Phaseolin a Major Evolutionary Marker

Phaseolin is the major seed storage protein of common bean and it can be used to trace the evolutionary origin of common bean genotypes. The electrophoretic variability of phaseolin of wild-growing common beans from Mesoamerica and the Andes was compared with landraces of the same region. The wild common bean accessions of different geographic origin could be distinguished by their phaseolin type (Fig. 7.3). In Mesoamerica, the wild forms showed both the S type as well as M types. The Colombian wild common bean exhibited the CH and B types, whereas in the southern Andes, wild forms showed only the T type (Brown et al. 1981). There was a correspondence in the geographic distribution of phaseolin types between wild and cultivated common bean. The cultivars with S and T phaseolin patterns predominated in Mesoamerica and in the southern Andes, respectively. The B phaseolin type was present only in wild and cultivated common bean from Colombia. On the other hand, the C, H, and A phaseolin types were found not only among landraces of the Andes, but also among wild forms (Koenig et al. 1990). Therefore, multiple domestications are thought to be the primary cause for parallel geographical phaseolin variation between wild and cultivated common bean forms. Secondarily, occasional outcrosses between wild and cultivated common beans may have also contributed to this parallel distribution of phaseolin types. Given the low frequency of B phaseolin cultivars, Colombia might, however, only be a minor or more recent domestication region (Gepts et al. 1986)

A relationship was observed between phaseolin pattern and seed type. Cultivars with T, C, H, and A phaseolin types on the average have larger seeds than cultivars

PCR amplification of the 15-bp region in phaseolin

Heteroduplex

Homoduplex

Fig. 7.3 Polymerase chain reaction amplification of part of the phaseolin genes surrounding the 15 repeat. 1: T phaseolin (Andean); 2: S phaseolin (Mesoamerican); 3: I phaseolin (Ecuador and northern Peru) (Kami et al. 1995)

with S and B phaseolin types (Gepts et al. 1986). This relation provides evidence for exchange of germplasm between Mesoamerica and the Andes. It makes it possible to follow the world-wide dispersal of common bean cultivars. Mesoamerican cultivars were introduced into North-eastern USA, the Iberian Peninsula, Western Europe, and Africa. In these regions, they formed, however, only a minor component compared to the cultivars of Andean origin. The latter may have had a competitive advantage over genotypes of other origins because of a more adequate photoperiodic adaptation due to similar latitudes. The Mesoamerican and Andean gene pools were introduced in Europe at different times (Zeven 1997; Santalla et al. 2002) but the amount and distribution of the two gene pools in Europe is not well-known (Papa et al. 2005).

Previous studies have indicated that most of the Iberian cultivars may have been introduced from Chile due to a high frequency of the C phaseolin pattern (Gepts and Bliss 1988; Gil and De Ron 1992). However, most recent studies (Escribano et al. 1998; Rodino et al. 2003) indicated a high frequency of the T phaseolin pattern among Iberian cultivars. The T phaseolin type was also observed in Western Europe (Gepts and Bliss 1988). Subsequently, the Iberian Peninsula landraces could have been introduced in other parts of Europe such as Greece, Cyprus, and Italy, as indicated by the high proportions of T and C types in these areas (Lioi 1989; Limongelli et al. 1996; Santalla et al. 2002; Logozzo et al. 2007). These studies have provided evidence for the existence in the Iberian Peninsula of the two major gene pools, Andean and Mesoamerican. A much lower diversity of European common bean, due to a strong founder effect during the introduction into Europe, was suggested. However, recent studies show that the reduction of a genetic diversity was not as strong as expected and gene flow between the two American gene pools has been suggested (Santalla et al. 2002; Papa et al. 2005; Sicard et al. 2005)

Heteroduplex

Homoduplex

Fig. 7.3 Polymerase chain reaction amplification of part of the phaseolin genes surrounding the 15 repeat. 1: T phaseolin (Andean); 2: S phaseolin (Mesoamerican); 3: I phaseolin (Ecuador and northern Peru) (Kami et al. 1995)

and the new genetic variation in the common bean was showed (Rodino et al. 2006). The variation in bean-growing environments, cropping systems, and consumer preferences for seed types in this area might have played a significant role in the common bean crop diversity and could give rise to the preservation of a large variation in the characteristics of domesticated common beans. Different distribution of the C type compared to the T types was observed. While T type are more frequent in most of the European countries the C type are prevalent in Italy and Spain where the common bean first reached Europe (Logozzo et al. 2007).

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