Tinkering with worm sex to shed light on evolution

This post contributed by Nadine Lymn, ESA Director of Public Affairs The roundworm Caenorhabditis elegans (C. elegans) is a tiny laboratory animal that researchers have worked with for decades.  As a hermaphrodite, C. elegans makes both sperm and eggs and can reproduce by self-fertilization.  In contrast to humans, where hermaphrodites are rare, for C. elegans, this is its normal state.   However, male individuals, with only male gonads, can also occur and these individuals must mate with a hermaphrodite in order to reproduce, as shown in the video below. A central question in evolution is how variations in the genes responsible for determining gender can exist since there seems to be so little room for error; if the mutation goes awry and negatively affects reproductive ability, a species could be in serious trouble. A new paper published in the journal Evolution took a closer look.   Michigan State University researchers Christopher Chandler and Ian Dworkin and colleagues at Iowa State University used worms that had already been mutated in previous experiments.  One mutation determined that at a specific temperature, the larvae will become a hermaphrodite.  At a higher specific temperature, the larvae—while still genetically a hermaphrodite—becomes a male, and at temperatures in between, intersex individuals arise, sporting both male and hermaphrodite characteristics.   These intersex individuals are different from the normal, hermaphrodite C. elegans, in that they are truly mixed up—they have some characteristics of both the hermaphrodite and male versions of C. elegans.   Chandler, Dworkin, and colleagues exposed the worms to the intermediate temperatures, creating populations of intersex individuals. As described in a press release about the research, these characteristics made reproduction difficult, though still possible. Chandler and colleagues allowed these populations to reproduce for 50 generations, creating a strong selection for individuals still able to function sexually.  Then the researchers measured the later generations’ sex ratio and fertility.  They found that these later populations had more typical C. elegans sex ratios and higher fertility, despite the fact that they were still subjected to the intermediate temperatures that had rendered their predecessors intersex animals.  As explained in the press release, other genes were evolving to compensate for changes in the sex determination genes, in a way that allowed individual worms to develop either as a male or a hermaphrodite, instead of as an intersex animal. In other words, C. elegans was able to make up for the negative mutations brought out by the artificial conditions created by the researchers.  In their paper’s conclusion, the authors note that their experiment, funded by the National Science Foundation, demonstrates that “organisms can accommodate deleterious developmental mutations on relatively short time scales” and that the...

Read More

Chickenpox sweeties and the social ecology of infectious disease

This post contributed by Liza Lester, ESA communications officer   No one speaks for the endangered poliomyelitis. No one raises money to protect the last survivors, as health workers stalk the virus through its last redoubts in India, Pakistan, Nigeria and Afghanistan. On the contrary, the WHO spends billions on hunting it to extinction. But the virus has held out longer than expected. Joshua Michaud, policy analyst at the Kaiser Foundation, thinks the polio fighters are falling behind. Guinea worm will be the next scourge to fall, he said on an AAAS panel engaged to discuss Infectious Disease: Challenges to Eradication on Monday. Why have efforts with guinea worm been so successful? a precocious Georgetown student wanted to know. Biology was on our side. There is no vaccine for guinea worm, and no medicine to cure infection. To extract the worm, you must wind it slowly around a stick as it emerges through a sore in your leg (an oft-repeated story holds that the treatment has not changed since the Egyptians of the XVIII dynasty described it in 1550 BCE, though the source appears to have been exaggerated). The process is excruciating, and it takes weeks. But we know key details of the worm’s biology that the ancient Egyptians did not. Basic technology and careful hygiene can defeat the worm. Larvae harbor in the bodies of invisible copepods, “water flies” tiny enough to swallow. Once swallowed, female larvae nestle against the long limb bones of their hosts, growing up to a meter in length over the course of a year. They surface inside a burning ulceration that sends their victims running for a dip in a cool pond—and the next generation of larvae escape to start the cycle of life anew. The good news, said Michaud, is that guinea worm does not have another host. It has no environmental bolt hole to hide in while under siege, only to emerge when health forces are not looking. It needs humans. And affected people are visibly affected. Break the cycle for one year, and you can free a communal water source, and its community, from the worm. Copapods may be microscopic, but a simple nylon strainer on the end of a drinking tube saves you from swallowing them (although not bacterial and viral parasites that might also lurk there, interjected Dennis Carroll, in charge of avian flu and other emerging threats at USAID). Help the infected, persuade them to stay out of drinking water sources when their worm breaches, and you break the cycle. Success requires the help and good will of village elders. The Carter Foundation has been courting good...

Read More

Worm brain sheds light on the evolution of the cerebral cortex

The last time humans and the marine ragworm Platynereis dumerilii shared a common ancestor was roughly 600 million years ago, according to scientists from the European Molecular Biology Laboratory in Heidelberg, Germany (EMBL). That is, researcher have discovered a true counterpart of the cerebral cortex, also called the pallium, in this relative of the earthworm. This finding, explained the scientists, could be the key to unraveling the evolution of this important area of the human (and all vertebrates) brain.

Read More