Zhao X P, Si Y, Hanson R E, Crane C F, Price H J, Stelly D M, Wendel J F, Paterson A H
Plant Genome Mapping Laboratory, Department of Soil and Crop Science, Texas A&M University, College Station, Texas 77843-2474, USA.
Genome Res. 1998 May;8(5):479-92. doi: 10.1101/gr.8.5.479.
Polyploid formation has played a major role in the evolution of many plant and animal genomes; however, surprisingly little is known regarding the subsequent evolution of DNA sequences that become newly united in a common nucleus. Of particular interest is the repetitive DNA fraction, which accounts for most nuclear DNA in higher plants and animals and which can be remarkably different, even in closely related taxa. In one recently formed polyploid, cotton (Gossypium barbadense L.; AD genome), 83 non-cross-hybridizing DNA clones contain dispersed repeats that are estimated to comprise about 24% of the nuclear DNA. Among these, 64 (77%) are largely restricted to diploid taxa containing the larger A genome and collectively account for about half of the difference in DNA content between Old World (A) and New World (D) diploid ancestors of cultivated AD tetraploid cotton. In tetraploid cotton, FISH analysis showed that some A-genome dispersed repeats appear to have spread to D-genome chromosomes. Such spread may also account for the finding that one, and only one, D-genome diploid cotton, Gossypium gossypioides, contains moderate levels of (otherwise) A-genome-specific repeats in addition to normal levels of D-genome repeats. The discovery of A-genome repeats in G. gossypioides adds genome-wide support to a suggestion previously based on evidence from only a single genetic locus that this species may be either the closest living descendant of the New World cotton ancestor, or an adulterated relic of polyploid formation. Spread of dispersed repeats in the early stages of polyploid formation may provide a tag to identify diploid progenitors of a polyploid. Although most repetitive clones do not correspond to known DNA sequences, 4 correspond to known transposons, most contain internal subrepeats, and at least 12 (including 2 of the possible transposons) hybridize to mRNAs expressed at readily discernible levels in cotton seedlings, implicating transposition as one possible mechanism of spread. Integration of molecular, phylogenetic, and cytogenetic analysis of dispersed repetitive DNA may shed new light on evolution of other polyploid genomes, as well as providing valuable landmarks for many aspects of genome analysis.
多倍体的形成在许多植物和动物基因组的进化过程中发挥了重要作用；然而，令人惊讶的是，对于那些新组合在一个共同细胞核中的DNA序列的后续进化，我们却知之甚少。特别令人感兴趣的是重复DNA部分，它在高等植物和动物的核DNA中占了大部分，而且即使在亲缘关系很近的分类群中，其差异也可能非常显著。在最近形成的一个多倍体棉花（陆地棉；AD基因组）中，83个非交叉杂交的DNA克隆含有分散重复序列，据估计这些序列约占核DNA的24%。其中，64个（77%）在很大程度上局限于含有较大A基因组的二倍体分类群，它们共同构成了栽培的AD四倍体棉花的旧世界（A）和新世界（D）二倍体祖先之间DNA含量差异的大约一半。在四倍体棉花中，荧光原位杂交分析表明，一些A基因组的分散重复序列似乎已经扩散到了D基因组的染色体上。这种扩散也可能解释了为什么只有一个D基因组二倍体棉花，即海岛棉，除了正常水平的D基因组重复序列外，还含有中等水平的（否则为）A基因组特异性重复序列。在海岛棉中发现A基因组重复序列，为之前仅基于单个基因座证据提出的一个观点提供了全基因组范围的支持，即该物种可能要么是新世界棉花祖先现存的最近后代，要么是多倍体形成的混杂遗迹。多倍体形成早期分散重复序列的扩散可能为识别多倍体的二倍体祖先提供一个标记。尽管大多数重复克隆与已知的DNA序列不对应，但有4个与已知的转座子对应，大多数含有内部亚重复序列，并且至少有12个（包括2个可能的转座子）与棉花幼苗中以易于识别的水平表达的mRNA杂交，这表明转座是一种可能的扩散机制。对分散重复DNA进行分子、系统发育和细胞遗传学分析的整合，可能会为其他多倍体基因组的进化带来新的启示，同时也为基因组分析的许多方面提供有价值的标记。
