The molecular foundations of zygosis

Abstract

Zygosis is the generation of new biological individuals by the sexual fusion of gamete cells. Our current understanding of eukaryotic phylogeny indicates that sex is ancestral to all extant eukaryotes. Although sexual development is extremely diverse, common molecular elements have been retained. HAP2-GCS1, a protein that promotes the fusion of gamete cell membranes that is related in structure to certain viral fusogens, is conserved in many eukaryotic lineages, even though gametes vary considerably in form and behaviour between species. Similarly, although zygotes have dramatically different forms and fates in different organisms, diverse eukaryotes share a common developmental programme in which homeodomain-containing transcription factors play a central role. These common mechanistic elements suggest possible common evolutionary histories that, if correct, would have profound implications for our understanding of eukaryogenesis.

Keywords: Archaea; Evolution; Homeoproteins; Mitochondria; Reproduction; Syngamy.

Fig. 1

The rudiments of zygosis. Zygosis involves the fusion of two gametes (shown here immediately after the initial fusion of their membranes), followed by nuclear fusion of their nuclei to form a zygote, a new individual with twice the original genetic content (2n). Zygotes can either enter the mitotic cell cycle (in diplontic and haplo-diplontic organisms) or progress immediately to meiosis to regenerate haploid progeny (in haplontic organisms). Typically, but not always, gametes are monoploid and zygotes diploid

Fig. 2

a Zygosis in Chlamydomonas. Two gametes, one of the plus mating type (upper) and one of the minus mating type (lower) fuse, ultimately forming a zygospore. bGSP1 mutants fail to complete zygosis. After fusion with a minus gamete, GSP1 mutant cells fail to limit mitochondrial and chloroplast inheritance in the normal fashion and do not differentiate into zygospores, resulting in vegetative diploid cells containing organelle genomes from both parents