Graduate Student Research Projects in the Lab
Evolution Of Complexity in the Volvocine Algae: Transitions in Individuality
Through Darwin's Eye
PDF of
Matt Herron and
Rick Michod's recent paper in Evolution
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 they
diverged relatively recently from unicellular relatives and 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, this jump in complexity can be reduced to a series of small steps that
cumulatively describe a gradual transition between the two levels. We use
phylogenetic reconstructions of ancestral character states to trace the
evolution of steps involved in this transition in volvocine algae. The history
of these characters includes several well-supported instances of multiple
origins and reversals. The inferred changes can be understood as components of
cooperation–conflict–conflict mediation cycles as predicted by multilevel
selection theory. One such cycle may have taken place early in volvocine
evolution, leading to the highly integrated colonies seen in extant volvocine
algae. A second cycle, in which the defection of somatic cells must be
prevented, may still be in progress.
Artificial Selection on Body Size in Volvocine Algae by
Matt Herron
Two trends are apparent in the relationship between size and level of
specialization in volvocine algae: larger colonies have higher levels of
specialization, and, within the germ-soma specialized forms, larger species have
higher proportions of somatic cells. Trends among species suggest that
trade-offs between motility and fecundity have been involved in the evolution of
terminally differentiated somatic cells. However, trends among species can
differ substantially from those within species. If the trends observed among
volvocine species reflect selective pressures within species imposed by
trade-offs between motility and fecundity, these trade-offs should also be
measurable within species. I am using artificial selection experiments to
address the following questions: (1) Does reproduction impose a cost on
volvocine colonies in the form of reduced motility? (2) Does the cost of
reproduction increase with increasing colony size? (3) Do colonies change their
investment in soma in response to changes in size? (4) Does the response to
change in size differ in environments in which motility differs in importance?
(5) What constraints restrict the response to selective pressures on size and
motility? To increase the genetic variability in colony size to a point
sufficient to measure changes in the cost of reproduction, colonies of
Pleodorina starrii are being selected for increasing and decreasing colony size.
By including environmental treatments in which motility is more or less
important, I will test whether colonies are able to adjust their relative levels
of investment in motility and fecundity
DNA damage accumulation in zygospores of the facultatively sexual green
alga, Chlamydomonas reinhardtii by Deborah Shelton
Abstract: Facultatively sexual species (FSS) must gain a short-term benefit
from reproducing sexually under particular conditions. Therefore, understanding
sex in FSS is relevant to the more general question of why sex is adaptive and
nearly universal. The DNA repair hypothesis posits that the adaptive value of
sex is homologous recombinational repair (HRR) of damaged DNA during meiosis. In
the facultatively sexual green alga Chlamydomonas reinhardtii, fusion of haploid
gametes produces a dormant zygospore. Meiosis (and HRR) occurs just prior to the
germination of a haploid alga from the diploid zygospore. Therefore, the DNA
repair hypothesis predicts that DNA damages accumulate in the diploid zygospore
and are repaired prior to germination by meiosis. This study will use
single-cell gel electrophoresis ("comet assay") to test whether DNA damages
accumulate over time in C. reinhardtii zygospores and are repaired by meiosis.
Transitions in Individuality and Origin of Multicellularity in Volvocalean
Green Algae
We have developed a life history approach to the evolution of
multicellularity. Several students have worked on this project both at the
University of Arizona and in Paris resulting in the following paper.
Michod, R. E., Y. Viossat, C. A. Solari, A. M. Nedelcu, and M. Hurrand. 2006.
Life history evolution and the origin of multicellularity. Journal of
Theoretical Biology. 239:257-272.
PDF
Cristian Solari used Volvocalean green algae to study
the origin of multicellularity and evolutionary transitions in complexity.
Cristian developed a mathematical model of the origin of cooperative somatic cells in
primitive colonial groups and is testing the model by measuring fitness
components (motility, productivity) in different environments using
available mutants in Volvox carteri. Click
here for more on Cristian's work.
Denis Roze is studying mathematical
models of how new evolutionary individuals--with the properties
of variation, fitness and heritability--emerge from lower level individuals.
Specifically, how do multi-cellular organisms emerge from groups of cells
A Comparative Study of the Evolution of Multicellularity in the Volvocales
With Matt Herron. PDF
of Poster. The volvocine algae (Volvox and its close relatives) represent a
unique opportunity for the study of origins of multicellularity. Several major
evolutionary transitions have occurred within this relatively young group, which
spans a large range of sizes and several levels of complexity. To explore the
historical and adaptive bases of these major evolutionary transitions, we
inferred phylogenetic relationships in the colonial volvocine algae using DNA
sequence data. Using maximum parsimony reconstructions of ancestral character
states, we traced the evolution of key innovations such as coloniality, large
size, cellular differentiation, and novel developmental programs.
Origin of the Eukaryotic Cell
We are working on the origin of the eukaryotic cell, specifically
the origin and evolution of early associations between the mitochondrion and its
host. Preliminary work on this is described in this
book chapter. We are in the process of translating the many scenarios for the origin of
the protomitochondrion and its host into the language of multilevel selection
theory and game theory. We then plan to approach this problem using mathematical modeling and
experiments.
Evolution of Sex
Aurora Nedelcu is studying the role of stress and reactive oxygen
species (ROS) in the development and evolution of sex in Volvox carteri.
Click here for more on this work and visit the
Volvocales Information Project.
Rick Hudson's Ph.D. thesis several years ago concerned laboratory studies in
Bacillus subtilis, comparative studies of sexual life cycles and phylogenetic studies of
the distribution of sex in bacteria. Rick mapped transformation and its component
processes onto existing ribosomal DNA phylogenies. He concluded that transformation is
distributed widely and is likely ancient. The world was born sexual! The second part of
his thesis concerns life history aspects of development of the sexual stage. Rick asked
which hypotheses for the evolution of sex can best explain the timing of sex in bacteria.
His results show that dispersal and sex are alternative strategies used when conditions
deteriorate. This runs contrary to the commonly accepted wisdom which is that sex is
always positively associated with dispersal.
Evolution of Cooperation and Individuality in Slime Molds
Rick Hudson and other recent EEB students have been using slime molds to study the
evolution of the organism and individuality (see R. E Hudson, J. E. Aukema, C.
Rispe, and D. Roze. Altruism, cheating, and anticheater adaptations in cellular
slime molds. Am.Nat. 160:31-43, 2002. Download PDF file
here)
