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Zygotes of the unicellular green alga Chlamydomonas reinhardi occasionally divide mitotically to give rise to stable diploid vegetative strains. As well as by their mode of origin, these strains are distinguished from haploids by cell and nuclear size, DNA content per nucleus, and chromosome number. Diploid strains heterozygous for a variety of mutant genes are phenotypically wild type and mating-type minus. Thus these mutant genes are recessive to their wild-type alleles, and the mating-type-minus is dominant over the mating-type-plus allele.

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SUMMARYTwo laboratory strains of the green algaChlamydomonas reinhardtii137cdiffer 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 inChlamydomonas.

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