4.3: Life Cycle of the Unicellular Eukaryote
The life cycle of a unicellular organism begins with syngamy: one cell unites with cell having different genotype. To recognize each other, cells which are going to fuse ( gametes ) frequently use surface proteins, like cells of our immune system. If these proteins are same (same genotype), gametes will not fuse. Two fused gametes form a zygote, new diploid organism. Many unicellular protists use a zygote as a wintering stage. On spring, zygote splits with meiosis, and four haploid spores start four new organisms which reproduce all summer with mitosis (vegetative reproduction, cloning):
\[X+X \rightarrow XX \rightarrow I+I+I+I \rightarrow X \rightarrow I+I \rightarrow ...\]
Despite its simplicity, this life cycle has all three possible ways of reproduction: sexual (ploidy doubles: syngamy), asexual (ploidy reduces: meiosis of zygote) and vegetative (ploidy does not change: mitotic divisions). To mark these ways of reproduction, we will use “ R! ” shortcut for the meiosis, and “ Y! ” shortcut for syngamy (Figure \(\PageIndex{1}\)). It should be noted that before every mitosis (and meiosis), cell DNA goes through duplication (S-stage of the cell cycle).
Figure \(\PageIndex{1}\) The life cycle of unicellular eukaryote.