Evolutionary Ecology Research, 2012, 14: 707–727
Distributions of reproductive and somatic cell numbers
in diverse Volvox (Chlorophyta) species
Deborah E. Shelton1, Alexey G. Desnitskiy2 and Richard E. Michod1
1
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, USA and
2
Department of Embryology, St. Petersburg State University, St. Petersburg, Russia
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. Volvox species display diverse patterns of development that give rise to the
two cell types.
Questions: For Volvox species 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?
Data description: For each of five Volvox species, 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.
Search method: 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 in Volvox
compared with more complex organisms. Volvox species 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 all Volvox species.
Keywords: cellular differentiation, development, eco-devo, evolution, multicellularity,
reproductive allocation, robustness, Volvox.
Correspondence: D.E. Shelton, Department of Ecology and Evolutionary Biology, University of Arizona,
1041 E. Lowell St., Tucson, AZ 85721, USA. e-mail: [email protected]
Consult the copyright statement on the inside front cover for non-commercial copying policies.
© 2012 Deborah E. Shelton
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