Endospory in plants is the retention and development of

gametophytes A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has o ...

, partially or entirely, within the walls of the generative

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

. This is a trait present in many heterosporous plant species.

Origin

There is debate as to whether endospory or

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 isos ...

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 The Devonian ( ) is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at million years ago ( Ma), to the beginning of the succeeding ...

. 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 ''Selaginella'', also known as spikemosses or lesser clubmosses, is a genus of lycophyte. It is usually treated as the only genus in the family Selaginellaceae, with over 750 known species. This family is distinguished from Lycopodiaceae (th ...

, have been recognized as a potential intermediate in the morphological evolution to endospory due to its megaspores' potential for

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

and rhizoids extending from the trilete structure.

Ovule In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (or remnant of the sporangium, megasporangium), ...

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 Vascular plants (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified tissue ( ...

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.

Endosporic gametophytes

Endosporic megagametophytes extend only rhizoids and the

archegonium An archegonium (: archegonia), from the Ancient Greek ''ἀρχή'' ("beginning") and ''γόνος'' ("offspring"), is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female gamet ...

from the spore wall, they often lack

chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...

, and they do not acquire nutrients from the soil. Endosporic megagametophyte evolution directly correlates with endosporic microgametophytes, which are extremely reduced, and release flagellated sperm after their complete development and production of the antheridia within the spore wall.

Evolutionary benefits

During gametophyte development, endosporic gametophytes are dependent on their

sporophyte A sporophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the biological life cycle, life cycles of plants and algae. It is a diploid multicellular organism which produces asexual Spo ...

parent. The development of the gametophyte within the spore wall directly reduces the environmental impacts on the gametophytic gene expression resulting in higher genetic variation and rates of diversification. The retention of gametophytes within the spore wall additionally provided advantages for selection in ecological settings after fertilization. The support provided by the spore wall, which is similar but not as advanced as an ovule, increased reproductive success allowing for strong selective advantages during

competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indi ...

. Larger, enclosed megaspores were able to respond independently to the environment in regards to habitat and resources.

Ecological benefits

The development of gametophytes within spore walls provided improvements in sexual function as well as protection from harsh conditions. Nutrient dependence during gametophyte growth is fully supplied by the spore wall, resulting in endosporic megagametophytes increased the ability to store metabolites, lengthening the time a spore could live without water and the ability to populate new and disturbed habitats.

References

{{Reflist Botany Plant reproductive system