Endospory in plants

Origin

There is debate as to whether endospory or heterospory evolved first. Some debate centers upon the requirement of endospory to develop before heterospory. Endospory is assumed to follow heterospory but it has been suggested that without endospory, early plant species dependency on water fertilization and environmental impacts on gametophytic gene expression would have reduced the chances of heterospory in the Late Devonian. Heterospory and endospory are often found co-occurring and the origin of endospory is drawn from comparisons in extant species. Fossils provide evidence of the origin of heterospory in the middle to late Devonian with earliest record of fossil taxa being Cyclostigma and Bisporangiostrobus, late Devonian genera. Early fossil records of endospory have not been discussed in literature, but the oldest extant lineage with heterospory, the Selaginella, have been recognized as a potential intermediate in the morphological evolution to endospory due to its megaspores' potential for photosynthesis and rhizoids extending from the trilete structure.

Ovule structures began diversifying during the late Devonian, suggesting that endospory originated in around this time. It is possible that in some lineages, heterospory was an consequence of endospory through developmental changes of endospory. In tracheophytes specifically, endospory and heterospory may have evolved separately a number of times.

It has been suggested that heterospory and endospory may be adaptively linked, but with independent developmental control. Phylogenetic inference of hornworts demonstrates that endospory is homoplastic. This is observed in the separate origins of endospory across multiple orders of liverworts.