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, candlewax-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.
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