Sexual Sporulation in the Zygomycota
- Some species within the Zygomycota are HOMOTHALLIC (self-fertile), while others are HETEROTHALLIC (self-sterile).
- Sexual sporulation in HETEROTHALLIC species requires the presence of TWO PHYSIOLOGICALLY COMPATIBLE COLONIES (mycelia), usually designated + (plus) and - (minus); remember that mycelia of fungi belonging to the Zygomycota are HAPLOID.
- Specialized aerial hyphae, called ZYGOPHORES, form small branches called PROGAMETANGIA, which are mutually attracted and grow towards one another - see diagram below.
- Upon contact with one another, the TIPS OF THE PROGAMETANGIA ENLARGE.
- A cross-wall (solid septum) develops behind the tip of each progametangium, dividing each into a GAMETANGIUM and a SUSPENSOR CELL.
- The WALLS of the two contacting gametangia BREAKDOWN at the point of contact and the TWO PROTOPLASTS FUSE - the NUCLEI then PAIR and FUSE (karyogamy) to form a DIPLOID ZYGOTE.
- The zygote cell enlarges, its wall thickens and becomes pigmented and in many species the wall becomes highly sculptured - this structure is the maturing ZYGOSPORE.
- After a period of dormancy the zygospore germinates, producing a sporangiophore and sporangium (GERM SPORANGIUM).
- MEIOSIS occurs during formation of the GERM SPORANGIUM - so it contains HAPLOID SPORANGIOSPORES, which will upon germination give rise to haploid mycelia (colonies).