Life without sex: so, how many millions of years has it been?
Yes, there really is a scientific version of the barroom lament, and it's a serious inquiry. Biologists have long held that asexuality is an evolutionary dead end because sex purges the genes of detrimental mutations, provides the genetic variation for coping with environmental change or both. But new methods of genetic analysis are suggesting that certain groups of species have avoided sex and still have done quite well over the eons, thank you very much.
"The question of, 'Why sex?' is a very central one to biology," says David Mark Welch of the Marine Biological Laboratory in Woods Hole, Mass.
Three years ago, Mark Welch and Matthew Meselson of Harvard University reported genetic evidence that an entire class of organisms, containing 360 species, seems to have evolved perfectly well without sex. This group of tiny water creatures, called bdelloid rotifers, is thriving in fresh water and soggy land worldwide despite, seemingly, no sex for at least 40 million years.
Since that publication, geneticists and paleontologists have been focusing their most advanced methods on questions of asexuality. The scientists are refining tests that detect sexuality and searching for other celibate lineages.
SEX SIGNS So far, biologists have found some 2,000 living species in which they haven't seen a trace of sexual behavior. Yet keen-eyed study has, on occasion, dashed many previous claims of asexuality. A living insect species once thought not to have male forms, for example, turned out to include males that were perfectly functional but very, very tiny.
Biologists have mused that there might be something funny about the sexual history of bdelloid rotifers, one of the classes of a phylum of little stalk-like water animals crowned with a characteristic circle of hairlike cilia. The father of microscopy, Antonie van Leeuwenhoek, wrote about them some 300 years ago. They may be only 0.1 to 1 millimeter long, but they have differentiated parts, such as nerve ganglia, muscles, light-sensitive structures, digestive organs, and ovaries.
Those ovaries make eggs, but by an unusual means. In sexual organisms, gonads split the genome in half when making eggs and sperm. The ovaries in bdelloid rotifers, however, create eggs containing the full genome. These eggs require no sperm to develop into adults that are essentially genetic copies of their mothers.
The fossil record so far hasn't said much about the history of bdelloid rotifers--only that some specimens found in amber dating from 35 to 40 million years ago didn't include any obvious males.
To explore bdelloid history further, Meselson and Mark Welch looked at modern rotifers. No males have turned up in the whole bdelloid group, suggesting a long evolutionary history of asexuality. For a genetic test, the researchers proposed that one distinctive sign of millennia without sex-reproduction might be a pattern of abundant variability between copies of genes.
The researchers focused on versions of the same gene in different species. They reported in 2000 that the two copies of the gene in the asexual species differ from each far more than do copies of genes in rotifers that evolved with sexual reproduction (SN: 5/20/00, p. 326). The simplest explanation of the patterns, the researchers contended, was that in the asexuals, there had been no reshuffling of the genome, as occurs during sex, so an individual rotifer's two copies of a gene had each had plenty of time to independently build up quirky mutations.
TESTING, TESTING After the unveiling of the bdelloid oddity, researchers threw themselves into testing more predictions about the genetics of asexuality.
Meselson and Irina Arkhipova, a geneticist in his lab, looked at the bdelloid pattern of transposable elements. These snippets of genetic material shoehorn themselves into a variety of spots in a host's DNA. Such pushiness can secure an element s place in subsequent generations, but it can also mess up the gene in which it lands. Arkhipova refers to transposable elements as sexually transmitted parasites.
Two decades ago, a theorist predicted that a species that changed from sexual to asexual reproduction would lose transposable elements because clones without disadvantageous insertions would persist longer than clones bearing such elements.
To explore this prediction, Arkhipova and Meselson surveyed genetic sequences from representatives of 24 phyla of animals, including the lab fruit fly Drosophila melanogaster, the tiny lab worm Caenorhabditis elegans, and the waterborne pathogen Giardia lamblia. Traces of two large families of transposable elements that copy themselves and proliferate when a species reproduces sexually showed up in most of the animals but not in the bdelloid rotifers, the researchers reported in 2000.
Signs of a different kind of transposable element, called mariner-like elements, did show up in the bdelloids as well as many other organisms. There's evidence that these elements can somehow proliferate without sex. Also, Arkhipova says, they're not as damaging as the other two element families are to the neighbors of the gene where they intrude. The pattern observed among these three families of transposable elements strengthens the case that bdelloids evolved asexually, says Arkhipova.
Mark Welch and Meselson tested another genetic prediction about asexuality. Some scientists had speculated that asexual species would accumulate more mutations than sexual species do because the genetic reshuffling of sexual reproduction allows some offspring to escape genetic typos.
Mark Welch and Meselson addressed this notion by comparing sequences of a gene in bdelloids with its counterpart in sexual rotifers. Contrary to the theory, though, the sexual rotifers showed about the same abundance of glitches in the gene, the researchers reported in the May 29, 2001 Proceedings of the National Academy of Sciences.
WHO'S NEXT? Now that biologists regard asexuality as being as interesting as sexuality, researchers are looking for examples among creatures as varied as clams, mites and fungi.
One tantalizing case comes from the work of Ian Sanders of the University of Lausanne in Switzerland and his colleagues. They study arbuscular mycorrhizae, a group of Glomales fungi that buddy up with plant roots for mutual nutritional benefit. These fungi have been proposed as ancient asexuals, but figuring out how to make sense of the fungi's genomes is tricky. For example, an individual fungus can receive hundreds of nuclei from its parents.
A more emphatic claim for ancient asexuality comes from researchers studying the widespread fossils of millimeter-long crustaceans called darwinulid ostracods. Most biologists who study the extant populations agree that they're asexual. As paleontologist David Home of the Natural History Museum in London recalls events, "Other people were making all those flamboyant claims about bdelloid rotifers, and we said, 'Hang on a minute.'"
The paired outer shells of fossilized darwinulid ostracods turn up by the thousands in ancient lake sediments around the world, so these species offer the unusual advantage of a potential asexual with an abundant fossil record.
Extant ostracods range widely throughout salt and fresh water and include lineages that certainly do have sex. "They are renowned for their rather large [male] copulatory organ," says Horne. The complex paired appendages can take up as much as a third of the volume of the whole animal. Sperm in one freshwater species can be up to 15 millimeters long, losing out to only water bugs for the title of world's longest sperm.
Some female ostracods, including darwinulids, care for their brood, carrying the youngsters in a bulge at one end of the adult's protective, paired shells. Thus, the relatively large female shells show a bump at one end and other asymmetries.
Among the darwinulid ostracods, all available modern specimens show these motherly characteristics. There was a 19th-century report of a male, lacking the bulge, in a common extant species of darwinulid. But Horne has been unable to locate that specimen, and no researcher has reported a male since then.
Fossil darwinulid ostracods have been more controversial because it's difficult to agree on what would represent subtle differences between the sexes. Yet, a systematic review of fossils shows no evidence of males for at least 200 million years, Horne and his colleagues argue in the April 7 Proceedings of the Royal Society of London B.
Ostracods have very different genetic characteristics from those of bdelloids, says Isabelle Schon of the Royal Belgian Institute of Natural Sciences in Brussels. She and her Brussels colleague Koen Martens started analyzing darwinulid ostracod genes as part of a European multicenter research project during the mid-1990s. Schon and Martens wondered whether they'd find the extra variation between copies of an individual's genes that's been observed in bdelloids, what's now sometimes called the Meselson effect.
"We saw the opposite," Schon says. The genes showed remarkably little variation. "It was a very puzzling surprise," she recalls. In the April 22 Proceedings of the Royal Society of London B, she and Martens report on three genes that have far less variety compared with counterparts in a sexual ostracod species.
This finding hasn't made ostracod scientists doubt that the darwinulids evolved asexually, but it has inspired speculation on why these organisms seem to have been spared the need for sex. Last year, the Schon-Martens group argued that a representative species, Darwinula stevensoni, has "a general-purpose genotype." It can grow in distilled water, seawater, and every salinity tested in between, the researchers say in the July 2, 2002 Oecologia. It copes with temperatures from 10[degrees]C to 30[degrees]C. With a genome flexible enough to do all that, maybe actual variation in the genes isn't as important as it is in many other organisms.
The ancient asexuals then, if they really are asexuals, are giving mixed results for the theories explaining sex. Bdelloids seem to be getting rid of menacing transposable elements, but so far as biologists know, don't seem bothered by an abundance of other mutations. This undermines the view that mutation buildup is a major problem solved by sex.
The darwinulid ostracods show a different genetic pattern, with extreme uniformity instead of bdelloids' rampant variation. These ostracods may get their environmental flexibility from a route other than variation in genes fostered by sex.
Clearly, science won't resolve the barstool speculation any time soon.
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|Title Annotation:||evolution of organisms despite no sexual activity|
|Date:||Jun 28, 2003|
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