My new paper in Evolutionary Ecology Research is currently featured on the NASA Astrobiology Institute website (“Algae Fitness and Multicellular Life“). This was the final chapter of my Ph.D. dissertation, and it describes an artificial selection experiment using Pleodorina starrii. The paper is co-authored by my Ph.D. advisor, Rick Michod, and two (then) undergraduates, Susma Ghimire and Conner Vinikoor.
A 32-celled colony of Pleodorina starrii with 12 somatic cells.
Pleodorina is considered “partially differentiated,” meaning that some of its cells are of the ancestral, undifferentiated type (like those of Eudorina) and some are differentiated as somatic cells. These somatic cells never grow much, and they never divide to form daughter colonies.
One of the reasons this paper took so long to publish is that the results were mixed. By some measures, the proportion of sterile somatic cells responded to selection on colony size as expected, but some of the changes weren’t statistically significant, and some were even in the “wrong” direction.
There were some interesting results, though. First, the environment in which the algae evolved seemed to have a larger impact on proportion of soma than intentional selection for size. In the largest colonies (64-celled colonies), both the mean (24.7 vs. 23.3) and the modal number (28 vs. 24) of somatic cells was higher in colonies from the still environment, where motility was hypothesized to be more important. Cell number was surprisingly variable, even within genotypes: some evolved isolates included 4-, 8-, 16-, 32-, and 64-celled colonies within the same culture. Other isolates were much less variable, with as little as a 2-fold range of cell numbers. Finally, the highest proportions of soma were not in the largest colonies but the smallest: 4- and 8-celled colonies had, on average, 50% somatic cells! That these small colonies are giving up half of their reproductive capacity for a motility advantage they probably don’t need suggests that some sort of developmental constraint may be in play, and we suggest a (speculative and not very detailed) mechanism for such a constraint.
IANA biologist, and I am not clear on this. I think by “somatic cells” you mean cells that do not contribute to the sexual reproduction process? And by, ” they never divide to form daughter colonies”, are you saying that the ancestral-type cells do divide (thus growing the organism), but it always remains a single organism and keeps the same number of somatic cells it was “born” with?
Pleodorina has a sexual cycle, but here I’m talking about their asexual reproduction. The ancestral-type cells do divide, and each one develops into a daughter colony similar to the parent. The daughters typically have similar proportions of reproductive and somatic cells as the parent, though the total number of cells may be the same, half, or double that of the parent. When the daughter colonies are close enough to maturity, they are released from the mother colony, which is left as a husk with its somatic cells and holes in the colonial boundary where the daughters have left. Kind of sad, really.