Plant Genome III Conference

GENOME MAPPING IN LEGUMES: INSIGHTS INTO MOLECULAR EVOLUTION, GENE MAPPING, AND POSITIONAL, CLONING.

Comparative mapping with DNA markers provides insights into genomic blocks conserved among sexually isolated taxa. Knowing about conserved genomic blocks makes it possible to capitalize on mapping information from different species, potentially increasing marker density in regions of interest. In three cultivated species of the legume tribe Phaseoleae (Vigna radiata, Phaseolus vulgaris, and Glycine max), comparative mapping demonstrated that large segments are conserved between V. radiata and P. vulgaris, but only short blocks are conserved between these two species and G. max (Boutin et al, in review). Our lab is interested in resistance to the soybean cyst nematode (SCN; Heterodera glycines), but the soybean map was poorly populated in the region near the most important SCN resistance gene when our study began. Comparative genome mapping indicated that several Vigna and Phaseolus markers might be tightly linked to SCN resistance in Glycine. Linkage analysis confirmed this, leading to an increase in marker density near the SCN resistance gene from one marker every 10 centimorgans to one marker every 2 centimorgans. Physical mapping based on pulsed field gel electrophoresis (PFGE) further confirmed tight linkage among the markers originally identified by comparative mapping. In fact, two markers not yet linked by segregation analysis in soybean, but known to be tightly linked in Phaseolus, were shown to be physically linked in soybean by PFGE. Conservation at the physical level may be especially helpful in positional gene cloning in soybean since the genome size of V radiata is approximately 40% that of G. max.