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Richard E Michod

Publications and source records attributed to Richard E Michod.

10 recordsLinked to original sources
PubMed2006

The group covariance effect and fitness trade-offs during evolutionary transitions in individuality.

Transforming our understanding of life is the realization that evolution occurs not only among individuals within populations but also through the integration of groups of preexisting individuals into a new higher-level individual, that is, through evolutionary transitions in individuality. During evolutionary transitions (such as during the origin of gene networks, bacteria-like cells, eukaryotic cells, multicellular organisms, and societies), fitness must be reorganized; specifically, it must be transferred from the lower- to the higher-level units and partitioned among the lower-level units that specialize in the fitness components of the new higher-level individual. This paper studies the role of fitness trade-offs in fitness reorganization, the evolution of cooperation, and the conversion of a group into a new individual during the origin of multicellular life. Specifically, this study shows that the fitness of the group is augmented over the average fitness of its members according to a covariance effect. This covariance effect appears to be one of the first emergent properties of the group and a general aspect of groups with multiplicative properties that are themselves averages of properties of lower-level units. The covariance effect allows groups to break through the constraints that govern their members, and this effect likely applies to group dynamics in other fields.

Animals↗
PubMed2006

The evolutionary origin of an altruistic gene.

Although the conditions favoring altruism are being increasingly understood, the evolutionary origins of the genetic basis for this behavior remain elusive. Here, we show that reproductive altruism (i.e., a sterile soma) in the multicellular green alga, Volvox carteri, evolved via the co-option of a life-history gene whose expression in the unicellular ancestor was conditioned on an environmental cue (as an adaptive strategy to enhance survival at an immediate cost to reproduction) through shifting its expression from a temporal (environmentally induced) into a spatial (developmental) context. The gene belongs to a diverged and structurally heterogeneous multigene family sharing a SAND-like domain (a DNA-binding module involved in gene transcription regulation). To our knowledge, this is the first example of a social gene specifically associated with reproductive altruism, whose origin can be traced back to a solitary ancestor. These findings complement recent proposals that the differentiation of sterile castes in social insects involved the co-option of regulatory networks that control sequential shifts between phases in the life cycle of solitary insects.

Algal Proteins↗
PubMed2006

A hydrodynamics approach to the evolution of multicellularity: flagellar motility and germ-soma differentiation in volvocalean green algae.

During the unicellular-multicellular transition, there are opportunities and costs associated with larger size. We argue that germ-soma separation evolved to counteract the increasing costs and requirements of larger multicellular colonies. Volvocalean green algae are uniquely suited for studying this transition because they range from unicells to multicellular individuals with germ-soma separation. Because Volvocales need flagellar beating for movement and to avoid sinking, their motility is modeled and analyzed experimentally using standard hydrodynamics. We provide comparative hydrodynamic data of an algal lineage composed of organisms of different sizes and degrees of complexity. In agreement with and extending the insights of Koufopanou, we show that the increase in cell specialization as colony size increases can be explained in terms of increased motility requirements. First, as colony size increases, soma must evolve, the somatic-to-reproductive cell ratio increasing to keep colonies buoyant and motile. Second, increased germ-soma specialization in larger colonies increases motility capabilities because internalization of nonflagellated germ cells decreases colony drag. Third, our analysis yields a limiting maximum size of the volvocalean spheroid that agrees with the sizes of the largest species known. Finally, the different colony designs in Volvocales reflect the trade-offs between reproduction, colony size, and motility.

Biological Evolution↗
PubMed2006

Multicellularity and the functional interdependence of motility and molecular transport.

Benefits, costs, and requirements accompany the transition from motile totipotent unicellular organisms to multicellular organisms having cells specialized into reproductive (germ) and vegetative (sterile soma) functions such as motility. In flagellated colonial organisms such as the volvocalean green algae, organized beating by the somatic cells' flagella yields propulsion important in phototaxis and chemotaxis. It has not been generally appreciated that for the larger colonies flagellar stirring of boundary layers and remote transport are fundamental for maintaining a sufficient rate of metabolite turnover, one not attainable by diffusive transport alone. Here, we describe experiments that quantify the role of advective dynamics in enhancing productivity in germ soma-differentiated colonies. First, experiments with suspended deflagellated colonies of Volvox carteri show that forced advection improves productivity. Second, particle imaging velocimetry of fluid motion around colonies immobilized by micropipette aspiration reveals flow fields with very large characteristic velocities U extending to length scales exceeding the colony radius R. For a typical metabolite diffusion constant D, the associated Peclet number Pe = 2UR/D >> 1, indicative of the dominance of advection over diffusion, with striking augmentation at the cell division stage. Near the colony surface, flows generated by flagella can be chaotic, exhibiting mixing due to stretching and folding. These results imply that hydrodynamic transport external to colonies provides a crucial boundary condition, a source for supplying internal diffusional dynamics.

Algal Proteins↗
PubMed2005

Life-history evolution and the origin of multicellularity.

The fitness of an evolutionary individual can be understood in terms of its two basic components: survival and reproduction. As embodied in current theory, trade-offs between these fitness components drive the evolution of life-history traits in extant multicellular organisms. Here, we argue that the evolution of germ-soma specialization and the emergence of individuality at a new higher level during the transition from unicellular to multicellular organisms are also consequences of trade-offs between the two components of fitness-survival and reproduction. The models presented here explore fitness trade-offs at both the cell and group levels during the unicellular-multicellular transition. When the two components of fitness negatively covary at the lower level there is an enhanced fitness at the group level equal to the covariance of components at the lower level. We show that the group fitness trade-offs are initially determined by the cell level trade-offs. However, as the transition proceeds to multicellularity, the group level trade-offs depart from the cell level ones, because certain fitness advantages of cell specialization may be realized only by the group. The curvature of the trade-off between fitness components is a basic issue in life-history theory and we predict that this curvature is concave in single-celled organisms but becomes increasingly convex as group size increases in multicellular organisms. We argue that the increasingly convex curvature of the trade-off function is driven by the initial cost of reproduction to survival which increases as group size increases. To illustrate the principles and conclusions of the model, we consider aspects of the biology of the volvocine green algae, which contain both unicellular and multicellular members.

Animals↗
PubMed2005

John Maynard Smith.

John Maynard Smith was one of the most original thinkers in evolutionary biology of the post neo-Darwinian synthesis age. He was able to define new problems with clarity and by doing so open up new research directions. He did this in a number of areas including game theory and evolution, the evolution of sex, animal behavior, evolutionary transitions and molecular evolution. Although he is best known for his research and his ideas, he was a great expositor and wrote many books, including introductory texts in the areas of evolution and genetics, ecology and mathematical modeling, as well as advanced expositions of research problems.

Biological Evolution↗
PubMed2004

Sex as a response to oxidative stress: a twofold increase in cellular reactive oxygen species activates sex genes.

Organisms are constantly subjected to factors that can alter the cellular redox balance and result in the formation of a series of highly reactive molecules known as reactive oxygen species (ROS). As ROS can be damaging to biological structures, cells evolved a series of mechanisms (e.g. cell-cycle arrest, programmed cell death) to respond to high levels of ROS (i.e. oxidative stress). Recently, we presented evidence that in a facultatively sexual lineage--the multicellular green alga Volvox carteri--sex is an additional response to increased levels of stress, and probably ROS and DNA damage. Here we show that, in V. carteri, (i) sex is triggered by an approximately twofold increase in the level of cellular ROS (induced either by the natural sex-inducing stress, namely heat, or by blocking the mitochondrial electron transport chain with antimycin A), and (ii) ROS are responsible for the activation of sex genes. As most types of stress result in the overproduction of ROS, we believe that our findings will prove to extend to other facultatively sexual lineages, which could be indicative of the ancestral role of sex as an adaptive response to stress and ROS-induced DNA damage.

Algal Proteins↗
PubMed2003

Sex as a response to oxidative stress: the effect of antioxidants on sexual induction in a facultatively sexual lineage.

The evolution of sex is one of the long-standing unsolved problems in biology. Although in many lineages sex is an obligatory part of the life cycle and is associated with reproduction, in prokaryotes and many lower eukaryotes, sex is facultative, occurs in response to stress and often involves the formation of a stress-resistant dormant form. The proximate and ultimate causes of the connection between stress and sex in facultatively sexual lineages are unclear. Because most forms of stress result in the overproduction of cellular reactive oxygen species (ROS), we address the hypothesis that this connection involves ROS and possibly reflects the ancestral role of sex as an adaptive response to the damaging effects of stress-induced ROS (i.e. oxidative stress). Here, we report that two antioxidants inhibit sexual induction in a facultatively sexual species - the multicellular green alga, Volvox carteri. Furthermore, the nature of the sex response and the effect of an iron chelator on sexual induction are consistent with sex being a response to the DNA-damaging effects of ROS. In addition, we present preliminary data to suggest that sex, cell-cycle arrest and apoptosis are alternative responses to increased levels of oxidative stress.

2,2'-Dipyridyl↗
PubMed2003

Cooperation and conflict in the evolution of individuality. IV. Conflict mediation and evolvability in Volvox carteri.

The continued well being of evolutionary individuals (units of selection and evolution) depends upon their evolvability, that is their capacity to generate and evolve adaptations at their level of organization, as well as their longer term capacity for diversifying into more complex evolutionary forms. During a transition from a lower- to higher-level individual, such as the transition between unicellular and multicellular organisms, the evolvability of the lower-level (cells) must be restricted, while the evolvability of the new higher-level unit (multicellular organism) must be enhanced. For these reasons, understanding the factors leading to an evolutionary transition should help us to understand the factors underlying the emergence of evolvability of a new evolutionary unit. Cooperation among lower-level units is fundamental to the origin of new functions in the higher-level unit. Cooperation can produce a new more complex evolutionary unit, with the requisite properties of heritable fitness variations, because cooperation trades fitness from a lower-level (the costs of cooperation) to the higher-level (the benefits for the group). For this reason, the evolution of cooperative interactions helps us to understand the origin of new and higher-levels of fitness and organization. As cooperation creates a new level of fitness, it also creates the opportunity for conflict between levels of selection, as deleterious mutants with differing effects at the two levels arise and spread. This conflict can interfere with the evolvability of the higher-level unit, since the lower and higher-levels of selection will often "disagree" on what adaptations are most beneficial to their respective interests. Mediation of this conflict is essential to the emergence of the new evolutionary unit and to its continued evolvability. As an example, we consider the transition from unicellular to multicellular organisms and study the evolution of an early-sequestered germ-line in terms of its role in mediating conflict between the two levels of selection, the cell and the cell group. We apply our theoretical framework to the evolution of germ/soma differentiation in the green algal group Volvocales. In the most complex member of the group, Volvox carteri, the potential conflicts among lower-level cells as to the "right" to reproduce the higher-level individual (i.e. the colony) have been mediated by restricting immortality and totipotency to the germ-line. However, this mediation, and the evolution of an early segregated germ-line, was achieved by suppressing mitotic and differentiation capabilities in all post-embryonic cells. By handicapping the soma in this way, individuality is ensured, but the solution has affected the long-term evolvability of this lineage. We think that although conflict mediation is pivotal to the emergence of individuality at the higher-level, the way in which the mediation is achieved can greatly affect the longer-term evolvability of the lineage.

Adaptation, Physiological↗