Chromosomes of Villadia and Altamiranoa (Crassulaceae)

Villadia, ranging from Texas to Peru with some 25 species, has a rather distinctive thyrsoid to spicate inflorescence, and we keep it as a genus separate from Sedum. Twenty species show every gametic chromosome number from 9 to 17 and also 20–22 and higher. Chromosome pairing in hybrids shows that the species differ by many translocations and that species with 21 or lower are effectively diploid. More specialized species tend to have fewer and larger chromosomes, suggesting that through time translocations have rearranged the ancestral genome into fewer units. We suspect that relocated genes may be programmed differently, affecting phenotype. Thus Villadia is like Echeveria in having a remarkably long descending series of evidently diploid chromosome numbers. Altamiranoa, often included in Villadia, with about 15 species from Mexico south, more closely resembles Sedum in its broadly cymose inflorescence. It appears polyphyletic, with no clear boundary from Sedum, and we disperse its species in Sedum. The ten species studied have gametic numbers from 20 to 29 that probably are effectively diploid, with a few higher and probably polyploid. Again, chromosome pairing in hybrids shows that the species differ by many translocations. Putative relatives in Sedum section Leptosedum have n = 26 to 31. Thus cytologically as well as morphologically Altamiranoa has remained more similar than Villadia to its Sedum relatives.