Plant & Animal Genome VI Conference

W72

THE DIVERSE ORGANIZATION OF MULTIGENE FAMILIES IN COTTON SUBGENOMES

Department of Agronomy and Range Science, University of California, Davis, CA 95616-8515

The study of gene structure and organization of multigene families in the cultivated allotetraploid cotton Gossypium hirsutum L. (2n = 4x = 52), especially in a physiologically relevant context, poses a unique challenge compounded by the complexity of the genome. On the most basic level, closely related genes are representatives of homeologous loci predictably derived from the A and D subgenomes following the relatively recent evolution of allotetraploid cotton. However, this simplistic model for gene organization is complicated further by the fact that extant ancestral progenitors of present day diploid species were themselves paleopolypoloids, resulting in a larger number of gene family members than may otherwise be expected. RFLPs and/or the size and nucleotide composition of introns has been used successfully to assign the origin of closely related genes to each of the subgenomes.

A case in point is the structure and organization of a cotton gene superfamily encoding the vacuolar H+-ATPase (V-ATPase) catalytic, or A subunit, which is composed of two diverse, but closely related multigene families. Interestingly, each gene family includes 4-6 putative homeologous A and D loci that are more closely related to each other, even though they are derived from different subgenomes, than are A (or D) homologs between families. Structural analysis revealed that an ancient gene duplication event of an ancestral gene occurred following the bifurcation of plants and yeast, giving rise to the two genes long before the evolution of the Gossypium genus. The promoter regions of representatives from both gene families are quite diverse, indicating that the two genes have evolved the means to perform similar functional roles, but in response to a discrete set of environmental, physiological and developmental cues during plant growth, depending on the individual needs of the cell. This scenario is supported by expression work in which quantitative RT-PCR analysis demonstrated that the two gene families are differentially expressed in developing cotton fibers in a manner that is likely related to the customized assembly of V-ATPase holoenzymes with defined roles in cell expansion.