DISPERSED REPETITIVE DNA HAS SPREAD TO NEW GENOMES SINCE POLYPLOID FORMATION IN COTTON

Polyploid formation has played a major role in the evolution of many p lant and animal genomes; however, surprisingly little is known regardi ng the subsequent evolution of DNA sequences that become newly united in a common nucleus. Of particular interest is the repetitive DNA frac tion, 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 th ese, 64 (77%) are largely restricted to diploid taxa containing the la rger A genome and collectively account for about half of the differenc e in DNA content between Old World (A) and New World [D) diploid ances tors of cultivated AD tetraploid cotton. In tetraploid cotton, FISH an alysis showed that some A-genome dispel sed repeats appear to have spl ead to D-genome chromosomes. Such spread may also account for the Fin ding that one, and Only one, D-genome diploid cotton, Gossypium gossyp ioides, contains moderate levels of (otherwise] A-genome-specific repe ats in addition to normal levels of D-genome repeats. The discovery of A-genome repeats in C. gossypioides adds genome-wide support to a sug gestion previously based on evidence from only a single genetic locus that this species may be either the closest living descendant of the N ew World cotton ancestor, or an adulterated relic of polyploid formati on. Spread of dispersed repeats in the early stages of polyploid Forma tion may provide a tag to identify diploid progenitors of a polyploid. Although most repetitive clones do not correspond to known DNA sequen ces, 4 correspond to known transposons, most contain internal subrepea ts, and at least 12 (including 2 of the possible transposons) hybridiz e to mRNAs expressed at readily discernible levels in cotton seedlings , implicating transposition as one possible mechanism of spread. Integ ration of molecular, phylogenetic, and cytogenetic analysis of dispers ed repetitive DNA may shed new light on evolution of other polyploid g enomes, as well as providing valuable landmarks for many aspects of ge nome analysis.