Since the summer of 1991, I have been studying the genetics and molecular biology of the unicellular green alga, Chlamydomonas reinhardtii. This work is being done in collaboration with Carol Dieckmann, Telsa Mittlelmeier, and Mike Rice at the University of Arizona.
Chlamydomonas is a photosynthetic organism, so it is advantageous for the cell to be able to swim toward light at an appropriate level. At moderate light intensities, the cell swims toward the light (positive phototaxis). At high light intensities, the cell stops swimming briefly (photophobic behavior) and then swims away from the light (negative phototaxis). We are studying positive phototaxis using the tools of genetics and molecular biology. This problem can be broken down into three major areas of study: How does the cell perceive the direction and intensity of the light? How does the cell transmit the information about the light to the flagella? How do the flagella respond to that information?
What is currently known about light signal reception in Chlamydomonas?
The cell organelle that is responsible for light reception is the eyespot. This structure is located at a precise place in the cell and is thought to be composed of structures in the chloroplast and structures in the portion of the plasma membrane directly above chloroplast portion of the eyespot. The plasma membrane is thought to contain the pigment responsible for light reception, rhodopsin. This is the same pigment involved in vertebrate vision. The chloroplast portion of the eyespot is made up of 2-4 layers of lipid-carotenoid granules each separated by a layer of thylakoid membrane. This is what gives the eyespot its characteristic orange appearance in the light microscope. We are interested in how the eyespot is assembled because it is assembled in a precise place in the cell after each cell division and because it is assembled in two different cellular compartments: the chloroplast and the plasma membrane. We are taking a genetic approach to the problem. In the fall of 1991, we assembled a collection of 170 mutants that failed to swim toward light. Each of those mutants was examined by light microscopy to look for potential eyespot assembly mutants. We found 4 mutants with reduced (mini) eyespots, 5 mutants with multiple eyespots, and 16 mutants with no eyespots. We've done the basic genetics on these mutants and we know that all the mini-eyespot mutants are in a single complementation group and all the multi-eye mutants are in a second complementation group. The eyeless mutants fall into two complementation groups. One of these complementation groups is represented by a single mutant. The mini- and multi-eye mutants are located on the same linkage group (chromosome) as is the eyeless mutant with a single representative. The other eyeless complementation group is located on linkage group #10 and, until we got around to doing the genetics, we had assumed that our mutants were identical with the previously identified gene ey-1 (which also has an eyeless phenotype). Our new eyeless mutant complements ey-1 and its alleles ey-550 and ey-627. We're still trying to figure that one out because an ey-1 mutant crossed with our eyeless strains failed to produce wild type recombinants.
Where is the project headed? What could you be a part of if this interests you?
Roberts DGW, Lamb MR, and Dieckmann CL. 2001. Characterization of the EYE2 gene required for eyespot assembly in Chlamydomonas reinhardtii. Genetics 158: 1037-1049.
Lamb MR, Dutcher SK, Worley CK, and Dieckmann CL. 1999. Eyespot-assembly mutants in Chlamydomonas reinhardtii. Genetics 153: 721-9.