ORGANIZATION AND EVOLUTION OF THE DIPLOID COTTON D GENOME

¹ Plant Genome Mapping Laboratory, University of Georgia, 111 Riverbend Road, Athens, GA 30602

To further investigate early hints from a tetraploid genetic map that modern "diploid" cottons may actually be ancient polyploids, we have identified DNA probes that detected non-homoeologous duplications in tetraploid cotton and mapped these in a highly-polymorphic cross between diploid D-genome genotypes. A total of 195 DNA probes identified 450 RFLP loci covering almost the entire D genome. The duplicated loci were not randomly arranged on the chromosomes. Each chromosome can be divided into several blocks according to collinear relationships with other chromosomes. Some of the conserved regions showed a syntenic relationship with two or more blocks. These duplications were also accompanied by much chromosomal rearrangement, mainly as a result of inversion. These data suggest that the cotton D genome may be derived from ancient duplication of an ancestor with 6 or 7 chromosomes, followed by extensive chromosomal rearrangement. From the modern genetic map, we can predict to some degree what the genome of such a putative ancestor may have looked like. Such a "predicted" gene order should show greater similarity to those of taxa that diverged from the cotton lineage prior to this duplication, a possibility that we will test by comparison to the Arabidopsis sequence. These analyses are expected to clarify the comparative organization of the cotton and Arabidopsis genomes, and foster leveraging of Arabidopsis sequence data by cotton scientists.