Heterospory

• Similar plant structures (both have strobili) • Heterospory plants differ in that sporangium have either a large number or small spores or a small number of large spores whereas homosporous plants have the same number of small spores in each sporangium

• Size differentiation: megaspores and microspores • Unisexuality of the gametophytes that develop from each spore type (megaspores only become female megagametrophyte and vice versa) • Endoscopic development of gametophyte As gametophyte develops inside the spore, it is dependent on spore reserves (cannot photosynthesise) Next generation sporophytes entirely depend on unused megaspore reserves

• Usually a 1000 to a few 1000 times greater volume than microspore • Microspores are a similar size to homosporous spores

Heterospory occurs in some lycophytes, some monilophytes and all seed plants

• First arose about 370 million years ago in the lycophyte lineage • Fossils of cyclostigma and bisporangiostrobus • evolved independently multiple times (suggests it is an adaptation)

• Microspores are like pollen and megaspores are like ovules of a pine • Heterospory was a necessary precursor to the evolution of seeds

• Size represents nutrient reserve for sporophyte generation and development of gametophyte • In a shadier, more productive areas there is more competition • Spore size increased from Silurian to Carboniferous (at the same time there was more shade and crowding developing because of increasing vasculature) • Large spores (especially heterospory) seems to be an adaptation to novel regeneration niches created by plants themselves • Heterospory seems to be an adaptation to shady regeneration niches

seeds are what freed plants from dependence on water for reproduction