Overview

Our understanding of life is being transformed by the realization that evolution occurs not only among individuals within populations, but also through the integration of groups of individuals into new higher-level individuals. Indeed, the major landmarks in the diversification of life and the hierarchical organization of the living world are consequences of a series of evolutionary transitions in individuality: from genes to gene networks to the first cell, from prokaryotic to eukaryotic cells, from cells to multicellular organisms, from asexual to sexual populations, and from solitary to social organisms. We are interested in understanding the diversity of life by understanding the evolution of interactions among individuals and how these interactions may create new kinds of evolutionary individuals. We are especially interested in cooperation and conflict during the origin of multicellularity and the evolution of sex. We are interested in the consequences of sex in terms of coping with genetic error (mutation and damage), and how sex affects evolutionary transitions in individuality. The methods used in our work involve mathematical and computer models, experiments with micro-organisms, bioinformatics, comparative genomics, molecular biology, and philosophical analysis. Our ongoing work concerns the evolution of cooperation, sex, multicellularity and complexity in volvocine green algae. We are grateful for past support from the National Science Foundation and the National Institute of Health, and to NASA Exobiology and the National Science Foundation for our current support.

The Volvocine Green Algae

The Volvocine Green Algae diverged from a unicellular Chlamydomonas –like ancestor approximately 240 mya (Herron et al. 2009). They range from simple unicellular species like Chlamydomonas reinhardtii to large complex species with thousands of cells and cellular differentiation, as seen in Volvox carteri. Intermediate forms range from simple sheets of 8-16 cells as seen in Gonium, to larger 32-64 celled spheroids as in Eudorina, and 128 celled spheroids with somatic cells as in Pleodorina.

Evolution of the VARL Gene Family

The evolution of germ-soma differentiation is a critical component of the evolutionary transition in individuality from unicellularity to multicellularity. This is because somatic cells give up their reproductive ability for the betterment of the group and germ cells lose the ability to survive independently. Thus, germ and somatic cells must operate in concert to survive and produce the next generation. In Volvox carteri the development of somatic cells is controlled by regA, a transcription factor that regulates chloroplast biogenesis. regA is a member of the VARL gene family and part of a tandem duplication of four to five genes known as the regA gene cluster. Currently, we are trying to understand when the regA and the regA gene cluster evolved in the Volvocine algae and if it is present in organisms lacking somatic cells. To do this we are using a variety of molecular biological and comparative genomic techniques. This project will increase our understanding of the evolution of cellular differentiation and complexity in the Volvocine algae and more broadly as well.

Volvocales Genome Project

The Michod Lab is part of a multi-lab collaboration (Dr. Bradley J. S. C. Olson; Kansas State University, Dr. Hisayoshi Nozaki; University of Tokyo, Dr. Pierre Durand; University of Witwatersrand) and a NSF Collaborative Research Project (Dr. Bradley J. S. C. Olson; Kansas State University) to sequence the genomes of several volvocine algae. While the genomes of unicellular Chlamydomonas reinhardtii and multicellular Volvox carteri are previously published (Merchant et al. 2007, Prochnik et al. 2010), we are interested in a more detailed understanding of the genetic basis for the evolution of multicellularity. We are also interested to understanding how the order of genetic change relates to Dr. David Kirk’s 12 Step Program to Evolving Multicellularity at the phenotypic level. These projects will utilize the Volvocine algae, the most recent example of multicellular evolution, as a model system, with the goal of addressing a major gap in our knowledge of the genetic basis of multicellular evolution.

Mathematical Modeling

Evolution of Cooperation

The evolution of cooperation is a fundamental process in the evolutionary transitions of individuality. Because lower level units have an evolutionary advantage if they cheat and gain lower-level fitness at the expense of the group, mechanisms to mediate this conflict are necessary for evolutionary transitions in individuality. The Michod Lab has investigated evolutionary mechanisms to mediate lower-level conflict such as germ-soma segregation, mutation rate, single-celled propagules thereby ensuring the maintenance of cooperation and the emergence individuality at the higher level.

NASA project: Steps to Multicellularity.

We have developed a general theory for ETIs and propose testing this theory using the volvocine green algae as an experimental model. Due to the morphological diversity in these algae, we can investigate the genetic basis for the evolution of key phenotypes spanning the unicellular to multicellular ETI. In no other experimental model is such an undertaking feasible. Germ and somatic cells correspond to the two major fitness components (reproduction and viability); their evolution is a critical part of explaining how and why a cell group sometimes evolves into a new evolutionary individual. We aim to understand the evolution of the regA gene family, which plays a central role in germ soma division of labor (G-S DOL) in the volvocine algae. We will determine the functions of these genes and test whether unicellular-level life history trade-offs drove G-S DOL at the multicellular level, as predicted by our theory. We will use a phenotypic screen for soma-less mutants to uncover other genes involved in G-S DOL. Genomic approaches to the evolution of multicellularity allow investigation into the origin and complexity of gene families in terms of their role in phenotypes associated with the ETI. With this understanding of genotype and multicellularituy, we will address the evolution of allocation to germ and soma in a range of environments (i.e., the genotype-phenotype map for GS-DOL) using measurements of cell type allocation in diverse species, artificial selection experiments, and a study of the suppression of reproduction in repsonse to stress. We will subsequently explore the evolution of the genotype-phenotype map for germ-soma allocation using mathematical modelling.