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[edit]Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. Heterospory was evolved from isospory independently by several plant groups; the clubmosses, the arborescent horsetails,[1] and progymnosperms[1] in the Devonian period[2] as part of the process of evolution of the timing of sex differentiation.[3] Heterosporic plants produce small spores called microspores which either germinate to become free-living male gametophytes or have reduced male gametophytes packaged within them, and larger spores called megaspores that either germinate into free-living female gametophytes, or which have a female gametophyte packaged within them which is retained in and nurtured by the sporophyte phase, a condition referred to as endospory. Heterosporous species are thus usually dioicous, a condition that promotes outcrossing. Some heterosporous species produce micro- and megaspores in the same sporangium, a condition known as homoangy, while in others the micro- and megaspores are produced in separate sporangia (heterangy). These may both be borne on the same monoecious sporophyte or on different sporophytes in dioecious species.
Continuing the background
[edit]Following the development of homospory within the original sporophytes, some species began to produce spores of varying sizes. This was done independently in many vascular plant species, with significantly varying features between them. What has remained in common between them are important features of heterospory: The size differentiation between the smaller microspores and larger megaspores, endospory and unisexuality in which male gametophytes develop from microspores and female gametophytes from megaspores. Endospory refers to the development of gametophytes within the wall of a spore which is the case for heterporic spores. These gametophytes heavily rely on resources and nutrients contained within the walls for growth. This is commonly found across species of moss.[4]
Origin of Heterospory
[edit]The evolution of heterospory is very similar to how anisogamy, the production of gametes of different size, evolved. The difference between the two is that heterospory came to be in the context of the life cycle for plants, the alternation of generations.[4]
Heterospory developed due to natural selection pressures that encouraged an increase in propagule size. This first led to an increase in spore size and ultimately resulted in the species producing that larger megaspore as well as the original smaller microspore.[5]
While heterospory has evolved from homospory many times, the vast majority of the resulting species have since gone extinct leaving behind a relatively few number of species. Of these, the heterosporic plants that produce seeds have been the most successful and widespread.[5] Seed producing plants constitute the largest subsection of heterosporus plants.[4]
Reproduction
[edit]Heterospory was a key event in the evolutionary transition of both fossil and surviving plants. The retention of megaspores and the dispersal of microspores allow for both dispersal and establishment reproductive strategies. This adaptive ability of heterospory increases reproductive success as any type of environment favors having these two strategies. Heterospory stops self-fertilization from occurring in a gametophyte, but does not stop two gametophytes that originated from the same sporophyte from mating.[4] This specific type of self-fertilization is termed as sporophytic selfing, and it occurs most commonly among angiosperms. This means while heterospory stops extreme inbreeding from occurring, it does not prevent inbreeding altogether as sporophytic selfing can still occur.[4]
A complete model for heterospory's origin, known as the Haig-Westoby model,[6] establishes a connection between minimum spore size and successful reproduction of bisexual gametophytes. For the female function, as minimum spore size increases so does the chance for successful reproduction. For the male function, reproductive success does not change as the minimum spore size increases. This means there exists an optimal spore size where both eggs and sperm can be produced by the bisexual gametophyte.
See Also
[edit]References
[edit]- ^ a b Stewart, W.N.; Rothwell, G.W. (1993). Paleobotany and the evolution of plants (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-38294-7.
- ^ Bateman, R.M.; Dimichele, W.A. (1994). "Heterospory - the most iterative key innovation in the evolutionary history of the plant kingdom" (PDF). Biological Reviews of the Cambridge Philosophical Society. 69: 315–417. Retrieved 2010-12-30.
- ^ Sussex, I.M. (1966) The origin and development of heterospory in vascular plants. Chapter 9 in Trends in Plant morphogenesis, ed. by E.G. Cutter, Longmans.
- ^ a b c d e Petersen, Kurt B.; Burd, Martin (2016-10-01). "Why did heterospory evolve?". Biological Reviews: n/a–n/a. doi:10.1111/brv.12304. ISSN 1469-185X.
- ^ a b Haig, David; Westoby, Mark (1989-11-01). "Selective forces in the emergence of the seed habit". Biological Journal of the Linnean Society. 38 (3): 215–238. doi:10.1111/j.1095-8312.1989.tb01576.x. ISSN 1095-8312.
- ^ Haig, David; Westoby, Mark. "A model for the origin of heterospory". Journal of Theoretical Biology. 134 (2): 257–272. doi:10.1016/s0022-5193(88)80203-0.
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Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. Heterospory was evolved from isospory independently by several plant groups; the clubmosses, the arborescent horsetails,[1] and progymnosperms[1] in the Devonian period[2] as part of the process of evolution of the timing of sex differentiation.[3] Heterosporic plants produce small spores called microspores which either germinate to become free-living male gametophytes or have reduced male gametophytes packaged within them, and larger spores called megaspores that either germinate into free-living female gametophytes, or which have a female gametophyte packaged within them which is retained in and nurtured by the sporophyte phase, a condition referred to as endospory. Heterosporous species are thus usually dioicous, a condition that promotes outcrossing.
Some heterosporous species produce micro- and megaspores in the same sporangium, a condition known as homoangy, while in others the micro- and megaspores are produced in separate sporangia (heterangy). These may both be borne on the same monoecious sporophyte or on different sporophytes in dioecious species.
References
[edit]- ^ a b Stewart, W.N.; Rothwell, G.W. (1993). Paleobotany and the evolution of plants (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-38294-7.
- ^ Bateman, R.M.; Dimichele, W.A. (1994). "Heterospory - the most iterative key innovation in the evolutionary history of the plant kingdom" (PDF). Biological Reviews of the Cambridge Philosophical Society. 69: 315–417. Retrieved 2010-12-30.
- ^ Sussex, I.M. (1966) The origin and development of heterospory in vascular plants. Chapter 9 in Trends in Plant morphogenesis, ed. by E.G. Cutter, Longmans.
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