For decades, biologists thought the reason they captured only females of some whiptail lizard species was because the males were better at hiding. It wasn't until the 1960s that they realized the males didn't exist: female whiptails—a cross between the male of one lizard species and the female of another related species—carried on the lineage without sexual reproduction. Fifty years later, HHMI early career scientist Peter Baumann has now revealed the molecular basis for whiptail reproduction.
There are various ways that other organisms reproduce without the mixing of genes from two distinct sexes, but they often lose their genetic diversity after a number of generations—only one version of each gene eventually is present. But the asexual types of whiptail lizards, over several thousand generations, have maintained the mix of genes from the two species that spawned them. Baumann hypothesized they must have a different strategy.
“I went looking in the literature to find out what was known about this and found very little,” says Baumann. “So I turned to classic cell biology.” Baumann and his colleagues at the Stowers Institute for Medical Research, in Kansas City, isolated nuclei from whiptail eggs and examined the contents.
The eggs of sexually reproducing lizards contain 23 chromosomes each, so when they fuse with sperm, the resulting offspring have the correct number of chromosomes—46—in their nonreproducing, or somatic, cells. In the asexual whiptails, Baumann's lab group found, meiosis (the cell division process that produces eggs and halves the normal number of chromosomes) begins with cells containing twice the normal number of chromosomes: 92. The final egg, therefore, contains 46 chromosomes—double the number in sexually reproducing lizards—to compensate for the fact that it won't be fertilized.
Moreover, during the stage of cell division in which parts of matching chromosomes, originally derived from each parent, usually recombine, the asexual whiptail eggs have only identical chromosomes to pair up—those duplicated an extra time before meiosis. No matter how much they mix, these matching chromosomes maintain their identity. During the formation of eggs for sexual reproduction, however, a different approach is taken—the matching, though nonidentical, chromosomes pair up to shuffle genes around purposefully. The study results appeared online in Nature on February 21, 2010.