ABSTRACT
More than 200 tuber-bearing species of genus Solarium are known, of which seven are cultivated forms (see Hawkes 1990; Dodds 1965 for reviews). However, the predominant cultivated form worldwide is Sola rium tuberosum ssp. tuberosum (Hawkes 1990) or Solarium tuberosum Group Tuberosum (Dodds 1965). Tuberosums are tetraploids, 2n = 4x = 48, believed to have evolved in Europe from introductions of the primitive cultivated form S. tuberosum ssp. andigena, or Group Andigena, via the Canary Islands during the late 16th century (Hawkes and Francisco-Ortega 1993). The means by which Andigena cultivars were bred and selected by the indigenous people of the Andes is a matter of conjecture. How ever, this tetraploid species is believed to have arisen several thousand years ago, either by spontaneous doubling of the somatic chromosome number of a diploid cultivated form, S. stenotomum, 2n = 2x = 24, or the doubling of chromosome number of a hybrid(s) between Stenotomum and a wild diploid species, S. sparsipilum (Hawkes 1990) (Fig. 3.1). Either way, Tuberosum is assumed for most purposes to be an auto tetraploid and usually demonstrates tetrasomic inheritance. Early cytological stud ies of meiosis (Swaminathan and Howard 1953), plus the fact that mod em breeders have introgressed genetic variation from several wild spe cies of varying degrees of ploidy (Ortiz 1998), suggest that modem culti vars may in fact be segmental allopolyploids. This could, in part, explain the disturbance of segregation ratios observed in modem molecular stud ies of tetraploid populations (Bradshaw et al. 1998).
