Identification of the agg1 mutation responsible for negative phototaxis in a “wild-type” strain of Chlamydomonas reinhardtii

The unicellular green alga Chlamydomonas reinhardtii is a model organism for various studies in biology. CC-124 is a laboratory strain widely used as a wild type. However, this strain is known to carry agg1 mutation, which causes cells to swim away from the light source (negative phototaxis), in contrast to the cells of other wild-type strains, which swim toward the light source (positive phototaxis). Here we identified the causative gene of agg1 (AGG1) using AFLP-based gene mapping and whole genome next-generation sequencing. This gene encodes a 36-kDa protein containing a Fibronectin type III domain and a CHORD-Sgt1 (CS) domain. The gene product is localized to the cell body and not to flagella or basal body.