Seven mostly long-stemmed species of Echeveria Series Nudae and many intermediate forms from the Andes of Venezuela to northern Peru have gametic chromosome numbers of 21 or 22, and a few are tetraploids with 42 or 44. They hybridize freely with each other in cultivation. In the hybrids most of their chromosomes form pairs at meiosis. All species in Series Nudae appear to be very closely related and probably descended from progenitors that arrived only recently in South America. They can also hybridize with species of most genera of Mexican Crassulaceae and belong with them in the same vast comparium of more than 200 species. In these hybrids, segments of their chromosomes can pair with segments of chromosomes from other species and genera, but no part of any of them can pair with any other chromosome of their own basic (conspecific) genomes. Each of the 21 or 22 gametic chromosomes differs from one another, and there is no evidence of any polyploidy in their ancestry. Acaulescent or short-stemmed species, mostly from farther south, are classed with a different series, the Racemosae.