User:Hrobertson2/Heterospory

Origin of Heterospory
Heterospory developed due to natural selection pressures that encourage an increase in propagule size. Heterospory is believed to have evolved from homosporous (same size spores) plants. Heterosporous plants, similar to anisosporic plants, produce two different sized spores in separate sporangia that develop into separate male and female gametophytes. It is proposed that the emergence of heterosporous plants started with the separation of sporangia ,which allowed for the development of two different spore types; numerous small spores that are easily dispersed, and fewer, larger spores that contain adequate resources to support the developing seedling. During the Devonian era there were many species that utilized vertical growth to capture more sunlight. It is believed that heterospory and separate sporangia evolved in response to competition for light. Disruptive selection within species resulted in there being two separate sexes of gamete or even the whole plant. 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 is advantageous in that having two different types of spores increases the likeliness that plants would successfully  produce offspring. Heterosporous spores can respond independently to selection by ecological conditions in order to strengthen male and female reproductive function. Heterospory evolved from homospory many times, but the species in which it first appeared are now extinct. Heterospory is thought to have emerged in the Devonian era, mostly in wet/damp places based on fossil record evidence. In addition to being an outcome of competition for light, it is thought that heterospory was more successful in wetter areas because the megaspore could move more easily around in an aquatic environment while microspores were more easily dispersed by wind. Differing sized spores have been observed in many fossilized plant species. For example, the species Lepidophloios, also known as the scale tree, have been shown in fossils to have been heterosporous ; The scale tree had separate cones containing either male or female spores on the same plant. Modern heterosporous plants such as many ferns  exhibit endospory, in which a megagametophyte is fertilized by a microgametophyte all while still inside the spore wall, gaining nutrients from the inside of

spore. Both heterospory and endospory seem to be one of the many precursors to seed plants and the ovary. '''Heterosporic plants that produce seeds are their most successful and widespread descendants. Seed plants constitute the largest subsection of heterosporic plants. '''