The genetic basis of unisexuality in Isotoma fluviatilis

I.fluviatilis occurs in diploid hermaphrodite, tetraploid hermaphrodite, and diploid dioecious populations. Studies of seed production and germination showed significant but incomplete genetic barriers between the various sex and ploid forms. Male and female sterility are controlled by recessive genes which appear to be linked with a recessive sublethal. Interaction between the sex-determining genes results in plants genotypically neuter being phenotypically male. The plants used in the crosses were shown to be heterozygous for at least some of the sex-determining genes and for genes controlling several phenotypic characters not associated with the sex system. Extensive gene hybridity, and consequently heterosis as a component of the genetic system of the species, was also suggested by the observation that the percentage seed germination and seedling survival fell with increased inbreeding. It is concluded that the diploid hermaphrodite, tetraploid hermaphrodite, and diploid dioecious races should be treated as separate subspecies and the female plants from the gynodioecious population as a form of the species