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April 20, 2010

Algae and Developing Genders

An interesting question in evolution has always been how the two genders evolved. Most unicellular animals don't have district genders; they can reproduce both sexually and asexually. Two little single celled algae look pretty much the same, right down to the gametes that combine to create a new algal cell. Most multicellular animals, however, can be classified pretty easily as either male or female. The majority of these are only able to reproduce sexually (there are some species of reptile that get around this, but that's a discussion for another post). Plants, too, have a distinct male and female. Considering sexual dimorphism -the development of a distinct male and female in a species- is a trait uncommon in single cell organisms and common in multicellular ones, there must have been some evolutionary change which made different genders far more beneficial. Unfortunately, neither single cell organisms nor genetic material preserve in the fossil record, so we cannot look there for answers.

Volvox carteri
A comparison of two closely related algal species has given researchers some interesting clues on this evolutionary gap. Volvox carteri is a multicellular green algae, and its cousin, Chlamydomonas reinhardtii, is single celled. Their genomes are pretty much identical, with one giant exception. In the area of the genetic code that serves as the algal version of X and Y chromosomes, Volvox had some of the same genes and Chlamydomonas, along with a wide variety more. In fact, this section is five times larger in Volvox. This was a huge surprise. In the past, researchers had though that the "sex chromosomes," X and Y, had developed as a section of the genome deteriorated. These algae suggest that the opposite is true: that, for whatever reason, the bits of DNA involved with gender and reproduction can change and diversify very quickly.

There is a lot more study that has to be done to map out any sort of evolutionary path for this change. The team is looking more closely at the "new" genes in Volvox, as well as comparing both species to a third, Gonium, which is an intermediate between Volvox and Chlamydomonas. It's one of the first breakthroughs in determining how sexual dimorphism originally around. It will take time, but it's a very interesting first step to solving this evolutionary mystery.

Source: Science Daily- Lessons from the Pond: Clues from green algae on the origin of males and females