The American Society of Naturalists Haploid and Diploid Generations

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Abstract
In an otherwise wholly haploid organism, the diploid zygote can under special conditions develop without meiosis into a new multicellular individual. This possibility may in consequence of a single mutation become the regular behavior of the zygote under ordinary conditions. The new diplont can display only characters already represented in the genetic constitution of the haplont. In the simplest case, this implies a duplication by the diplont of the structure of the haplont. An alternation of generations seems to have arisen by duplication in green and brown algae; possibly also in the ancestors of pteridophytes and of seed plants. If the haplont has developed a distinct post-zygotic phase leading I to spore-formation, the newly established diplont displays the characters of this phase. The post-zygotic phase is transferred from haplont to diplont. The 2 generations are then unlike from the start. An origin of alternation by transfer seems to have occurred in the ancestry of bryophytes; and in that of pteridophytes and seed plants if these derive from bryophytic ancestors. Alternation in long-cycled red algae apparently came about through both transfer and duplication.[long dash]Alternation being established, the general tendency is toward an elaboration of the diplont and a reduction of the haplont. Reduction has led, in Codium and apparently in the [female] line in a few angiosperms, to the complete elimination of the haplont save for the cell generation that includes the gametes. This is essentially the condition in animals.[long dash]But in one species of Cladophora and possibly in a race or races of Ectocarpus it appears that an elimination of the haplont has come about in a single step, the final cell products of the diplont by a mutation affecting their response to external conditions taking on the function of gametes. In view of the absence of intermediate stages, so abundant in plants, it is probable that in the ancestry of the metazoa the haploid generation was thus eliminated in one step rather than by a gradual process of reduction.

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