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ABSTRACT
How the variation in phenotypic traits like cell size and motility impacts predator-induced cellular aggregation is not known. Furthermore the genetic composition of cell groups in mixed populations of Chlamydomonas has not been investigated. An examination of these two questions will not only enhance our understanding of Chlamydomonas ecology, but also shed light on the primordial steps before integrated multicellular groups were established. Group living comes with viability and reproductive costs and it is not known how these are shared if groups are genetically heterogeneous. We observed that the natural predator Peranema trichophorum (Euglenoidea) induced clumping in Chlamydomonas. When co-cultured with P. trichophorum cells protected themselves by forming facultative groups (reverting back to a unicellular lifestyle once predators were removed). The dynamics of group formation in different Chlamydomonas species and strains correlated with cell size and swimming speed. Small or less motile strains aggregated more readily than large, fast-swimming ones. Interestingly, Chlamydomonas groups were both intra-species and inter-species chimaeric. This suggests that the predator-induced group formation in Chlamydomonas involved cells coming together rather than staying together and during aggregation cells showed little or no discrimination between self and non-self. These data demonstrate that the dynamics of cell aggregation, in unicellular volvocines at least, depends on phenotypic traits like cell size and motility and high genetic relatedness is not mandatory at this initial stage. These findings further our understanding of aggregation in mixed Chlamydomonas populations and have implications for understanding the very first steps on the road to simple multicellularity.
ACKNOWLEDGEMENTS
SS acknowledges a URC postdoctoral fellowship from the University of Witwatersrand, Johannesburg, South Africa. This work was supported by grants to PMD from Medical Research Council and the University of Witwatersrand, Johannesburg, South Africa and National Aeronautics and Space Administration (#NNX13AH41G) grant to Richard E. Michod, Patrick J. Ferris and PMD at the University of Arizona. SS acknowledges financial support from the Durand Foundation for Evolutionary Biology and Phycology. The authors thank Cristian Solari and Stuart Sym for helpful comments.
Disclosure statement
No potential conflict of interest was reported by the author(s).
SUPPLEMENTARY INFORMATION
The following supplementary material is accessible via the Supplementary Content tab on the article’s online page at http://dx.doi.org/10.1080/09670262.2015.1107759
Supplementary figs S1–4. Genetic heterogeneity in clumps formed by Chlamydomonas in the presence of Peranema.
Supplementary movie 1. Cell motility of a small strain C. moewusii UTEX 9.
Supplementary movie 2. Cell motility of a large strain C. reinhardtii CC2342.
Supplementary movie 3. Cell motility of a large strain C. reinhardtii CC2931.
Additional information
Notes on contributors
Santosh Sathe
S. Sathe: original concept, study design, experiments, data analysis and manuscript preparation; P.M. Durand: original concept, study design and manuscript preparation.
Pierre M. Durand
S. Sathe: original concept, study design, experiments, data analysis and manuscript preparation; P.M. Durand: original concept, study design and manuscript preparation.