Back in December, I wrote about two studies that compared global patterns of gene expression between germ cells and somatic cells in Volvox carteri, one by Benjamin Klein, Daniel Wibberg and Armin Hallmann from the University of Bielefeld in Germany and one by Gavriel Matt and Jim Umen from Washington University in St. Louis and the Donald Danforth Plant Science Center, respectively. The Matt & Umen paper has also been highlighted on the Genetics Society of America blog, Genes to Genomes.
The post, by Nicole Haloupek, summarizes the main findings of the paper and places them in the larger context of the evolution of cellular differentiation:
Among evolution’s greatest innovations are germ cells. These specialized reproductive cells—familiar to us as sperm and eggs in humans—set the stage for complex multicellular life because they free up all the other cells in the body (known as somatic cells) to specialize for many other functions. Because they appeared so long ago in our evolutionary history, the way our germ cells emerged has been obscured, leaving many questions about this momentous biological turning point.
In some ways, gametes (sperm and eggs) are a good analogy for the germ-some differentiation we see in Volvox. In both cases, there is just one type of cell (in a given body) that is responsible for reproduction. Many theorists, my Ph.D. advisor among them, think that this is a fundamentally important development. The division of labor between reproductive functions and somatic functions means that organisms are indivisible in an important way: the somatic cells aren’t capable of producing new organisms, and the reproductive cells can’t survive without the somatic cells.
In another way, though, sperm and eggs are importantly different from the germ cells in Matt & Umen’s comparison. Sperm and eggs are, of course, involved in sexual reproduction, but the germ cells we are talking about in Volvox are for asexual reproduction. Volvox actually has sperm and eggs as well, during the sexual phase of the life cycle, and they evolved them independently from those of animals.
The most recent common ancestor of Volvox and humans probably had a life cycle that alternated sexual and asexual reproduction, just as Volvox does. The isogamous (equal-sized) gametes that were around back then evolved into sperm and eggs in some ancient ancestor of animals, and again in the volvocine algae (more than once, actually). Volvox gonidia are something else, something without a perfect analogue in humans, since we don’t reproduce asexually.
None of this is to say that understanding the evolution of germ-soma differentiation in Volvox can’t help us to understand germ-soma differentiation in animals (including humans). Sperm and eggs are the closest analogues in humans to Volvox gonidia, even if gonidia aren’t the closest analogue in Volvox to human sperm and eggs. The principle of dividing reproductive and somatic functions is the same in both cases, and understanding how and why that situation came to be is crucial to understanding the evolution of complex multicellularity.