Origin and evolution of highly polymorphic rDNA sites in Alstroemeria longistaminea (Alstroemeriaceae) and related species.

Alstroemeria (Alstroemeriaceae) displays a conserved and highly asymmetric karyotype, where most rDNA sites can be properly recognized by the size and morphology of the chromosomes. We analyzed the intraspecific variation of rDNA sites in A. longistaminea and compared with their distribution in other species (A. caryophyllaea and A. piauhyensis) and a representative of a sister genus, Bomarea edulis. All three species of Alstroemeria presented 2n = 16, and one to six B chromosomes were found in some individuals of A. longistaminea. There was a set of 12 conserved rDNA sites (four 5S and eight 35S) and up to 11 variable sites. B chromosomes were almost entirely covered by 35S signals, coupled with tiny 5S sites. Noteworthy, most rDNA sites found in A. caryophyllaea and A. piauhyensis were localized in chromosome positions similar to those in A. longistaminea, suggesting the existence of conserved hotspots for rDNA accumulation. Some of these hotspots were absent in Chilean Alstromeria as well in B. edulis. We propose that insertions of rDNA sequences on chromosomes do not occur randomly but rather on preferential sites or hotspots for insertions. The maintenance of these arrays, however, may be favored/constrained by different factors, resulting in stable or polymorphic sites.