No need for sex: all-female species of lizards cross their chromosomes to make babies
Since the 1960s, scientists have known that some species of whiptail lizards need a male even less than a fish needs a bicycle. These species of female lizards (from the Aspidoscelis genus) from Mexico and the southwestern United States manage to produce well-bred offspring without the aid of male fertilization.
But how do they – and the 70 other vertebrate species that spread this way – without the genetic monotony and susceptibility to disease that often results from asexual reproduction? “It remained unclear” and “was the subject of much speculation”, reports a team of researchers who aimed to answer precisely this question. Their findings were published online Feb. 21 in the journal Nature. (American scientist is part of Nature Publishing Group.)
These lizards and other “parthenogenetic species are genetically isolated,” explains Pierre Baumann, a research associate 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 this asexually when necessary, but some species, like these tiny lizards, have no choice. “They cannot exchange genetic material, and this loss of genetic exchange is a major disadvantage for them in a changing environment,” he says. Unless an animal can recombine DNA it already has, it will produce offspring with an identical set of chromosomes, in which any genetic weaknesses, such as susceptibility to disease or physical mutation, would not have. no chance of being replaced by outside genetic material from a comrade.
New research from Baumann and his team reveals that these lizards maintain their genetic richness by starting the reproductive process with twice as many chromosomes as their sexually reproducing cousins. These celibate species result from the hybridization of different sexual species, a process that instills in parthenogenetic lizards a great deal of genetic diversity to begin with. And the researchers found that these species could maintain diversity by never owning their homologous chromosomes (as sexual species do by taking one set of chromosomes from each parent) but rather combining their sister chromosomes instead. “Recombination between pairs of sister chromosomes maintains heterozygosity” across the entire chromosome, note the authors of the study, led by Aracely Lutes, postdoctoral researcher in Baumann’s laboratory.
This discovery, which had not previously been confirmed in the reptile world, means that “these lizards have a way of distinguishing sister chromosomes from homologs”, explains Baumann. How do they do? This is something the band is currently studying.
Another big unknown is precisely how lizards end up with double chromosomes in the first place. Baumann suspects this could happen over two rounds of replication or if two sex cells combine forces before the division process begins.
While asexual reproduction may sound boring — and one that can have questionable genetic results unless done right — it also has its benefits, Baumann notes. “You greatly increase the chances of populating a new habitat if it only takes one individual,” he says, citing the example of the blind Brahmin snake (Ramphotyphlops braminus), another parthenogenetic species. “If she has a way to reproduce without the help of a male, that’s an extreme advantage.” Indeed, it is – the brahminy has already colonized six continents.