Fusing chromosomes

Carl Zimmer recounts recent research by Evan Eichler’s group on the evolution of human chromosome 2, which represents a fusion of two separate chromosomes in ancient apes, which still remain separate in the living great apes: “The Mystery of the Missing Chromosome (With A Special Guest Appearance from Facebook Creationists)”. The research paper is by Mario Ventura and colleagues Ventura:2012.

The two chromosomes fused, and the cap was deleted, inclusing StSat. It could no longer spread around our genome, the way it did in chimpanzees and gorillas.
This study is an important advance in our understanding of how human chromosomes evolveda subject of medical significance, too, since the duplication of the DNA at the end of chromosomes can cause dangerous mutations that can cause genetic disorders. Plus, it is very cool to see how our chromosomes are, in fact, an ancient patchwork.

People often ask me when this chromosome fusion happened in ancient hominins. I think they attribute excessive importance to this event, reasoning that chromosome fusion may have been the cause of some reproductive isolation. For example, they often ask specifically about Neandertals and modern humans, figuring that when we show Neandertals had 48 chromosomes, it will at last explain why they are extinct.

In reality, the fusion must have happened within a population. The first person who carried it, and his immediate descendants, must have been able to mate and reproduce successfully with people who didn’t carry it. This outcome is not uncommon for chromosomal rearrangements. Many create reproductive incompatibility, and those that do are very unlikely to become common within a population. Some become moderately common but create problems for homozygotes who carry two copies of them. Others seem to be neutral and do not cause noticeable problems.

So why do related species with different chromosome numbers often have trouble producing fertile offspring, even if they can mate successfully? This is likely because many chromosomal rearrangements and other genetic changes have accumulated in each lineage after a long period of reproductive isolation. Each may have been near selectively neutral within the population where it first occurred. A few may start out deleterious in homozygotes, and later may become fixed in the population only after other genetic changes ameliorate (or “rescue”) these deleterious effects. Sometimes, positive natural selection can favor changes within one population that decrease carriers’ ability to reproduce with members of another population, and in these cases reproductive isolation can appear very rapidly. In other words, the evolutionary constraints on chromosome structure aren’t simple.

Whether fast or slow, as each of the emerging species becomes different from the ancestral genetic background, the potential for reproductive incompatibility increases. This evolution is not a single jump, but a series of steps that may result in gametic incompatibility, hybrid inviability, or hybrid sterility.

The series of events leading to the fusion of human chromosome 2 are genetically very interesting, as are the repeated instances of rearrangement that Ventura and colleagues illustrate in chimpanzees. But chromosome fusion has no special magical power, and whether it was connected to ancient speciations or other events in our evolution will take a lot of creative hypothesis testing.