Everything you’ve always wanted to know about sex – in yeast
Like many fungi and one-celled organisms, Candida albicans –a normally harmless microbe that can turn deadly – has long been thought to reproduce asexually. But a new study by Prof. Judith Berman of the University of Minnesota and Tel Aviv University and colleagues shows that the well-known bacterium, that causes yeast infections, is capable of sexual reproduction after all.
The finding, published recently in the journal Nature , represents an important breakthrough in understanding how this pathogen has been shaped by evolution and could suggest strategies for preventing and treating the often-serious infections it can cause.
The most common fungus that infects humans, C. albicans is part of the large community of microorganisms that live, for the most part harmlessly, within the human gut. But unlike many of its neighbors, this one-celled yeast can also cause disease, ranging from thrush and vaginal infections to systemic blood infections that cause organ failure and death and usually occur in people with immune defects related to HIV/AIDS, organ transplantation or chemotherapy. The bacteria are responsible annually for 400,000 deaths in the US alone.
Most single- celled organisms reproduce by dividing, but others reproduce asexually, parasexually or via sexual mating. Scientists have long believed that C. Albicans reproduce merely by splitting from one cell into two, without mating. Organisms that produce asexually or parasexually are diploid, which means they have two sets of chromosomes and thus can reproduce without a mate. Organisms that reproduce sexually are haploid, which means they have one set of chromosomes and need a mate to provide a second set. C. Albicans was believed to be diploid, but the new study shows that the yeast is sometimes haploid, and that these haploids are capable of sexual reproduction.
Sexual reproduction fuels the evolution of higher organisms because it combines DNA from two parents to create one organism. The haploid isolates discovered in Berman’s lab arise only rarely within a population and have been detected following propagation in the lab or in a mammalian host. These haploids can mate with other haploids to generate diploid strains with new combinations of DNA, which may provide the diversity required for fungus to evolve.
The haploids also pave the way for genetic studies of the pathogen, such as the construction of “libraries” of recessive mutant strains. In addition, the ability to perform genetic crosses between haploids will help produce modified diploid strains that should help scientists better understand interactions between the fungus and its host and how it transforms from a harmless microbe into a deadly pathogen.
THE IMPORTANCE OF BUG GROOMING
Like a self-absorbed teenager, bugs spend a lot of time grooming. But for insects, there are more benefits than just “looking good.”
North Carolina State University researchers found that that insect grooming – specifically, cleaning of the antennae – removes both environmental pollutants and chemicals produced by the insects themselves. The study, by Dr. Coby Schal, published in the Proceedings of the [US] National Academy of Sciences, shows that grooming helps insects maintain acute olfactory senses that are responsible for a host of functions, including finding food, sensing danger and even locating a suitable mate. The findings could also explain why certain types of insecticides work more effectively than others.
Since insects groom themselves incessantly, the US entomologists wanted to explore the functions of this behavior. They devised a simple set of experiments to figure out what sort of material insects were cleaning off their antennae, where this material was coming from, and the differences between how groomed and ungroomed antennae functioned.
The researchers compared cleaned antennae of American cockroaches with antennae that were experimentally prevented from being cleaned. They found that grooming cleaned microscopic pores on the antennae that serve as conduits through which chemicals travel to reach sensory receptors for olfaction. Cockroaches clean their antennae by using their forelegs to place the antennae in their mouths; they then methodically clean every segment of the antenna from base to tip.
The researchers found that both volatile and non-volatile chemicals accumulated on the ungroomed antennae of cockroaches, but most surprising was the accumulation of a great deal of cuticular hydrocarbons – fatty, candle-wax-like substances secreted by the roaches to protect them against water loss. The researchers compared their behavior with that of carpenter ants, houseflies and German cockroaches.
Although they groom a bit differently than cockroaches – flies and ants seem to rub their legs over their antennae to remove particulates, with ants then ingesting the material off their legs – the tests showed that these insects also accumulated more cuticular hydrocarbons when antennae went ungroomed. They concluded that grooming is necessary to keep the foreign and native substances at a particular level.
“Leaving antennae dirty essentially blinds insects to their environment,” said Schal.
There could be pest-control implications to the findings, as an insecticide mist or dust that settles on a cockroach’s antennae, for instance, should be ingested by the roach rather quickly due to constant grooming. That method of insecticide delivery could be more effective than relying on residual insecticides to penetrate the thick cuticle, for instance.
Finally, Schal says the study can also be used as a caution to other researchers who use insects in experiments. Gluing shut an insect’s mouth to prevent it from feeding, for example, could also prevent the insect from grooming its antennae. Experimental results could be skewed as a result of this sensory deprivation, Schal suggested.