Minisymposium 12: Genome evolution

Abs # 25003: Genetic and molecular characterization of the I locus of Phaseolus vulgaris

Presenter:	Vallejos, C. Eduardo , vallejos@ufl.edu
Authors	Vallejos, C. Eduardo  (A)   Astua-Monge, Gustavo  (A)   Jones, Valerie  (A)   Plyler, Tammy  (A)   Sakiyama, Nei  (B)   Vanhoutten, Wim  (C)   Mackenzie, Sally  (C)
Affiliations:	(A): Dept of Horticultural Sciences, University of Florida, Gainesville, FL (B): Dept Fitotecnia, Universidade Federal de Viçosa, Viçosa, MG, Brazil (C): Beadle Center for Genetics Research, University of Nebraska, Lincoln, NE

The I gene of the common bean confers resistance to non-necrotic strains of the Bean common mosaic virus (BCMV), and conditions bean plants to develop systemic necrosis following infection with necrotic strains of the virus. Plants that are homozygous recessive at this locus develop systemic mosaic after inoculation with either group of strains. We have constructed a BAC-based contig that comprises the I locus. Partial sequence analysis of several BAC termini, and hybridization analysis indicates this locus is associated with a multigene family of disease resistance genes that belongs to the TIR-NBS-LRR subclass. While resistant haplotypes display approximately 22-24 TIR-hybridizing fragments, susceptible haplotypes display half as many fragments. This difference in copy number creates hemizygosity at this locus, a condition that is probably responsible for suppression of recombination within this megalocus. Southern analysis of several Andean and Mesoamerican bean cultivars strongly suggests the I gene arose only once in the Mesoamerican gene pool. Furthermore, Southern analysis of wild accessions revealed that the most primitive beans display only two TIR-hybridizing fragments; this observation suggest this multigene family has evolved in great part by unequal crossing over and recombination. Finally, transcripts levels of this gene family appear to be expressed at a higher level in the resistant than in the susceptible genotypes, and appear to be up-regulated after inoculation with the virus.