Analysis of polimorphic NORs in Alouatta species (Primates, Atelidae)

Nucleolar organizer regions were analyzed in four different species of Alou atta (Atelidae, Platyrrhini) using silver staining. They showed a high vari ability, appearing in different numbers and positions, with a singular vari ation not found in other primate genera so far. Hence, the number of NOR-si tes varied from four to six, while the number of NOR bearing chromosomes we re two, four or six. The comparison of these data with previous ones sugges ts that A. fusca clamitans and A. caraya, with two small pairs of acrocentr ic chromosomes with a NOR-site in the proximal region of the long arm, have a conservative trend from a hypothetical ancestor, while A. belzebul belze bul and A. seniculus macconelli have derived states. Moreover, it seems tha t the processes of rDNA amplification sites occurred independently at least four times during the irradiation of this genus. These processes could be due to transposition events or to the amplification of inactivated NORs con taining a relatively small number of rDNA copies. Our results reinforce the chromosomal phylogeny proposed to this genus, corroborating the basal posi tion of A. caraya, and the position of A. belzebul nigerrima, closer to A. seniculus group.