The genus Lepidium is widely distributed throughout the world in a11 continents except Antarctica. The genus probably originates in the Mediterranean basin where most of the diploid species are found (Thellung 1906; Mummenhoff et al. 1992). Little is known about the time of origin of the genus and the mechanisms responsible for its worldwide distribution. However, most of the existing evidence indicates that long-distance dispersal during the late Tertiary or Quaternary, rather than continental drift, was responsible for the colonization of these species to the Americas and Australia. This seems to be the prevalent mechanism of distribution of other genera in the family such as Capsella and Cardamine (Mummenhoff et al. 1992). Common genetic features observed in the immigrant species of Lepidium are autogamy and polyploidy, which helps their establishment in new habitats, Although there are extensive taxonomic treatments of the Lepidium species of Australia (Hewson 1982) and North America (AI-Shehbaz 1986a, 1986b) as well as a general monograph on the genus (Thellung 1906), information is scarce on the species endemic to South America, and in particular about Andean Lepidium species, which belong mostly to the sections Dileptium and Monoploca (The11ung 1906).
These species are interesting because they grow at high-altitude habitats, up to 4500 m asl, and include the cultivated species maca. Probably maca was domesticated in; San Blas, Junín, between 1300 and 2000 years ago, but little is known about its origin (Mato s 1978; Rea 1992). It is believed that in the 16th and 17th centuries maca had a wider range of cultivation than today. The existence of wild species of different ploidies, in some cases sympatric with the cultivated taxon, indicates that extinction of possible ancestral species has not proceeded too far to prohibit understanding of the evolutionary history of the group. Consequently, taxogenetic studies may disclose the ancestral species of the cultivated taxon. The identification of related wild species could be applied to the gene tic improvement of maca, if these carry useful genes that could be transferred by hybridization.
Maca is an Andean crop of narrow distribution. It is restricted today to the suni and puna ecosystems (Bonnier 1986) of the Departments of Junín and Cerro de Paseo of Peru at elevations above 3500 m and often reaching 4450 m in the central Andes of Peru (León 1964; Te11o et al. 1992). The largest cultivated area is found around Lake Junín at Huayre, Carhuamayo, Uco, Ondares, Junín, Ninacana and Vicco. Apparently maca occupied wider areas of cultivation in the past (Johns 1981). In addition tú Junín and Cerro de Pasco, presumably, it also was grown in Cusco and in the Lake Titicaca watershed. Some of the writers of the time mention that many natives did not have any other food but maca. It was also used as payment of taxes to the Spanish administrators (Castro de León 1990). Its restricted cultivation today indicates that maca may have been in danger of being phased out as a crop.