The Sexual Cycle and Intercrossing in the Genus Astrephomene*

SYNOPSIS. The structure and certain aspects of the physiology of the sexual cycle, including mating and zygote germination, were studied in strains of the colonial green flagellate Astrephomene gubernaculifera. Mating type inheritance was analyzed in 4 crosses among strains with 7 chromosomes and 1 cross between strains with 4 chromosomes. Results in all cases indicated that mating type was under the control of a single pair of alleles. Twenty-six morphologically identical clones from separate geographic areas were studied with regard to sexual compatibility. On the basis of intercrossing, 6 sexually isolated groups were recognized. Complete gene flow (genetic compatibility) was found among the strains comprising intercrossing Group I.