Inheritance of Mating Type and a Lethal Factor in Cosmarium Botrytis Var. Subtumidum Wittr

Abstract

Summary.-The heterothallic strains of Cosmarium botrytis var. subtumidum Wittr. provide the first material for genetic investigations in the desmids. As aresult of the survival of only two non-sister-nuclei following the second division in meiosis, two gonesrather than four are producedby a germinating zygospore.The mating type of sexual strains is shown to be determined by a single gene,whichsegregatesinthenormal Mendelianfashion. The two mating typessegregate, as a rule, in thefirst divisioni of themeioticprocess; therefore, the twogonesfroma single zygospore are usually of different mating types. The abortion of all zygotes resulting from certain backerosses of gones with theminus parental strain is due to a single factor which is undetectable in thehaploid vegetative cell or in the heterozygous condition in the zygote. Zygotes formed bythefusion of two gametes, each carrying thelethal factor, always lyse shortly aftertheir formation. The lethal factor is inherited as a single gene,