Wild-type and genderswapped Volvox. Color differential
interference contrast images of V. carteri wild-type male (upper left),
wild-type female (upper right), pseudo-female (lower left) and
pseudo-male (lower right). Credit: Sa Geng and James …more
Throughout evolution, living things have repeatedly developed
physically distinct sexes, but how does this actually happen? A
discovery in the multicellular green alga, Volvox carteri, has revealed
the genetic origin of male and female sexes, showing how they evolved
from a more primitive mating system in a single-celled relative.
A team of scientists led by James Umen, Ph.D., Associate
Member, Enterprise Institute for Renewable Fuels at the Danforth Plant
Science Center, identified the master regulatory gene for sex
determination in Volvox and found that it has acquired new functions
compared to a related gene in its close relative, the unicellular alga
Chlamydomonas reinhardtii, which does not have physically
distinguishable (dimorphic) sexes. Their findings are publishing in the open access journal PLOS Biology on July 8, and may also provide a possible blueprint for how sexes in other multicellular organisms like plants and animals may have originated.
For plants and animals having male and female reproductive cells or
gametes is the norm, and the differences between the two types of
gametes are obvious. Male gametes are small motile sperm or pollen,
while female gametes are large egg cells. However, the evolutionary
origins of male and female sexes are unclear because the distant
unicellular relatives of plants, animals and other multicellular species
generally don't have distinct sexes, but instead have mating types - a
system in which gametes of one mating type can only fuse with those with
a different mating type, but the cells of each mating type are
indistinguishable from each other in size and morphology.
Unlike the case in plants and animals whose unicellular ancestors are
very distantly related, male and female sexes in Volvox evolved
relatively recently from mating types in an ancestor that was similar to
Chlamydomonas. During a previous study, Umen and co-workers –
postdoctoral fellows Sa Geng and Peter DeHoff – had identified a gene in
Volvox males called MID whose counterpart in Chlamydomonas was known to
control differentiation of its two mating types called "plus" and
"minus".
By forcing genetically female Volvox to express MID, the team led by
Umen was able to convert what would have been egg cells into packets of
functional sperm cells. Conversely, by using a method of gene
inactivation called RNA interference (RNAi), the Danforth scientists
were able to block MID expression in genetic males causing them to
develop with functional eggs in place of their sperm packets. The team
was even able to use their gender-swapped strains to carry out
successful matings between pairs of genetically male or genetically
female Volvox. Importantly, even though the MID genes from the two
species of algae are related, the Chlamydomonas MID gene was unable to
substitute for Volvox MID. The discovery of a master regulatory gene for
sexes and mating types in this group of green algae shows that these
two forms of reproduction share a common genetic origin, and hint that a
similar evolutionary scenario may underlie the origin of sexes in
animals, plants and other multicellular lineages.
In addition to the evolutionary insights gained by Umen's research
team, there are also practical implications for algal biotechnology.
"Just as is the case for crop plants, breeding will be an important tool
for making improved algal strains that can serve as biofuel feed stocks
or other purposes. However, sexual reproduction in most algal species
is poorly understood. The identification of a conserved regulatory gene
that controls sex and mating in the algae may lead to clues about how
sex is controlled in other related groups of algae that are used for
biotechnological applications," Umen said.
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