One of the most studied aspects of Volvox development is the differentiation of its 2000 or so cells into two types: a few (usually 12-16) large reproductive cells (germ) and the rest small, biflagellate cells that provide motility (soma). The main genes controlling this differentiation have long been known, but the details of how they work are still being worked out.
In a session chaired by Erik Hanschen, we heard three talks providing further details on the evolutionary history and mechanisms of germ-soma differentiation. Zach Grochau-Wright, who is working on his Ph.D. at the University of Arizona, explored the evolutionary history of a cluster of genes that includes regA, a transcription factor crucial for the determination of somatic cells in Volvox carteri (Zach’s related talk at AbSciCon is available here). Although the role of regA in V. carteri development has long been appreciated, it is still unknown whether it plays a similar role in other volvocine species with cellular differentiation. Zach is working toward answering this question by looking for regA and related genes in other volvocine species with and without cellular differentiation. Whether or not regA controls somatic differentiation in other species is a crucial question. Somatic cells are thought to have evolved multiple times independently in the volvocine algae, and a finding that they are determined by the same genes across these lineages would mean that either the inferred phylogenetic relationships are wrong (and soma only evolved once) or that the repeated origins of somatic cells were not only morphologically convergent but genetically parallel as well. The latter possibility would be particularly surprising, because whatever the developmental basis of somatic determination is in, for example, the members of section Volvox (a.k.a. Euvolvox), Pleodorina, and Astrephomene, it is certainly not the same as in V. carteri.*
The other two talks in the differentiation session addressed gene expression differences between germ and somatic cells in V. carteri. It has been known for some time that regA prevents expression of genes required for germ cell development, but exactly which genes are differentially expressed in germ versus somatic cells has not been fully worked out. Arash Kianionmomeni, from the University of Bielefeld, found that reproductive and somatic cells differ in their response to light (see “Expression and form: Arash Kianianmomeni on gene regulation” for more on Dr. Kianionmomeni’s work). Specifically, the expression of photoreceptor genes differs between the two cell types, including some that are expressed only in one cell type or the other. Light responses serve several critical functions in volvocine life cycles, including phototaxis, control of photosynthesis, and circadian control of the cell cycle. Since these functions are segregated between the two cell types in V. carteri, we should perhaps not be terribly surprised that light has different effects on their gene expression. Understanding these differences is an important first step in determining the mechanistic basis for at least some of the differences between cell types.
Gavriel Matt, from Washington University in St. Louis and the Donald Danforth Plant Science Center, presented a genome-wide analysis comparing gene expression between reproductive and somatic cells. This is not the first time cell-type specific gene expression differences have been identified in V. carteri, but the previous study (Tam & Kirk 1991) was carried out long before modern, massively parallel sequencing technologies became available. Using RNA-Seq, Gavriel and Jim Umen have increased the number of genes whose expression is known to differ between cell types by ~200 fold. I won’t steal their thunder here, as they’ll hopefully be publishing these results soon, but they were able to draw some intriguing conclusions by comparing the functional categories of genes expressed (or overexpressed) in the germ cells versus those expressed in the soma. Like any good study, their analysis opens up new avenues of research, and it will likely take years to work out what even a portion of these genes are doing.
*I may get into the reasons for this in a later post.