The transition from unicellular to differentiated multicellular organisms constitutes an increase in the level complexity, because previously existing individuals are combined to form a new, higher-level individual. The volvocine algae represent a unique opportunity to study this transition because extant species display a range of intermediate grades between unicellular and multicellular, with functional specialization of cells.
Following the approach Darwin used to understand “organs of extreme perfection” such as the vertebrate eye, evolution of multicellularity can be understood as a series of small steps that cumulatively describe a transition between the two levels. With Rick Michod, I used phylogenetic reconstructions of ancestral character states to trace the evolution of steps involved in this transition. The history of these characters includes several well-supported instances of multiple origins and reversals.
In contrast to the previously accepted date of 50-75 million years ago, I showed that the multicellular volvocine algae diverged from unicellular ancestors at least 200 million years ago. Developmental changes over this span have been sporadic, with a number of important changes occurring within a relatively short time after the divergence from unicellular ancestors. In contrast to this relatively rapid period of change, some lineages have undergone relatively little change over long time spans, and some extant species are living fossils whose basic body plans have changed little in the last 200 million years.
The evolution of mortal somatic cells was a critical step in the evolution of complex body plans and the major radiations of multicellular life. To test hypotheses about the origin of somatic cells, I subjected the volvocine alga Pleodorina starrii to selection on colony size in two different environments. The positive among-species relationship between colony size and proportion of soma was paralleled within the larger (16- to 64-celled) colonies of P. starrii, but not within the smaller (4- and 8-celled) colonies, which had the highest proportions of soma, suggesting the existence of an evolutionary constraint preventing optimization of soma in the smallest size classes.
Nozaki, H., N. Ueki, O. Misumi, K. Yamamoto, S. Yamashita, M. D. Herron and F. Rosenzweig. 2015. Morphology and reproduction of Volvox capensis (Volvocales, Chlorophyceae) from Montana, USA. Phycologia 54:316-320. (pdf)
Herron M. D. and A. M. Nedelcu. 2015. Volvocine algae: from simple to complex multicellularity. pp. 129-152 in A. M. Nedelcu & I. Ruiz-Trillo (eds.) Evolutionary transitions to multicellular life: Principles and mechanisms. Springer. (pdf)
Herron M. D., S. Ghimire, C. R. Vinikoor and R. E. Michod. 2014. Fitness trade-offs and developmental constraints in the evolution of soma: an experimental study in a volvocine alga. Evolutionary Ecology Research 16:203-221. (pdf)
Leliaert, F., D. R. Smith, H. Moreau, M. D. Herron, H. Verbruggen, C. F. Delwiche, and O. De Clerck. 2012. Phylogeny and molecular evolution of the green algae. Critical Reviews in Plant Sciences 31(1):1-46. (pdf)
Herron, M. D., A. G. Desnitskiy, and R. E. Michod. 2010. Evolution of developmental programs in Volvox (Chlorophyta). Journal of Phycology 46(2):316-324. (pdf)
Herron, M. D., J. D. Hackett, F. O. Aylward, and R. E. Michod. 2009. Triassic origin and early radiation of multicellular volvocine algae. Proceedings of the National Academy of Sciences, USA 106(9):3254-3258. (pdf) (Supporting Information)