PETER BAUMANN
Since the 1960s scientists have known that some species of whiptail
lizards need a male even less than a fish needs a bicycle. These
all-lady lizard species (of the Aspidoscelis genus) from Mexico and the U.S. Southwest manage to produce well-bred offspring without the aid of male fertilization.
But how do they—and the other 70 species of vertebrates that propagate
this way—do it without the genetic monotony and disease vulnerability
that often results from asexual reproduction? "It has remained unclear"
and "has been the topic of much speculation," report a team of
researchers who aimed to answer just that question. Their results were
published online February 21 in the journal Nature. (Scientific American is part of Nature Publishing Group.)
These lizards and other "parthenogenetic species are genetically isolated," explains Peter Baumann,
an associate investigator at the Stowers Institute for Medical Research
in Kansas City, Mo., and co-author of the study. Species as diverse as Komodo dragons and hammerhead sharks
do it asexually if necessary, but some species, like these little
lizards, don't have a choice. "They can't exchange genetic material, and
this loss of genetic exchange is a major disadvantage to them in a
changing environment," he says. Unless an animal can recombine the DNA
they already have, they will produce an offspring with an identical set
of chromosomes, in which any genetic weakness, such as disease
susceptibility or physical mutation, would have no chance to be
overridden by outside genetic material from a mate.
The new research by Baumann and his team reveal that these lizards maintain genetic richness
by starting the reproductive process with twice the number of
chromosomes as their sexually reproducing cousins. These celibate
species resulted from the hybridization of different sexual species,
a process that instills the parthenogenetic lizards with a great amount
of genetic diversity at the outset. And the researchers found that
these species could maintain the diversity by never pairing their
homologous chromosomes (as sexual species do by taking one set of
chromosomes from each parent) but rather by combining their sister
chromosomes instead. "Recombination between pairs of sister chromosomes
maintains heterozygosity" throughout the chromosome, noted the authors
of the study, which was led by Aracely Lutes, a postdoctoral researcher
in Baumann's lab.
This discovery, which had until now been unconfirmed in the reptile
world, means that "these lizards have a way of distinguishing sister
from homologous chromosomes," Baumann says. How do they do it? That's
something the group is now investigating.
Another big unknown is precisely how the lizards end up with double the
amount of chromosomes in the first place. Baumann suspects that it could
happen over two rounds of replication or if two sex cells combine
forces before the division process starts.
Although asexual reproduction
might seem like a bore—and one that can have questionable genetic
outcomes unless done right—it has its benefits, too, Baumann notes.
"You're greatly increasing the chances of populating a new habitat if it
only takes one individual," he says, citing the example of the brahminy
blind snake (Ramphotyphlops braminus), another parthenogenetic
species. "If she has a way of reproducing without the help of a male,
that's an extreme advantage." Indeed it is—the brahminy has already
colonized six continents. VIA; http://www.scientificamerican.com/article/asexual-lizards/
