∞ generated and posted on 2016.12.30 ∞

Union of gametes resulting in formation of a zygote.

Syngamy, also known simply as fertilization, is most familiarly recognized as the fusion of egg and flagellated sperm. This process, however, does not always involve dissimilar gametes nor necessarily, if it involves sperm, does that sperm have to possess flagella. Indeed, for many organisms, such as many algae, sperm can possess more than one flagellum!

Regardless of the details of what kind of cells combine with what, syngamy always represents a doubling in ploidy, usually going from haploid to diploid. This contrasts with the process of meiosis, which always involves a halving of ploidy, usually going from diploid to haploid.

To reiterate, in keeping track of ploidy:

germ-line cells (diploid) → meiosis → gametes (haploid) → syngamy → zygote (diploid) → mitosis → germ-line cells

Above and in the figure below is what sexual cycles look like in animals.

Figure legend: Fertilization in its most familiar form, the fusion of sperm with egg.

This process is slightly different in plants due to alternation of generations:

sporophyte (diploid) → meiosis → spores (haploid) → mitosis (germination and so forth) → gametophyte (haploid) → mitosis → gametes (haploid) → syngamy → zygote (diploid) → mitosis → sporophyte (diploid)

The sporophyte and the gametophyte are (as indicated) the diploid and the haploid multicellular plants, respectively.

In most multicellular fungi the process instead consists of:

thallus (diploid) → meiosis → spores (haploid) → mitosis (germination and so forth) → thallus (diploid)

The thallus, in turn, is the fungus body, i.e., the mycelia plus fruiting bodies.