Tuesday, December 13, 2005

Hooray for Humanzee

John Wilkins points us to the inspiration for King Kong -- a Russian scientist’s failed attempts to create a human-chimpanzee hybrid. The conversation over at Pharyngula shifted to obstacles of hybridization between humans and chimps. PZ Myers thinks that differences in gene regulation between hybrids would present a greater barrier to hybridization than genome rearrangements. I agree with him that rearrangements themselves would not pose a problem for hybridization -- many species, including humans, are polymorphic for rearrangements. Also, the difference in chromosome number (humans have 23 pairs, chimps have 24 pairs) is not all that important because it results from a fusion of two chromosomes along the human lineage; both species have equivalent amounts of genomic information, it’s just arranged slightly different.

The rearrangements may still be important if they harbor genes responsible for the reproductive isolation between the two species. This is well supported in Drosophila and has been suggested in apes, but not without some disagreement. The theory posits that inversions will prevent the transfer of reproductive isolating factors between speciating populations by suppressing recombination. The problem with applying this theory to humans and chimps is that the model assumes range overlap during speciation, such that rearrangements are necessary to prevent gene flow between the two species in regions of the genome containing hybrid incompatibility factors. From what I understand, an important event during hominid-chimp speciation was the emigration of hominids from rain forests to open grass lands meaning the speciation occurred allopatrically. Hence, we have no reason to believe that the genomic rearrangements differentiating the human and chimp genomes play an important role in the reproductive isolation of the two species.

Human-chimp hybrids and evolutionary intermediates have interested scientist and lay-people alike for quite a long time. One particularly famous case involves a predominantly bipedal chimp named Oliver who was often referred to as a Humanzee (for human-chimpanzee hybrid). Oliver had many behaviors that seemed more human than chimp, leading some people to believe he was either the product of a human-chimp hybridization event or some evolutionary missing link. It turns out, via a simple molecular assay, that Oliver was 100% chimpanzee, and his human-like behaviors were often exaggerated. Oliver represents a good piece of evidence for the role of developmental plasticity in anatomical evolution -- one could imagine that his upright stance could have influenced other members of his community, thereby changing the selective pressures on the morphology of the population.

Studying the genetics of speciation represents a major paradox: the study of genetics (from Mendel to Morgan to current research done today) requires the crossing of different individuals to determine how traits are inherited, but species boundaries prevent those requisite matings. Research on the genetics of speciation can be done using everyone’s favorite model system, Drosophila, by creating mutant flies that can mate across species boundaries. Surprisingly, mutations in single genes can break down those species boundaries, allowing for viable hybridization between D. melanogaster and its close relatives. Geneticists can create inter-specific hybrids using these mutant flies and study the effects of transcriptional regulatory elements on the differences in gene expression (something would obviously interest Dr. Myers). So far, a few speciation genes have been identified, but their protein products do not fall into a particular class -- they include transcription factors and a component of the nuclear pore complex.

It’s hard to refute the idea that transcriptional regulation is important in speciation, given both the analysis of expression differences between species and the identification of transcription factors that prevent hybridization. I guess I’m going to have to agree with the developmental biologist on this one -- the human-chimpanzee hybridization experiments probably failed due to regulatory differences (along with problems in sperm-egg recognition proteins). Also, God probably wouldn’t have wanted it to happen anyway.


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