Megasporen

Megaspores are haploid spores produced by meiosis of the megasporocyte within a megasporangium. They represent the female side of heterospory, the condition in which plants produce distinct microspores (males) and megaspores (females). Megaspores are typically larger than microspores.

In seed plants, the megasporangium is located inside the ovule. The diploid megasporocyte undergoes meiosis to produce four haploid megaspores; usually only one becomes functional, while the others degenerate. The functional megaspore then germinates and, by successive mitotic divisions, develops into the female gametophyte, or megagametophyte. In angiosperms this megagametophyte is the embryo sac, which houses the egg cell and, in many species, a central cell formed from two polar nuclei.

Following fertilization, the egg cell forms the zygote, and the central cell typically integrates into the endosperm in angiosperms, providing nutrition to the developing embryo. In gymnosperms, the megagametophyte remains within the ovule and serves as the nutritive tissue for the developing embryo.

Megaspores are a defining feature of heterospory and play a central role in the reproductive biology and evolutionary history of seed plants. They are contrasted with microspores, which give rise to male gametophytes. Some heterosporous ferns also produce megaspores that develop into female gametophytes.