Microspore in the context of Alternation of generations

Megaspores, also called macrospores, are a type of spore that is present in heterosporous plants. These plants have two spore types, megaspores and microspores. Generally speaking, the megaspore, or large spore, germinates into a female gametophyte, which produces egg cells. These are fertilized by sperm produced by the male gametophyte developing from the microspore. Heterosporous plants include seed plants (gymnosperms and flowering plants), water ferns (Salviniales), spikemosses (Selaginellaceae) and quillworts (Isoetaceae).

A microsporangium (pl. microsporangia) is a sporangium that produces microspores that give rise to male gametophytes when they germinate. Microsporangia occur in all vascular plants that have heterosporic life cycles, such as seed plants, spike mosses and the aquatic fern genus Azolla. In gymnosperms and angiosperm anthers, the microsporangia produce microsporocytes, the microspore mother cells, which then produce four microspores through the process of meiosis. Microsporocytes are produced in the microsporangia of gymnosperm cones and the anthers of angiosperms. They are diploid microspore mother-cells, which then produce four haploid microspores by meiosis. These become pollen grains, within which the microspores divide twice by mitosis to produce a very simple gametophyte.

Heterosporous plants that produced microspores in microsporangia and megaspores in separate megasporangia evolved independently in several plant groups during the Devonian period. Fossils of these plants show that they produced endosporic gametophytes, meaning that their gametophytes were not free-living as in bryophytes but developed within the spores, as in modern heterosporic vascular plants.

Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. The smaller of these, the microspore, is male and the larger megaspore is female. Heterospory evolved during the Devonian period from isospory independently in several plant groups: the clubmosses, the ferns including the arborescent horsetails, and progymnosperms. This occurred as part of the process of evolution of the timing of sex differentiation. Four extant groups of plants are heterosporous; Selaginella, Isoetes, Salviniales and seed plants.