Origem triássica e radiação antiga das algas multicelulares volvocine

Triassic origin and early radiation of multicellular volvocine algae

1. Matthew D. Herron,1,
2. Jeremiah D. Hackett,
3. Frank O. Aylward,2 and
4. Richard E. Michod

Author Affiliations

1. Department of Ecology and Evolutionary Biology, University of Arizona, 1041 East Lowell Street, Tucson, AZ 85721

Edited by Francisco J. Ayala, University of California, Irvine, CA, and approved January 7, 2009 (received for review November 6, 2008)

Abstract

Evolutionary transitions in individuality (ETIs) underlie the watershed events in the history of life on Earth, including the origins of cells, eukaryotes, plants, animals, and fungi. Each of these events constitutes an increase in the level of complexity, as groups of individuals become individuals in their own right. Among the best-studied ETIs is the origin of multicellularity in the green alga Volvox, a model system for the evolution of multicellularity and cellular differentiation. Since its divergence from unicellular ancestors, Volvox has evolved into a highly integrated multicellular organism with cellular specialization, a complex developmental program, and a high degree of coordination among cells. Remarkably, all of these changes were previously thought to have occurred in the last 50–75 million years. Here we estimate divergence times using a multigene data set with multiple fossil calibrations and use these estimates to infer the times of developmental changes relevant to the evolution of multicellularity. Our results show that Volvoxdiverged from unicellular ancestors at least 200 million years ago. Two key innovations resulting from an early cycle of cooperation, conflict and conflict mediation led to a rapid integration and radiation of multicellular forms in this group. This is the only ETI for which a detailed timeline has been established, but multilevel selection theory predicts that similar changes must have occurred during other ETIs.

Keywords: evolution multicellurity multilevel selection transitions in individuality Volvox

Footnotes

1To whom correspondence should be addressed. E-mail:mherron@email.arizona.edu

Author contributions: M.D.H. designed research; M.D.H., J.D.H., F.O.A., and R.E.M. performed research; M.D.H. and J.D.H. analyzed data; and M.D.H. and R.E.M. wrote the paper.

2Present address: Department of Bacteriology, University of Wisconsin, Madison, 1550 Linden Drive, Madison, WI 53706.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at PNAS.

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