Sexually reproducing organisms range from simple single-celled systems to complex multicellular ones. They are isogamous, meaning instead of having male and female sexes, they belong to one of two “types”: plus or minus. Both types are identical to each other in form, but fusion can only take place between cells of opposite types.
As organisms get more complex, they evolved sexually dimorphic or anisogamous systems, meaning they produce distinct looking sex cells — small motile sperm and large stationary eggs.
The genetic basis of this transition from mating types to sexes is still a mystery and a study published in PLoS Biology on Tuesday may have overturned some pre-existing theories.
A team led by James Umen, from Danforth Plant Science Center, U.S., used two species of sexually-reproducing volvocine algae. The first, unicellular Chlamydomonas reinhardtii , was already known to have a gene called CrMID which determines if a cell was of plus or minus type.
The second was the multicellular Volvox carteri , which being evolutionarily ahead by about 200 million, has distinct sperm and egg cells. V. carteri has a gene similar to CrMID gene called in VcMID in its male cells. It seemed reasonable to test if VcMID plays a similar role in sex determination as its counterpart.
The team found that the addition of the VcMID gene to female V. carteri cells was enough to make them produce sperm and fertilise other females. Similarly, removal of VcMID from male cells caused them to behave like and be fertilised by sperm like normal egg cells, though not as efficiently.
Further tinkering established that VcMID does indeed work as a master sex determining gene in V.carteri , too. Attempts to substitute VcMID with the more archaic CrMID failed, proving that significant changes have occurred to the gene over the years.
The predominant theory until now was that other genes — like a gamete-size determining one — pitched in with the sex determining gene (in this case, MID) to allow the generation of distinct sex cells. But Umen’s work shows that changes within MID alone were largely responsible for the transition from mating types to sexes.
Studying the evolution of algal reproduction may be useful in coming up with improved varieties of algae, which are today important for biofuel production.
“Finding a common master regulator in volvocine algae indicates that such master regulators may exist in other groups of algae. It is even possible that sex determining genes in biotechnological important algae may be relatives of MID,” wrote Umen.
