Tuesday, April 17, 2007

Chromosomes and Primate Evolution

As more and more primate genomes are sequenced, we are gaining insight into the patterns involved in primate evolution. Many of these patterns involve large scale chromosomal rearrangements. A good summary of what we currently can infer about primate evolution from cytogenetics data is at Imprimat (http://www.inprimat.org/), a European Consortium focused on primatology including evolution.

If there is anyone who doubts the evolutionary relationship between humans and the great apes (chimps and gorillas), you might check out these banding pattern comparisons between human and chimpanzee chromosomes originally from:

Yunis, J.J. and Dunham, K. 1980. "The Striking Resemblance of High-Resolution G-Banded Chromosomes of Man and Chimpanzee". Science, 208, 1145-1148.

The Imprimat site has chromosome changes for primates inferred by FISH chromosome painting. In this technique, fluorescent DNA probes from different regions of human chromosomes are applied to chromosomes of other species. The probe will bind to regions of the other species chromosomes that have the same or very similar nucleotide sequences. See http://www.mun.ca/biology/scarr/FISH_chromosome_painting.htm for more information about this technique. Regions of different chromosomes that have the same basic nucleotide sequence and along with that corresponding genes are inferred to be homologous or what cytogeneticists call synteny. Synteny is a kind of homology referring to chromosome regions that have two or more corresponding loci. If you don't get the idea visit this interactive chromosome comparison between human chromosome 22 and the synteny with different mouse chromosomes.

The Imprimat site has this diagram showing the presumed syntenies between the great Apes (Chimps and gorillas) and human chromosomes; the numbering is for human chromosomes. Note for instance that human chromosome 2 is derived from the fusion of two chromosomes present in the great apes (Center box at bottom of figure).

Note that the primitive chromosome number is believed to be 48. Also note that chromosome 14 and 15 are believed to have been originally one chromosome that broke apart. Also in gorillas (GGO) a translocation or exchange of regions between chromosome 5 and 17 appears to have happened.

Another interesting diagram on this site is a cladogram showing inferred evolutionary relationships between the major groups of primates. Notice again the big difference between chimps and humans is the fusion event between what were two separate chromosomes to yield human chromosome 2.

Rhesus monkeys are in the family Cercopthidae on a separate branch from the human like primates which include the great apes, humans and gibbons (Hylobatidae). What's interesting is the great amount of chromosome rearrangement that apparently happened in the evolution of gibbons compared to Rhesus Monkeys.

A closer look at the chromosomes of a gibbon from Koehler et al (1995) is to the left. In their abstract, Koehler et al note that here is a primate that is closely related to humans with a "highly disturbed" pattern of chromosome rearrangements.

Other links:
Primate Info Net: Primate Evolution.

History of Primates.

A smooth fossil transition in a primate. (Just in case you don't think that transitional fossils exist)

Special online article on the Rhesus Monkey Genome.

Comparison of Human and Great Ape Chromosome 2. The molecular data is exactly what is predicted if human chromosome 2 really is the product of two fused chromosomes!

Lesson plans related to human/chimp chromosome comparisons.

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