ISOZYME EVIDENCE SUPPORTING A SINGLE GEOGRAPHIC ORIGIN FOR DOMESTICATED TEPARY BEAN

Crop domestication and evolution studies can provide valuable information about genetic diversity. The domestication pattern of tepary bean, Phaseolus acutifolius A. Gray, which is recognized as a possible source of genes for improving common bean (Phaseolus vulgaris L.), has yet to be resolved. We used starch gel electrophoresis to examine aconitase allozyme variation in seeds from 263 accessions of wild and domesticated tepary bean. Gene frequency data for wild and domesticated tepary bean were then compared in an attempt to clarify tepary bean domestication. Two ACO isozymes were detected. Whereas ACO-1 was monomorphic, two allozymes were detected for ACO-2. The allele encoding the slower migrating ACO-2 allozyme was present in domesticated tepary bean at a frequency of 0.997, whereas the frequency of the fast allele in wild tepary bean was found to be 0.96. This pattern of ACO-2 allozyme variation suggests that tepary bean was domesticated in a single geographic region, and predicts a genetic bottleneck during tepary bean domestication. Based upon our results, the Mexican states of Jalisco and Sinaloa are two potential geographic origins of domesticated tepary bean. This confirms the results of some previous studies and suggests that broad surveys of tepary bean germplasm for specific traits should focus on wild rather than domesticated forms of the species. In contrast, well-documented characteristics of tepary bean such as drought tolerance may be transferred to common bean by choosing only a few domesticated tepary bean parents that cross well with common bean.