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James Umen

 

James Umen

James Umen

Assistant Professor
Plant Molecular and Cellular Biology Laboratory

"My laboratory is interested in fundamental questions in cell biology—how do cells control growth and division, how do they determine their size, and how do the genetic pathways that control size and division evolve to generate life's incredible diversity?"

Sex chromosomes and sexual selection strongly influence every aspect of biology, including development, behavior, and even ecological interactions between species. Umen's research team has taken advantage of a unique group of green algae to begin answering a longstanding question in developmental and evolutionary biology: How do different sexes evolve?

While the single-celled alga Chlamydomonas has two sexes whose reproductive cells look identical, its close relative Volvox is multicellular and has evolved females and males that produce eggs and sperm, respectively. When Umen and his collaborators sequenced the sex-determining chromosomal region from both species, they found intriguing differences. Not only was the size of the sex-determining region from multicellular Volvox much larger than that of its unicellular relative Chlamydomonas, but the genes within this sex-determining region were found to evolve in a very different manner than those of the single-celled species. Unlike Chlamydomonas, the male and female versions of genes in the Volvox sex-determining region were so distinct that they looked as if they came from different species.

One striking example of this divergence was found in a gene that encodes the Volvox version of the retinoblastoma tumor suppressor, or RB. In both humans and green algae, the RB gene controls cell division, and Umen's laboratory had already shown that the absence of RB in Chlamydomonas causes loss of cell division control, just as it does when mutated in human cancer cells. The generation of eggs or sperm in Volvox entails sex-specific cell division reprogramming that is exactly what might be caused by the male-female differences in the Volvox RB gene. Interestingly, there is accumulating evidence that the RB pathway in humans and other animals may also play a special role in reproductive tissues. While the details of how Volvox and animals generate reproductive cells may be very different, the finding that similar genetic pathways are involved suggests that the RB pathway may have acquired a special role in germ cell formation very early in animal evolution.

Lab Photo

Left to right:
Sitting left: Cristina Lopez Standing: Yubing Li, Matt Zones, Jim Umen, Brad Olson, Harjivan Kohli Sitting right: Rhitu Rai

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James Umen

Faculty

James Umen

James Umen

Assistant Professor
Plant Molecular and Cellular Biology Laboratory

Cell Size Homeostasis and the RB Tumor Suppressor Pathway

Size homeostasis is a fundamental property of proliferating cells and is thought to be governed by cell size checkpoints. The multiple fission cell cycle of Chlamydomonas uncouples cell growth and division, and allows us unique access to a size checkpoint mechanism. A key regulator of this checkpoint is the Chlamydomonas retinoblastoma (RB) tumor suppressor pathway, whose function in cell size and cell cycle regulation is a major focus of investigation.

Cell Growth Regulation in Photosynthetic Eukaryotes

Cell growth in eukaryotes requires the coordinate regulation of cytoplasmic biosynthetic processes with those in chloroplasts and mitochondria, semi-autonomous organelles that contain their own protein biosynthetic machinery. Chloroplasts from higher plants and green algae represent a large fraction of cellular biomass, yet it is unknown how their growth is regulated with respect to cytoplasmic growth. The TOR (target of rapamycin) kinase signaling pathway is conserved in all eukaryotes where it functions as a nutrient sensitive modulator of growth rates. We are using Chlamydomonas as a simple model for how TOR signaling contributes to coordinated growth control in photosynthetic eukaryotes.

Evolution of Developmental Complexity

Chlamydomonas reinhardtii belongs to a diverse clade of green algae, some of which have undergone a remarkable transition to multicellularity. The best-characterized of the multicellular relatives is Volvox carteri, a species that embodies many of the hallmarks of multicellular metazoans or plants. These include terminally differentiated somatic cells, reproductive stem cells, complex embryonic patterning, and formation of sexually dimorphic germ cells (eggs and sperm), none of which are present in its unicellular relative Chlamydomonas. Indirect evidence suggests that the RB tumor suppressor pathway might be coupled to germ/soma differentiation and to dimorphic germ cell production. Our current work is aimed at cloning and characterizing the mating locus that is the genetic determinant for sperm and egg formation.

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