Endospory in plants is the retention and development of gametophytes, partially or entirely, within the walls of the generative spore.[1][2] This is a trait present in many heterosporous plant species.[2]

Origin

There is debate as to whether endospory or heterospory evolved first. Some debate centers upon the requirement of endospory to develop before heterospory.[2] 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.[2] 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.[3][4]

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.[5]  In tracheophytes specifically, endospory and heterospory may have evolved separately a number of times.[6]

It has been suggested that heterospory and endospory may be adaptively linked, but with independent developmental control.[5] Phylogenetic inference of hornworts demonstrates that endospory is homoplastic. This is observed in the separate origins of endospory across multiple orders of liverworts.[6]

Select extant classes exhibiting endospory[2][6]
Class Order Endospory present
Lycopsida (Clubmosses) Selaginales Yes
Anthocerotopsida (Hornworts) Dendrocerotales Yes
Sphenopsida (Horsetails) Equisetales Yes
Pteropsida (Ferns) Unknown
Polypodiopsida (Ferns) Salvinales Yes
Marsileales Yes
Filicales (Platyzoma) No
Gymnospermopsidia Seed Plants Yes

Endosporic gametophytes

Endosporic megagametophytes extend only rhizoids and the archegonium from the spore wall, they often lack chlorophyll,[1] and they do not acquire nutrients from the soil.[5] Endosporic megagametophyte evolution directly correlates with endosporic microgametophytes, which are extremely reduced,[7] and release flagellated sperm after their complete development and production of the antheridia within the spore wall.[1]

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.[1][5]

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. Larger, enclosed megaspores were able to respond independently to the environment in regards to habitat and resources.[1]

Ecological benefits

The development of gametophytes within spore walls provided improvements in sexual function as well as protection from harsh conditions.[1] Nutrient dependence during gametophyte growth is fully supplied by the spore wall,[4]  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.[3]

References

  1. 1 2 3 4 5 6 Cruzan, Mitchell B. (2018). Evolutionary biology : a plant perspective. New York, NY. ISBN 978-0-19-088268-6. OCLC 1050360688.{{cite book}}: CS1 maint: location missing publisher (link)
  2. 1 2 3 4 5 BATEMAN, RICHARD M.; DiMICHELE, WILLIAM A. (1994). "Heterospory: The Most Iterative Key Innovation in the Evolutionary History of the Plant Kingdom". Biological Reviews. 69 (3): 345–417. doi:10.1111/j.1469-185x.1994.tb01276.x. ISSN 1464-7931. S2CID 29709953.
  3. 1 2 L., Taylor, Thomas N. Smoot, Edith (1984). Paleobotany. Van Nostrand Reinhold/Scientific and Academic Editions. ISBN 0-442-28290-7. OCLC 9555786.{{cite book}}: CS1 maint: multiple names: authors list (link)
  4. 1 2 Petersen, Kurt B.; Burd, Martin (2016-10-11). "Why did heterospory evolve?". Biological Reviews. 92 (3): 1739–1754. doi:10.1111/brv.12304. ISSN 1464-7931. PMID 27730728. S2CID 5224364.
  5. 1 2 3 4 DiMichele, William A.; Davis, Jerrold I.; Olmstead, Richard G. (1989). "Origins of Heterospory and the Seed Habit: The Role of Heterochrony". Taxon. 38 (1): 1–11. doi:10.2307/1220881. hdl:2027.42/149713. ISSN 0040-0262. JSTOR 1220881.
  6. 1 2 3 Villarreal A., Juan Carlos; Campos S., Laura Victoria; Uribe-M., Jaime; Goffinet, Bernard (2012-03-01). "Parallel Evolution of Endospory within Hornworts: Nothoceros renzagliensis (Dendrocerotaceae), sp. nov". Systematic Botany. 37 (1): 31–37. doi:10.1600/036364412X616594. S2CID 86328103.
  7. ↑ Simpson, Michael G. (10 November 2019). Plant systematics. ISBN 978-0-12-812629-5. OCLC 1165240610.
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