Molecular, cultural, and ecological markers of genetic diversity in Mexican species of beans (Phaseolus)

coccineus ssp. coccineus (scarlet runner bean), P.coccineus ssp. darwinianus, P. acutifolius (tepary bean), and P. lunatus (Lima bean), all native to (and probably domesticated in) Mexico. Most of the studies required for improving bean genetics and cultivation are based on the common bean because of its commercial importance in world markets. On the other hand, the scarlet runner bean with its different biology (e.g., outbreeder rather than inbreeder) is poorly known even though it is an important staple crop to many isolated rural peoples of Mexico. Due to its importance to nourishment of rural people and to the lack of fundamental biological knowledge, detailed studies were conducted on scarlet runner bean in two regions of Mexico where indigenous peoples grow and manipulate this species and its wild relatives. As an initial step towards a long term study of the diversification, conservation, and development of scarlet runner bean, field and experimental data were analyzed in order to characterize the variation of this bean in western Chihuahua and northern Puebla. Characterization was based upon genetic (chloroplast DNA, isozyme and chromosomal features), cultural (ethnobotanical information on knowledge, cultivation, and use), and ecological (pollination, biology, and ecophysiology) studies of selected plants by an interdisciplinary team of researchers. Even though the two studies areas shared such features as mountainous terrain, indigenous cultures, and close proximity of cultivated and assumed wild relatives, the patterns of bean diversity in the two regions were different. The analyses of the chloroplast DNA (first for P. coccineus) suggested that the climbing P. coccineus ssp. darwinianus is more distantly related to the typical P. coccineus than currently accepted. The subsistence Nahuatl farmers of Puebla are diversifying the former in response to changing local market demands (i.e., early and late crops which do not compete with maize. The wild relatives do not provide additional genetic diversity today according to pollination and genetic data. On the other hand, the Tarahumara Indians in Chihuahua have selected a early maturing form which is correlated with its distinctive (relative to other P. coccineus forms) physiology and bush form which is cultivated in monoculture (rather than as a climber over the maize). In contrast to Puebla, the wild bean of Chihuahua in the surrounding pine�oak forests crosses with the cultivated forms. This natural hybridization system is a dynamic in situ conservation strategy in a rural area where native people are subject to short and often catastrophic cultivation seasons. In both cases, native farmers recognize and promote genetic diversity and its subsequent expression in adaptive growth behavior and subsistence production under changing human demands and limiting environmental conditions. (Author abstract)