More than 40 years ago, Bill Neaves, then a young Ph.D. student, discovered how an all-female, asexual species of the whiptail lizard came to be. He found that the lizard was a cross between the female species of one type of lizard and the male species of another.
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But what has puzzled him for years is how this all-female species maintains its high level of genetic variation, a contribution to evolutionary fitness that typically comes from sexual reproduction.
Despite reproducing without a male partner, this lizard species has a strong presence in the wild. Now, Aracely Lutes, a graduate student at the Stowers Institute for Medical Research in Kansas City, Mo., where Dr. Neaves works, has figured out the missing piece of the puzzle. The findings were reported Sunday in the journal Nature.
In a sexually reproducing lizard species, each lizard has 23 chromosomes from its mother, and 23 from its father.
During reproduction in the all-female species, Ms. Lutes found, all 46 of the mother’s chromosomes are duplicated, resulting in 92 chromosomes in each egg cell.
Those chromosomes then pair with their identical duplicates, and after two cell divisions, a mature egg with 46 chromosomes is produced. Since crossing-over during the cell divisions occurs only between pairs of identical chromosomes, the lizard that develops from the unfertilized egg is identical to its mother.
Importantly, each new lizard is also a replication of the original. Since the original was genetically diverse, pulling its chromosomes from two different species, its carbon copy descendants are, too.
It is because of this that the species has thrived over the generations. But unlike sexually reproducing species, these lizards will probably never evolve into a stronger species, Dr. Neaves said.
They “may be really well suited for the desert Southwest,” he said, “but when the next ice age comes, it may be that all of them get wiped out.”
