Chromosome painting in Callicebus lugens, the species with the lowest diploid number (2n=16) known in primates.

Cytogenetic studies have shown that New World primates are karyologically diverse and highly derived. The genus Callicebus is the best example of this karyological diversity, with diploid numbers ranging from 2n=50 to 2n=16. We report on Callicebus lugens, which has the lowest diploid number (2n=16) yet found in the primate order and represents a striking example of extreme karyotypic shuffling. To better understand the genomic rearrangements that have resulted in this extremely low diploid number, we mapped chromosome homologies between C. lugens and humans by in situ hybridization. The total number of hybridization signals was 42, excluding the Y chromosome, with a total of 34 syntenic associations not found in humans. This species has one of the most derived karyotypes among the Platyrrhini. Fusion has been the predominant mode of karyological evolution, although fissions and inversions have also transformed the C. lugens karyotype. Remarkably in such a highly rearranged karyotype, the synteny of 11 human chromosomes (4, 5, 9, 12, 13, 14, 17, 18, 20, 21, and X) was maintained intact, even if most of these human-homologous gene clusters were translocated. Other human syntenies, such as homologues to human chromosomes 10 and 16, were highly fragmented. Comparisons of the C. lugens-human homology map with those of other New World primates have not yet helped establish a phylogenic arrangement between congeneric species or link Callicebus with any other genus.