Induction of Heterothallic and Homothallic Zygotes in Dictyostelium discoideum by Ethylene

Pheromones do regulate sexual development in Dictyostelium discoideum

Based on negative data, Douglas et al refuted earlier work by the O'Day laboratory on the role of pheromones in the sexual development of Dictyostelium discoideum. This letter addresses some of the issues with their study.

Ethylene as a potent inducer of sexual development

A novel and critical function of ethylene, a potent plant hormone, has been well documented in Dictyostelium, because it leads cells to the sexual development (macrocyst formation) by inducing zygote formation. Zygote formation (sexual cell fusion) and the subsequent nuclear fusion are the characteristic events occurring during macrocyst formation. A novel gene, zyg1 was found to be predominantly expressed during the sexual development, and its enforced expression actually induces zygote formation. As expected, the zygote inducer, ethylene enhances the expression of zyg1. Thus the function of ethylene has been verified at all of individual (macrocyst formation), cellular (zygote formation), and molecular levels (zyg1 expression). Based on our recent studies concerning the behavior and function of the zyg1 product (ZYG1 protein), the signal transduction pathways involved in zygote formation are proposed in this review.

A novel function of ethylene

The cellular slime mold, Dictyostelium mucoroides-7 (Dm7) exhibits clear dimorphism; macrocyst formation as a sexual process and sorocap formation as an asexual process. These two life cycles are regulated by two regulators, ethylene and cyclic AMP (cAMP). This is the first report demonstrating a novel function of ethylene at the cellular level. That is, ethylene induces a zygote formed by cell fusion and subsequent nuclear fusion. Recently, the function of ethylene at the molecular level has been clarified as it induces zygote formation through an enhanced expression of a novel gene, zyg1. The signaling pathway for induction or inhibition of zygote formation is now trying to be clarified focusing on the ZYG1 protein.

Genes involved in Dictyostelium discoideum sexual reproduction

Macrocyst formation in the cellular slime moulds is a sexual process induced under dark and humid conditions. Normal development life cycle in these organisms involves proliferation by cell division and, upon starvation, formation of multicellular aggregates and fruiting bodies, consisting of spores and stalk cells. Macrocyst formation, cell division by binary fission and spore formation are thus three alternative modes of reproduction, for which it is of interest to understand how a choice is made. The genetic basis of asexual development and fruiting body formation is well known, by contrast information on the genetic control of sexual reproduction during macrocyst formation is scarce. In Dictyostelium discoideum, the most widely used species, several cell-surface proteins relevant to sexual cell fusion have been identified using cell fusion-blocking antibodies, but isolation of the relevant genes has been unsuccessful. Analysis of sexually deficient mutants, some of which are normal for asexual development, has shown that sexual reproduction is regulated by both specific genes and genes that are also involved in asexual development. Reverse genetic analysis of 24 genes highly enriched in a gamete-specific subtraction library has revealed four genes involved in the regulation of sexual cell interactions. One of them was found to be a novel regulator of the cAMP signalling pathway specific to sexual development. Studies on the molecular genetic control of the sexual cycle will be reviewed and their contribution to our understanding of the organization and function of the D. discoideum genome as a whole discussed.

Gamete fusion and cytokinesis preceding zygote establishment in the sexual process of Dictyostelium discoideum

Cells of Dictyostelium discoideum become sexually mature under submerged and dark conditions, and fuse with opposite mating-type cells to form zygote giant cells, which gather surrounding cells and finally develop into dormant structures called macrocysts. In the present study, we found that the multinuclear fused cells formed during this process frequently underwent cytokinesis driven by random local movements. The split cells were capable of re-fusion, and repeated cytokinesis. These radical behaviors continued until the extensive cell aggregation started around the giant cells. Thus, gamete fusion and initiation of zygote development do not coincide in the mating of D. discoideum. Analyses by confocal microscopy and flow cytometry indicated that the cessation of the random movement followed pronuclear fusion, and that microtubule organizing centers (MTOC), abundant in the fused cells at the beginning, gradually decreased and only one of them remained within the developed macrocyst. Some of the genes known to control cell movement, such as rasGEFB and rasS, increased shortly before the cessation of repeated fusion-cytokinesis and initiation of phagocytosis. These results suggest that the sequential molecular events are necessary in D. discoideum after gamete fusion to establish a new individuality of zygotes.