In situ hybridization (FISH) maps chromosomal homologies between Alouatta belzebul (Platyrrhini, Cebidae) and other primates and reveals extensive interchromosomal rearrangements between howler monkey genomes.

We hybridized whole human chromosome specific probes to metaphases of the black-and-red howler monkey Alouatta belzebul in order to establish chromosomal homology between humans and black-and-red howlers. The results show that the black-and-red howler monkey has a highly rearranged genome and that the human chromosome homologs are often fragmented and translocated. The number of hybridization signals we obtained per haploid set was 40. Nine human chromosome probes gave multiple signals on different howler chromosomes, showing that their synteny is disturbed in A. belzebul. Fourteen black-and-red howler autosomes were completely hybridized by one human autosomal paint, six had two signals, three had three signals, and one chromosome had four signals. Howler chromosomes with multiple signals have produced 12 chromosomal syntenies or hybridization associations which differ from those found in humans: 1/2, 2/20, 3/21, 4/15, 4/16, 5/7, 5/11, 8/18, 9/12, 10/16, 14/15, and 15/22. The hybridization pattern was then compared with those found in two red howler taxa and other mammals. The comparison shows that even within the genus Alouatta numerous interchromosomal rearrangements differentiate each taxa: A. belzebul has six unique apomorphic associations, A. seniculus sara and A. seniculus arctoidea share seven derived associations, and additionally A. seniculus sara has four apomorphic associations and A. seniculus arctoidea seven apomorphic associations. A. belzebul appears to have a more conserved karyotype than the red howlers. Both red and black-and-red howlers are characterized by Y-autosome translocations; the peculiar chromosomal sex system found in the red howler taxa could be considered a further transformation of the A. belzebul sex system. The finding that apparently morphologically similar or even identical taxa have such extreme genomic differences has important implications for speciation theory and neotropical primate conservation.