title: Distributions of reproductive and somatic cell numbers in diverseVolvox(Chlorophyta) species.
abstract: BACKGROUND
  Volvox(Chlorophyta) asexual colonies consist of two kinds of cells: a large number of small somatic cells and a few large reproductive cells. The numbers of reproductive and somatic cells correspond directly to the major components of fitness - fecundity and viability, respectively.Volvoxspecies display diverse patterns of development that give rise to the two cell types.
  
  
  OBJECTIVE
  ForVolvoxspecies under fixed conditions, do species differ with respect to the distribution of somatic and reproductive cell numbers in a population of asexual clones? Specifically, do they differ with respect to the dispersion of the distribution, i.e. with respect to their intrinsic variability? If so, are these differences related to major among-species developmental differences?
  
  
  METHODS
  For each of fiveVolvoxspecies, we estimate the number of somatic and reproductive cells for 40 colonies and the number of reproductive cells for an additional 200 colonies. We sampled all colonies from growing, low-density, asexual populations under standard conditions.
  
  
  METHODS
  We compare the distribution of reproductive cell numbers to a Poisson distribution. We also compare the overall dispersion of reproductive cell number among species by calculating the coefficient of variation (CV). We compare the bivariate (reproductive and somatic cell) dataset to simulated datasets produced from a simple model of cell-type specification with intrinsic variability and colony size variation. This allows us to roughly estimate the level of intrinsic variability that is most consistent with our observed bivariate data (given an unknown level of size variation).
  
  
  CONCLUSIONS
  The overall variability (CV) in reproductive cell number is high inVolvoxcompared with more complex organisms.Volvoxspecies show differences in reproductive cell number CV that were not clearly related to development, as currently understood. If we used the bivariate data and tried to account for the effects of colony size variation, we found that the species that have fast embryonic divisions and asymmetric divisions have substantially higher intrinsic variability than the species that have slow divisions and no asymmetric divisions. Under our culture conditions, the Poisson distribution is a good description of intrinsic variability in reproductive cell number for some but not allVolvoxspecies.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, USA.
authors.forename: Deborah E
authors.initials: DE
authors.lastname: Shelton
authors.affiliation: Department of Embryology, St. Petersburg State University, St. Petersburg, Russia.
authors.forename: Alexey G
authors.initials: AG
authors.lastname: Desnitskiy
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, USA.
authors.forename: Richard E
authors.initials: RE
authors.lastname: Michod
keywords: Volvox; cellular differentiation; development; eco-devo; evolution; multicellularity; reproductive allocation; robustness