Plant Genome III Conference

COMPARATIVE ORGANIZATION OF PLANT CHROMOSOMES: A FRAMEWORK FOR UNDERSTANDING EVOLUTION OF THE PHENOTYPE.

1. Dept. Soil & Crop Sci., Texas A &-M University, and
2. USDA-ARS, College Station TX 77843.

Comparative maps of plant chromosomes provide a means to assess whether phenotypic variation in different taxa can be accounted for by common genes.

Recent years have witnessed detailed evidence of fundamental similarity in gene repertoire and organization of higher plant genomes. Genetic analysis of conserved DNA elements in many groups of related taxa reiterate a theme of extensive similarity punctuated by occasional rearrangement, as reported in this meeting and elsewhere.

Comparative mapping can be applied not only to divergent taxa, but also to divergent chromosomes within a taxon. Nearly 30 years ago, Stebbins proposed that the progenitors of most higher plants had chromosomes numbers of 7-9, with extant chromosome numbers accounted for by polyploidy, chromosome losses, and chromosome fusion. While many results support this proposal, recent results from mapping of duplicated loci in Arabidopsis, together with both classical and recent results from mapping duplicated loci in maize, point to lower primordial chromosome numbers. Further investigation is needed to determine whether the course of chromosome evolution is merely more tortuous than anticipated, or whether a revision in current thinking about the organization of prototypical higher plant genomes is warranted. Finally, comparative mapping affords new opportunities to use chromosomal rearrangements as phylogenetic tools, inferring gene order in prototypical plants.

Similarities in composition of low-copy elements within and among genomes, are in stark contrast to the diversity of the repetitive fraction. In polyploids, repetitive DNA elements represent a valuable tool for determining the sub-genomic origin of chromosomes and large chromosome segments such as megabase DNA clones. These elements may prove valuable for map-based cloning in polyploids.

Finally, establishment that specific phenotypes can be accounted for by common genes in different taxa affords unprecedented opportunity to cross-utilize genetic information, and to select the optimal taxon for chromosome walking and map-based cloning (4 warranted). Examples of at least one such case will be presented.

We gratefully acknowledge financial support from the USDA- NRICGP Plant Genome Program, for aspects of this work.