Genetic investigation of a negatively phototactic strain of<i>Chlamydomonas reinhardtii</i>

Abstract

SUMMARY Two laboratory strains of the green alga Chlamydomonas reinhardtii 137c differ in their pattern of phototactic aggregation. One is positively phototactic under conditions where the other is negatively phototactic. The trait segregates 2:2 in tetrads and maps to a single locus. Heterozygous diploids are positively phototactic, showing that this allele is dominant. The aggregation pattern caused by either allele is not altered by the introduction of an unlinked gene that suppresses development of the eyespot. Probably the strains already differed in phototactic behaviour at the time they were first isolated. They may therefore reflect a genetic polymorphism common among soil algae. The genetic data allowed another significant observation not specifically related to phototaxis. Anomalous products from some crosses suggest that four nuclei sometimes fuse into a tetraploid zygote that then undergoes meiosis. The meiotic products that result are diploid. This represents a previously undescribed mechanism of diploid formation in Chlamydomonas .