Heterospory

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 arborescent horsetails, and progymnosperms. This occurred as part of the process of evolution of the timing of sex differentiation.

Heterospory developed due to natural selection pressures that encouraged an increase in propagule size. This may first have led to an increase in spore size and ultimately resulted in the species producing larger megaspores as well as smaller microspores.

Heterospory evolved from homospory many times, but the species in which it first appeared are now extinct. Heterosporic plants that produce seeds are their most successful and widespread descendants.Seed plants constitute the largest subsection of heterosporic plants.

Microspores are haploid spores that in endosporic species contain the male gametophyte, which is carried to the megaspores by wind, water currents or animal vectors. Microspores are nearly all nonflagellated, and are therefore not capable of active movement. The morphology of the microspore consists of an outer double walled structures surrounding the dense cytoplasm and central nucleus.

Megaspores contain the female gametophytes in heterosporic plant species. They develop archegonia that produce egg cells that are fertilized by sperm of the male gametophyte originating from the microspore. This results in the formation of a fertilized diploid zygote, that develops into the sporophyte embryo. While heterosporous plants produce fewer megaspores, they are significantly larger than their male counterparts.

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