Sexual Isolation in <i>Pandorina morum</i>*

Abstract

SYNOPSIS. The morphology of the vegetative phase and the sexual process in Pandorina morum have been studied in detail under controlled laboratory conditions. A number of strains from various areas of the United States, all essentially indistinguishable on morphological grounds, were analyzed for sexual compatibility. Forty‐seven heterothallic clones were found to represent 15 separate pairs of mating types, or a minimum of 15 syngens ( sensu Sonneborn, 1957). Two clones proved to be homothallic. Nine additional clones, which were not observed to mate with any strain, can be classified only after further collecting. The separate heterothallic pairs of mating types are sexually isolated by factors acting at the stage of gamete production; incompatible mating types produce no gametes when they are mixed together. The geographical distribution of the sets of mating types is very incompletely defined as yet, but they are not strictly endemic forms. Thirty heterothallic strains, representing each of the 15 sexually isolated sets of mating types of Pandorina morum , have been deposited with the Culture Collection of Algae maintained at Indiana University, Bloomington, Indiana, U.S.A.