megaspores

Megaspores are the larger spores produced by heterosporous plants. They originate in the megasporangia of the ovule or ovulate cone and are designed to give rise to the female gametophyte. In most plants, a megasporocyte undergoes meiosis to produce four haploid megaspores, but typically only one functional megaspore remains to develop further.

In seed plants, the functional megaspore germinates and divides mitotically to form the female gametophyte, which stays within the surrounding ovule tissues. In gymnosperms, the developing female gametophyte remains inside the integumented nucellus and ultimately produces the egg cell. In angiosperms, the mature female gametophyte, or embryo sac, usually contains eight nuclei organized into seven cells, including the egg cell and the central cell with two polar nuclei. The embryo sac then participates in double fertilization, producing the embryo and endosperm.

Meogaspores contrast with microspores, which are smaller spores that give rise to male gametophytes. The evolution of megaspores and heterospory is linked to the transition from free-sporing plants to seed plants, contributing to reduced water dependence in reproduction and enabling internal fertilization and seed formation. In many ferns and other early-diverging plants that exhibit heterospory, megaspores still function to develop the female gametophyte within the sporangium or associated tissues.