GENETICS OF GRACILARIA TIKVAHIAE (RHODOPHYCEAE). XI. FURTHER CHARACTERIZATION OF A BISEXUAL MUTANT1,2

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Abstract
A spontaneous bisexual mutant of Gracilaria tikvahiae McLachlan has been further characterized. Female plants that are carriers for the mutation, but do not themselves express bisexuality, have been identified among progeny derived from the original bisexual male plant. In crosses to normal males these carrier females yielded normal tetrasporophytes which in the subsequent gametophyte generation produced a 2 female: 1 male: 1 bisexual segregation. In crosses to bisexual males the carrier females produced unusual tetrasporophytes that formed cystocarps in addition to tetraspores. The gametophyte generation obtained from the tetraspores of these tetrasporophytes included only females and bisexuals, these being present in a 1:1 ratio. Other crosses, using bisexual male progeny, indicated that these have the same characteristics as the original bisexual mutant. All of the results are consistent with the genetic interpretation made previously that bisexuality results from a single recessive mutation, designated bi, in a gene distinct from the mt locus controlling male vs. female differentiation. From the phenotypes that have been observed it appears that the mutation does not cause bisexuality per se but rather results in unregulated female expression in males and tetrasporophytes where female‐specific genes are normally repressed. It is suggested that the normal bi+ allele plays an important role in that repression process.The origin of the bisexual mutation has been re‐examined by studying the progenitor stock of the mutation and stocks related to it. It appears that the bisexual mutation arose in a two‐step process, first to a low level of expression that is found in the progenitor stocks and then to the high level of expression found in mutant clone 1045(bi) and its descendants.