Molecular Genetic Studies of Bdelloid Rotifers

;​R o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o = 8w = 8w4 @ ？@ A B C D E F G H F Microsoft Word 6.0文档MSWordDoc。6;​Meselson蛭形轮虫是常见的淡水无脊椎动物，分4科18属350余种。雄性、雌雄同体和减数分裂是未知的，卵子是通过有丝分裂从卵母细胞产生的。蛭形总纲是唯一一个似乎完全是单性生殖的高级后生动物分类群，这使它们成为最有利的群体，可以在没有有性生殖和基因重组的情况下测试广泛进化的可能性。确定蛭形体是否无性进化的方法是基于这样一种预期，即数百万年前失去有性繁殖和基因重组的二倍体系将积累大量的中性杂合性，从而大大降低染色体对的同源性。这种杂合性将是可遗传的，因此其积累的古老起源将在无性群体物种间杂合核苷酸位点的系统发育中表现出来。超杂合性测试是通过确定从单卵生长的轮虫种群中大量独立分离的特定基因拷贝的核苷酸序列来进行的。基因拷贝是通过克隆聚合酶链反应产物和筛选基因组文库来分离的。到目前为止，对于蛭形虫Philodina roseola和Macrotrachela quadricornifera，结果与遗传的高杂合性一致，这种高杂合性在这两个属分化之前就开始积累了。这一发现的普遍性正在通过对来自两个不同家族的四个不同基因的序列研究进行检验。两科三属蛭形轮虫的基因。利用荧光原位杂交技术进行染色体同源性和发散性检测。如果染色体对的同源性大大降低，则分离基因对的一个成员将高度严格地只与一条染色体杂交，而这对基因对的另一个成员通常会与另一条染色体杂交。来自两个不同家族的蛭形轮虫正在以这种方式进行测试，使用的探针含有在序列研究中检测的相同基因。作为对照，上述测序和细胞遗传学测试是用单性轮虫进行的，其中雌性轮虫是二倍体，通常是孤雌生殖，但偶尔会进行减数分裂和有性繁殖，产生单倍体雄性，并表现出孟德尔分离。基因重组被认为是物种成功进化的必要条件。蛭形轮虫是一种常见的淡水无脊椎动物，卵母细胞通过有丝分裂产生卵子，目前还没有发现有性生殖的迹象。现在有可能在分子水平上寻找基因重组的任何迹象，并测试在没有有性繁殖和基因重组的情况下也能发生广泛进化的可能性。*** * @ @ Oh +' 0 $ H $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $D h R:\WWUSER\TEMPLATE\NORMAL。微软Word 6.0 2.0；​E = E w j j j j j j j j 1 %

; 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 %