Studies of the mutational instability presently residing in three mutant strains of Ulva mutabilis Føyn have indicated that, (I) it has a dominant behaviour in heterozygous conditions, (II) its inheritance seems not to be that of a single mendelian factor, and (III) the degree of instability as measured by the gametes is variable and dependent on the genetic constitution of the sporophyte generation prior to the gametophytes in question.
A new spontaneous mutant bubble (bu) with a marked effect on early morphogenesis in the multicellular green alga, Ulva mutabilis is described. It behaves as a recessive chromosomal mutant. The bu+ gene product formed in heterozygous sporophytes exerts a predetermining effect on morphogenesis of haploid gametophytes with the bu genotype. It is suggested that the product of the bu + gene affects the orientation of the mitotic spindles during early development.
Mutants were induced by ultraviolet light in haploid gametophyte germlings of the haplodiplontic multicellular alga Ulva mutabilis. When treated, they were from 2 to 10 days old and consisted of one cell (2 days old) to a few hundred cells (10 days old). Plants consisting exclusively of mutant cells as well as genetical chimeras consisting partly of mutant and partly of wild type cells were obtained. The frequencies of chimeras changed with the age of the irradiated germlings in a way characteristic of individual groups of genes. Chimeras involving mutations in a specific group of genes were confined to the 2‐cell stage of development. These genes are assumed to be active around the 2‐cell stage because mutations in genes which are active during a certain developmental period, will be expressed in the same generation only if the mutational event takes place before or within the active period.
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