As if being able to re-grow a tail isn't cool enough, some species of whiptail lizards (genus Cnemidophorus) have another trick: They can clone themselves. These species actually consist completely of females able to reproduce by parthenogenesis.
The original sexless females, known as parthenogens, come from the hybridization of two separate lizard lines. The parthenogen has one copy of chromosomes from its mother, and one analogous but slightly different copy from its father. It can give rise to offspring that are their exact clones, without their two genetic copies recombining.
Asexual whiptails have a special trick for making spermless reproduction work: The egg cells in other animals first double their choromosomes once and then divide twice, leaving them as haploid cells, with half the normal number of genetic material. But the whiptails' egg cells first double their chromosomes twice and then divide twice, leaving them with the normal number of chromosomes and rendering a sperm cell unnecessary.
Pairs of female whiptails sometimes engage in mock sex, which led to the nickname "lesbian lizards" and seems to encourage the production of egg cells. But they do seem to have some interest in the opposite gender: Sometimes these parthenogens are mate with males of different species, creating a species with 50 percent more genes than normal.
Bdelloid rotifers have survived 80 million years with no sex at all; no male of the species has ever been observed. And while they don't seem to be sharing genes with each other, they are definitely getting some on the side--from other organisms.
The rotifers act like genetic vacuums, scooping up genes around with abandon and putting them to use in their own bodies. The ones that help the rotifer survive are kept around in the population--some perform useful functions, like breaking certain chemical bonds--while those that don't help die off. To keep them around, the rotifers stitch these genes onto the ends of their genomes and pass them along to their genetically identical offspring. The variety with the best combination of genes wins the evolutionary race.
Some rotifers seemed to have developed an alternate strategy for evading parasites: Instead of evolving alongside them, forever developing new countermeasures, they beat them in a test of endurance. Rotifers are able to survive extremely hostile environments by slipping into a dehydrated dormant state. When the environment becomes more favorable, they rehydrate and spring alive again (sometimes months or years later), freed from infectious agents that don't have the rotifer's toughness.
Parthenogenesis has been observed in many types of sharks and snakes, though often the offspring have very shortened life spans. After an observation of pathogenesis in a hammerhead shark in captivity (which sadly didn't survive), researchers at the Belle Isle Aquarium of the Detroit Zoological Institute decided to try to hatch the seemingly unfertilized eggs their white spotted bamboo sharks would leave around the tank.
They were able to rear two pups successfully, and they were confirmed clones of their mother. Some researchers hypothesize that the ability of sharks to reproduce via parthenogenesis is what allowed them to become one of the oldest species on the planet: When males were scarce, females could just make progressively younger copies of themselves to wait for Mr. Right Shark to come around. (Which might even be a good idea for our species.)
They made these clones by a process called automatic parthenogenesis: The egg is formed normally (with half the species' usual number of chromosomes), then fertilized by the "polar body," a cell that is created during oogenesis and contains the same gene copies as the egg, resulting in the shark having half the genetic variation of its mother. This is why often the pups don't survivethey can be missing critical genes that the mother only had one copy of.
Female aphids are able to clone themselves without sex--producing up to 100 copies of herself that carry all of her DNA with them, instead of being muddled by some other pesky male's genes. Her offspring live up to 40 days and produce their own set of clones, so one single aphid can end up producing many billion clones by the end of the summer.
In colder climates, these aphids start producing male clones (which are identical to the mother except they lack a sex chromosome). The aphids do, however, take a lover every year or so to cleanse their bacterial palette by picking up a male's cohort of bacteria in the form of a sexually transmitted infection.
These bacteria are actually helpful for the aphid: They can protect them against parasites or enable them to eat different kinds of plants and withstand higher temperatures. The mothers can then pass these useful sexually transmitted diseases down to their clonal offspring.
Some animals are so good at regeneration that detached piece of one individual can grow into an entirely new organism all on its own. Some species of sea star (known commonly but erroneously as "starfish") use asexual reproduction via fragmentation as their main reproductive mode.
The nematode S. mediterranea can even re-grow its head and brain when decapitated. And the decapitated head can re-grow a completely new body. A slice down to about one cubic millimeter can create a whole new (while tiny) individual with the exact same genome, which can grow from a tiny fragment into a normal sized nematode.
The sea star's ability to regenerate is a little less impressive. It can only regenerate from an arm if the arm takes part of the animal's "central disk" with it. In some species the arm actually pulls away from the body and detaches itself, then heads off to form a new, genetically identical, organism.
Researchers are also studying how mammals would go about reproducing by parthenogenesis. So far, researchers have birthed mice spawned from two mothers (in 2004) or two fathers (2010) by uniting the genetic material of egg with egg or sperm with sperm. The tricky part was convincing the egg that when it fuses with another egg, this complimentary set of DNA (which would normally be provided by the male's sperm) actually does came from a male instead of a second egg, by "imprinting" it with modifications normally found in sperm DNA. This isn't really asexual reproduction, as the offspring has DNA from two parents, but it is interesting and strange nonetheless.
While getting mammals to undergo true asexual reproduction has thus far been beyond the power of science, researchers are trying to figure out why the union of sperm and egg is so important in mammalian reproduction, when all other domains of life have the ability to reproduce without sex. By using the techniques they develop, they might be able to produce embryonic stem cells from eggs alone, without the need for fertilization and all the moral quandaries that come with it.
In the plant kingdom, parthenogenesis and sex are far from mutually exclusive. Clones may be the way to go when nutrients and water are plentiful, but when the going gets tough, plants often switch to sex, which lets them try out different, possibly advantageous arrangements of genes.
Aspens are a good example of how sex can help an organism that usually clones itself to survive. Going by the name Pando (Latin for "I spread"), one particular colony of male aspen trees in Utah is not only the largest organism on earth (weighing in at over 6,000 tons), it is also the oldest, estimated to be 80,000 years old. The organism is actually an entire forest of an estimated 47,000 trees that are all genetically identical, and that are all feeding off a shared root system that covers over 10 acres. The whole thing grew from clones arising from a single tree.
Aspens do still have sexes, though. And recently researchers discovered that aspens can't actually clonally reproduce forever--at some point, to avoid petering out, they do need to have sex. It seems as they age, they become infertile, and stop being able to reproduce sexually, because genetic mutations build up over time with each new clone produced.
Researchers were able to date the age of each new clone back to the parent tree by comparing the everyday mutations that happen with known regularity during the process of cell division. By counting the number of these mutations they could tell which of the clones were newer and which were older, building a family tree of cloned trees. They found that the trees produced less pollen as they aged--and could be completely infertile by the tender young age of 20,000 years.
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