the nucleotide sequences of numerous independently is olated copies of specific genes from rotifer populations grown from single eggs. Gene copies are isolated by cloning the products of polymerase chain reactions and also by screening genomic libraries. Results thus far, with the bdelloids Philodina roseola and Macrotrachela quadricornifera, are consistent with heritable hyper heterozygosity that began to accumulate long before these two genera diverged. The generality of this finding is being tested by sequence studies of four different genes to two different families. genes in bdelloid rotifers of three different genera belonging to two different families. Tests for chromosome homology and divergence are being performed by fluorescence in situ hybridization. If the homology of chromosome pairs has become greatly reduced, one member of a diverged gene pair will hybridize at high stringency to only one chromosome-and the other member of the pair will generally hybridize to a different chromosome. Bdelloid rotifers from two different families are being tested in this manner, using probes containing the same genes that are being examined in sequence studies. As controls, the above sequencing and cytogenetic tests are being performed with rotifers of the Class Monogononta, in which females are diploids that usually reproduce parthenogenetically but which occasionally undergo meiosis and reproduce sexually, producing haploid males and exhibiting Mendelian segregation. %%% Genetic recombination is believed to be essential for the successful evolution of a species. The bdelloid rotifers, a common fresh-water invertebrate, produce eggs from oocytes by mitosis and no indication that sexual reproduction occurs has ever been found. It is now possible to look, at a molecular level, for any indication of genetic recombination and to test the possibility that extensive evolution can occur without sexual reproduction and genetic recombination. *** @ @ Oh +' 0 $ H l S u m m a r y I n f o r m a t i o n ( D h R:\WWUSER\TEMPLATE\NORMAL.DOT christine a savoy christine a savoy @ | r @ s @ w @ H' Microsoft Word 6.0 2 ; e = e w j j j j j j j 1 %