Evolution under the microscope
The tandem arrangement and structural similarities of the him and zim genes suggest they all arose from a common ancestor through gene duplication and subsequent selection. This inspired Dernburg and Phillips to look at similar genes in the related species C. remanei and C. briggsae. Like C. elegans, C. remanei has a total of four such genes, while C. briggsae has five. All code for zinc-finger proteins, all of which are distinct from one another.
"These different species of Caenorhabditis are evolving rapidly away from one another," says Dernburg. "The zinc-finger proteins bind to different sites in the Pairing Centers on their chromosomes, which prevents pairing during meiosis, so even those who mate can't produce fertile offspring."
The question remains whether new binding sites or new forms of zinc-finger protein lead rapid evolution in the worms. And there are other questions: in C. elegans, and also C. remanei, there are only four kinds of ZIM proteins that serve to initiate the pairing of six different chromosomes (in C. briggsae, only five proteins for six chromosomes), "so there must be more at work in helping the chromosomes that share the same ZIM figure out what's their proper partner," says Dernburg.
When it comes to the evolution of sex in C. elegans and its relatives, "we haven't yet figured out what makes each chromosome unique," Dernburg says. But the evidence is strong that the answer, as far as the worm is concerned, lies on the chromosomes' Pairing Centers.