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Sexual Life Cycles and Uniparental Inheritance

    We’ve discovered and illuminated a number of elegant features of the Chlamydomonas life cycle over the years, including the sexual adhesion mechanism, signaling via cAMP and flagellar membrane dynamics, the anatomy of zygotic cell fusion, and control over these traits via the complex mating-type locus and a key regulatory protein called Mid. Our current focus is on the haploid arrow diploid transition that is triggered by sexual fusion. We have shown that this transition is driven in Chlamydomonas by the heterodimerization of two homeoproteins, Gsp1 from the plus parent and Gsm1 from the minus parent. We are now asking whether a similar role is played by Gsp1 and Gsm1 orthologues in a second green algal radiation represented by Ostreococus and Micromonas, pikoprasinophytes that have a full complement of meiosis-related genes but have not (yet) been observed to interact sexually.
    A fascinating feature of the Chlamydomonas sexual cycle is its mode of uniparental inheritance of organelle DNA: chloroplast chromosomes deriving from the minus parent are selectively destroyed in the zygote, as are mitochondrial chromosomes deriving from the plus parent. The chloroplast pattern is determined by genes resident in the mating-type plus locus and by genes expressed in very early zygote development. We have identified 4 gene candidates involved in this process and are now asking how they act to protect the plus DNA from destruction and enable the destruction of minus DNA.
    These studies are being performed in the larger context of an on-going interest in understanding 1) what eukaryotic sex is “for” and 2) its roles in both the evolution of a species and the speciation process itself. Of recent interest is exploring the role played by the evolution of Gsm1/Gsp1, a kernel gene regulatory network, in sporophyte differentiation and the attendant green transition from water to land.
Cell Wall Glycoproteins
    Hyrdroxyproline-rich glycoproteins (HRGPs) can be thought of as plant collagens, assembling into long fibrils and meshworks and specifying form and growth patterns. In land plants, their activities are obscured by the co-proliferation of structural polysaccharides like cellulose and pectin, but in Chlamyomonas the walls are assembled from HRGPs alone. Moreover, vegetative and gametic cells form walls using one set of HRGPs while zygotes use an independent set. The system therefore represents a accessible venue to study both the protein interactions that govern HRGP assembly, including sexual adhesion, and the regulation of their expression during the haploid arrowdiploid transition. The highly repetitive fibrous domains also prove to be a rich resource for analyzing the mutational basis for balancing sequence diversity with sequence conservation.
cell wall
Wall in tangential fracure from intact cell. The cone-shaped central region is the P face of the plasma membrane, deeply pitted by the etching process. CS denotes the true cell surface. The major wall layers are numbered 1-7, X 65,000.
Lipid Bodies and Biodiesel

    Chlamydomonas had been considered “non-oleaginous,” but we have found ways to elicit a 60-fold increase in its production of TAG-filled lipid bodies (LBs), and seek to understand the biochemical, cell biological, and genetic basis for this induction. We have shown that a mutation knocking out the starch-biosynthesis pathway enhances LB production two-fold and that yield is influenced by light, exogenous carbon sources, and growth conditions. We have developed ways to purify LBs, allowing studies of their composition and in vitro properties. We are also collaborating with Dr. John Heuser at Washington University School of Medicine to visualize LB biogenesis and maturation by electron microscopy, and with Dr. Sabeeha Merchant at UCLA to characterize the transcriptomes of LB-producing cells. The long-term goal is to use mutant screens and transgenes to generate strains with enhanced TAG production. Hopefully these studies will be applicable to the production of algal biodiesel and an attendant reduction in atmospheric CO2 levels.


Lipid bodies (yellow) in wild-type (upper) and the sta6 mutant. Red is chlorophyll autofluorescence. Confocal sections (left) and z-series reconstruction (right).


sta6 cell with a lipid body L associated with the ER. C, chloroplast; N, nucleus; mito, mitochondrion. Quick-freeze deep-etch from John Heuser and Robyn Roth.


Previous Research Projects

    Previous projects in the Goodenough lab have focused on flagellar structure and function [click to relevant Research Articles] and on chloroplast biogenesis [click to relevant Research Articles]. These subjects remain of ongoing interest.


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