Electrophoretic analysis of seed storage proteins has proven to be a valuable tool in tracing the evolution of crop plants, especially for identification of the wild progenitors and gathering additional information on the evolutionary and domestication patterns.
The structural and functional features of phaseolin, the major seed protein of com- mon bean, make it a useful marker. This protein, accounting for 50% of total protein stored in the cotyledons and 35–46% of the total seed nitrogen, is coded by a cluster of closely related genes that may arise by successive duplication and diversification from an ancestral gene. The divergence processes include insertions, nucleotide substitutions, duplications or deletions of repeats (Kami & Gepts, 1994). In addition co- and post- translational modifications, including cleavage of the signal peptide, different glyco- sylation of polypeptides (Lioi & Bollini, 1984) and charge variation due to amino acid substitution resulted in the formation of slightly heterogeneous phaseolin polypeptides in the Mr 54–44 kDa, reflecting genotype divergence.
In a pioneering work by Gepts (1988), phaseolin was used as a marker in describing the domestication patterns and worldwide dissemination of common bean. Phaseolin electrophoretic analysis of wild and domesticated materials supported the hypothesis of multiple domestication events, thought to be the cause of parallel geographic phaseolin
variation between wild and cultivated forms. The Mesoamerican domestication gave rise to small-seeded S phaseolin cultivated materials, while large-seeded T, C, H and A phaseolin were observed in the southern Andes (Koenig, Singh, & Gepts, 1990). Moreover, it has been shown that phaseolin is a useful biochemical marker to follow the dispersal pathway of common bean from domestication areas into Europe. This revealed that the European common beans arose from the introduction of domesticated beans from both of the American gene pools. A higher frequency of Andean phaseolin types (76%) with respect to Mesoamerican ones (24%) was first recorded in European germplasm by Gepts and Bliss (1988). This was successively confirmed by Lioi (1989), analysing a large collection of accessions mainly from Italy, Greece and Cyprus. The prevalence of Andean types within the European common bean germplasm stored in some international gene banks has been recently confirmed by Logozzo et al. (2007), who analysed a collection of 544 accessions all from European regions, showing that the Andean phaseolin types T (45.6%) and C (30.7%) prevailed over the Mesoamerican S type (23.7%). A summary of the results from different studies are reported in Figure 2.5. Despite a large variation in sample sizes and sampling strategies among these inves- tigations, the presence of all three major phaseolin types (C, T and S) was observed in all the areas considered, suggesting a large seed exchange among the European coun- tries. Over a total of 1309 European accessions considered, a prevalence of Andean pha- seolin types at a single-country level was confirmed, with a global 79.6% versus 20.4% of Mesoamerican types. Differences in the frequencies of each Andean phaseolin type have also been observed. In the countries along the Mediterranean arc such as on the Iberian Peninsula, in Italy and the Balkan area, phaseolin C was the most common type. Conversely, in accessions from France, Central Europe and Sweden, the T type was the prevailing one. A relatively high frequency of Mesoamerican types was observed in Central Europe (27%) and France (30%) compared to Mediterranean countries, where the frequency is lower, reaching a mean value of 18%. European S types showed a larger seed size than those from the centre of domestication. Logozzo et al. (2007)
suggested two hypotheses to explain this finding: a preferential introduction of Durango and Jalisco races that, among Mesoamerican races, possess larger seeds, or a selection towards larger seeds within S types after introduction in Europe.
It has been suggested that crop expansion from America to Europe resulted in a reduction of diversity because a strong founder effects due to adaptation to new envi- ronments and consumer preferences, followed by evolution probably involving hybridi- zation and recombination between the Andean and Mesoamerican gene pools (Gepts, 1999). Papa et al. (2006) estimated a loss of diversity around 30% and a low differen- tiation between the gene pools in Europe, when compared with the differences in the Americas, suggesting a combination of greater gene flow or convergent evolution for adaptation to European environments. More recently Angioi et al. (2010) using six chlo- roplast microsatellite (cpSSR) markers, confirmed that European common beans arose from both gene pools, but the bottleneck effect of the introduction into Europe might not have been so strong. Moreover, they estimated that hybrids between the two gene pools occurred at higher frequencies in Central Europe and lower frequencies in Italy and Spain. Moreover they suggest that not only some of the countries therein, but the entire European continent can be regarded as a secondary diversification centre for P. vulgaris.
Molecular markers have been shown to be effective indicators for genetic varia- tion underlying agronomic traits with some advantages over morphological traits, such as the ability to distinguish among accessions with similar morphology and dis- criminate polymorphism over far more loci than isozymes or seed storage proteins. Molecular markers span broader genomic areas and present different types of inher- itance, so they have also been used to better estimate the levels of diversity and to understand the effects of migration and selection on the maintenance of polymor- phism in the European beans. There are several papers on the characterization of European germplasm of P. vulgaris using different molecular markers. Some stud- ies were based on random PCR markers, such as RAPDs (Mavromatis et al., 2010), inter-simple sequence repeats (ISSRs) and AFLP (Svetleva et al., 2006; Šustar-Vozlicˇ, Maras, Kavornik, & Meglicˇ, 2006). Other molecular markers such as SSR, which are Figure 2.5 Distribution (%) of phaseolin type frequencies across Europe. Number in parentheses next to the geographical region name refers to sample size.
Source: Data from Gepts and Bliss (1988), Rodiño et al. (2001), Šustar-Vozlicˇ et al. (2006),
more specific in target, were used to assess diversity among landraces (Lioi et al., 2005). Moreover, recently some studies were carried out to fingerprint specific lan- draces using different molecular markers (Lioi et al., 2012; Paniconi, Gianfilippi, Mosconi, & Mazzuccato, 2010).