Endospory in plants is the retention and development of

gametophytes A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the sex ...

, partially or entirely, within the walls of the generative spore. This is a trait present in many

heterosporous 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 is ...

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. 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'' is the sole genus of vascular plants in the family Selaginellaceae, the spikemosses or lesser clubmosses. This family is distinguished from Lycopodiaceae (the clubmosses) by having scale-leaves bearing a ligule and by having ...

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

megaspores Megaspores, also called macrospores, are a type of spore that is present in heterosporous plants. These plants have two spore types, megaspores and microspores. Generally speaking, the megaspore, or large spore, germinates into a female gametoph ...

' potential for

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

and

rhizoids Rhizoids are protuberances that extend from the lower epidermal cells of bryophytes and algae. They are similar in structure and function to the root hairs of vascular land plants. Similar structures are formed by some fungi. Rhizoids may be uni ...

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 megasporangium), and the ...

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 (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...

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 Homoplasy, in biology and phylogenetics, is the term used to describe a Phenotypic trait, feature that has been gained or lost independently in separate lineages over the course of evolution. This is different from Homology (biology), homology, wh ...

. This is observed in the separate origins of endospory across multiple orders of liverworts.

Endosporic gametophytes

Endosporic megagametophytes extend only rhizoids and the archegonium from the spore wall, they often lack chlorophyll, 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 An antheridium is a haploid structure or organ producing and containing male gametes (called ''antherozoids'' or sperm). The plural form is antheridia, and a structure containing one or more antheridia is called an androecium. Androecium is also ...

within the spore wall.

Evolutionary benefits

During gametophyte development, endosporic gametophytes are dependent on their sporophyte 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, indiv ...

. 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