Shuffling genes without sex

Researchers have discovered one way that asexually reproducing organisms maintain variation in their DNA. Female whiptail lizards can actually double their own chromosomes during meiosis, according to a study published online today in Nature. A checkered whiptail lizard Image: Peter Baumann "It's a great piece of work," said linkurl:Charles Cole,;http://www.amnh.org/science/divisions/vertzoo/bio.php?scientist=cole a herpetologist with the American Museum of Natural History in New York who was not involved in the study. "This study shows us that evolution came up with some really fancy gymnastics here that fifty years ago we never would have guessed these lizards could be doing." Parthenogenesis, a form of asexual reproduction that occurs without the contribution of male genetic material, has been well described in many invertebrates species and a few vertebrates. But since gametes in these species only receive one set of chromosomes, researchers have wondered how offspring maintain genetic diversity, especially in more complex vertebrate species. Whiptail lizards (genus Aspidosceli), have been long studied for their asexual capabilities - about one-third of 50 species reproduce exclusively by parthenogenesis. At first, scientists suspected the lizards might be biological flukes destined for extinction, but studies showed the contrary: the lizards maintained excellent genetic diversity, but no one knew how they did it. When linkurl:Peter Baumann,;http://www.stowers-institute.org/labs/baumannlab.asp a biochemist with Stowers Institute for Medical Research in Kansas City, Missouri, and senior author on the study, first heard about the lizards, he wondered if they somehow modified meiosis to produce a diploid egg. "I had been thinking about the problem for some time," Baumann said. "And it occurred to me that the imaging and microscopy technology has really advanced enough to allow us to address the questions in a very straightforward manner." Baumann and colleagues gathered lizards in the southwestern United States representing five different species of the genus Aspidoscelis - two sexually reproducing species and three parthenogenic ones. They isolated the oocytes and then measured the chromosome volume in the cells by employing laser scanning and transmission electron microscopy as well as fluorescent in situ hybridization (FISH). "We looked just before meiotic division when the chromosomes are highly condensed," said Baumann. "And this really conclusively showed us that they went into meiosis with twice the number of chromosomes." Further imaging showed these sister chromosomes recombining - explaining how variation in their offsprings' DNA persisted. Cole said there had been a fair amount of discussion about this possibility over the years, but there were no real solid observations or data to support it. "By providing the evidence, they really nailed it in this article," he said. "This is really an example of 'if it can happen in nature then it will happen,'" said linkurl:Joseph Gall,;http://www.bio.jhu.edu/Faculty/Gall/Default.html a cell biologist at the Carnegie Institution of Science in Baltimore, Maryland, who was not involved in the study. While he noted that this study nicely explains one example of how parthenogenic lizards can successfully thrive, he said, it may not be a general mechanism for parthenogenesis in other species. Other species might have other solutions, he noted,since "the diversity of phylogenetic mechanisms out there is extreme." Baumann is currently exploring whether other lizard species can double their chromosomes during meiosis and he is also examining how exactly these lizards produce and rearrange their extra chromosomes. "Now that they have the techniques to do this, they can now go ahead and figure out how the lizards reached this point - what steps were involved to go from traditional methods of reproduction to parthenogenesis," Cole said. "And I think they will find some very interesting steps there." **__Related stories:__***linkurl:Egg size matters for lizard sex;http://www.the-scientist.com/blog/display/55749/ [4th June 2009]*linkurl:Genetic benefits of asexuality?;http://www.the-scientist.com/news/display/53697/ [11th October 2007]*linkurl:1st mouse by parthenogenesis?;http://www.the-scientist.com/article/display/22124/ [4th April 2004]

Shuffling genes without sex

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