Tracing the wild genetic stocks of crop plants.

A number of attempts have recently been made to identify the wild populations that form the founder germplasm stock of our crop plants (Huen et al. 1997; Molina-Cano et al. 1999; Ladizinsky 1999; Zhou et al. 1999; Badr et al. 2000). Some of these works were based mainly on comparative analyses and statistical treatments of genomic DNA polymorphism data (Huen et al. 1997; Molina-Cano et al. 1999; Zhou et al. 1999). This approach can lead to biased or, at times, conflicting conclusions because the criteria for comparisons are traits that are polymorphic both in the wild and among the cultivated material. Therefore, the end result of these comparisons is a set of values expressing relative difference (or similarity) between the accessions studied, presented in most cases as a dendrogram or set of dendrograms. Therefore, any relative approach analyses using isozyme, random amplified polymorphic DNA (RAPD), restriction fragment length polymorphism (RFLP), amplified fragment length polymorphism (AFLP), simple sequence repeats (SSR), or single nucleotide polymorphisms might yield different results when carried out under similar conditions. The inconclusiveness of the genetic distance approach has been demonstrated in several crops, of which lentil is one example. Ranges of genetic similarity among accessions of cultivated lentil (Lens culinaris Medikus), its wild progenitor subsp. orientalis (Boiss.) Hand.-Mazz., and a more distantly related L. odemensis Ladiz. are 0.71–1.00, 0.76–1.00, and 0.75–0.95, respectively, with 0.76–1.00 between the cultivated lentil and its wild progenitor and 0.75–0.95 between the cultivated lentil and L. odemensis (Hoffman et al. 1986). Another difficulty in using relative genetic distance to identify the wild genetic stock of a crop plant is the genetic bottleneck typical of most domestication events (Ladizinsky 1985). Usually, domesticated crops are thought to have originated as a result of mutations in the seed dispersal loci of a few individuals. In a wild population encompassing high DNA polymorphism, such mutations did not necessarily occur in the most frequent DNA morph at any one locus. Genetic distance analysis was recently utilized to suggest Morocco as a centre of origin of domesticated barley (Hordeum vulgare L.) (Molina-Cano et al. 1999). The main problem with this contention has to do with the identification of the so-called wild barley (Hordeum spontaneum C. Koch) from Morocco. These populations were collected in a restricted area in the Djebel Siroua range, where they grow exclusively in cultivated fields and never spread into the adjacent natural habitats dominated by the wild desert plant Artemisia herba-alba Asso. (Molina-Cano et al. 1982). Moreover, in their analysis these authors included a tough rachis type, indicating their failure to take into account the differences between wild and domesticated plant forms. Yet another disturbing point (in addition to the title), is the conclusion suggesting Morocco as a possible centre of origin of cultivated barley. It is hard to understand how the high level of internal similarity of the Moroccan barley, which is likely to be a mere result of a founder effect (Ladizinsky 1985), supports such a conclusion. In our view, the spontaneous barley types from Morocco originated from back mutations in one or two of the brittlerachis genes (bt1, bt2) of cultivated barley grown in that area. This would provide a plausible explanation for the high degree of internal similarity of the Moroccan barley, its adaptation to cultivated fields, and its inability to survive in the surrounding natural habitats (Molina-Cano et al. 1982). Recently, AFLP markers were used to estimate genetic distance between wild and cultivated barley (Badr et al. 2000). In this study, accessions from Israel, Jordan, Lebanon, Syria, Turkey, Iraq, Iran, northern Africa, central Asia, and the Himalayas were compared. Interestingly, the hypothesis of Molina-Cano et al. (1999) regarding Morocco as a center of origin of cultivated barley was not confirmed by Badr et al. (2000). Based on their DNA marker analyses, Badr et al. (2000) suggest that certain populations in the central and northern parts of Israel along with several Jordanian populations are the possible origin of cultivated barley. Inspection of the number of Israeli and Jordanian accessions versus the numbers of lines from the rest of the geographic regions relative to the actual area of the respective territories reveals the following: Israel and Jordan, while holding less than one thirtieth of the area of the other regions, are represented by 132 wild accessions. The remainder of the wild barley distribution range (including northern Africa, central Asia, the Himalayas, Turkey, Iran, Iraq, Syria, and Lebanon) are represented by 185 accessions. Had the Israeli–Jordanian wild barley genepool been represented by merely 6 lines in accordance with its relative area, what might have been the