megaspore

A megaspore is the larger type of spore produced by heterosporous plants. It is formed by meiosis of a diploid sporocyte within a megasporangium and, in most cases, only one of the four resulting megaspores remains viable while the others degenerate. This functional megaspore then develops into the female gametophyte.

In seed plants and some ferns, the megaspore develops inside the ovule or sporangial tissue and gives rise to the female gametophyte, which provides the female reproductive cells. In gymnosperms and angiosperms, the resulting female gametophyte is typically embedded within the nutrient-rich maternal tissue. In angiosperms, the megaspore undergoes mitotic divisions to form the embryo sac, usually containing eight nuclei arranged into seven cells, including the egg cell and central cell. In heterosporous ferns and lycophytes, the megaspore germinates to produce a separate, photosynthetic or non-photosynthetic female gametophyte that bears archegonia.

Megaspores are characteristic of heterospory, a reproductive strategy that also involves microspores, the smaller spores produced by the male sporophyte. The differentiation into megaspores and microspores is an important step toward the evolution of seeds, as the female gametophyte in seeds is often retained within the ovule and provides nourishment to the developing embryo.

Examples of organisms with megaspores include certain lycophytes (such as Selaginella and Isoetes) and all seed plants (gymnosperms and angiosperms).