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gametophytes. It is proposed that the emergence of heterosporous plants started with the separation of sporangia, which allowed for the development of two different spore types; numerous small spores that are easily dispersed, and fewer, larger spores that contain adequate resources to support the developing seedling. During the
Devonian period there were many species that utilized vertical growth to capture more sunlight. Heterospory and separate sporangia probably evolved in response to competition for light. Disruptive selection within species resulted in there being two separate sexes of gamete or even the whole plant. This may first have led to an increase in spore size and ultimately resulted in the species producing larger megaspores as well as smaller microspores.
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Heterospory evolved from homospory many times, but the species in which it first appeared are now extinct. Heterospory is thought to have emerged in the
Devonian era, mostly in wet/damp places based on fossil record evidence. In addition to being an outcome of competition for light, it is thought that heterospory was more successful in wetter areas because the megaspore could move more easily around in an aquatic environment while microspores were more easily dispersed by wind. Differing sized spores have been observed in many fossilized plant species. For example, the species
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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. This specific type of self-fertilization is termed as sporophytic selfing, and in extant plants it occurs most commonly among
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A complete model for the origin of heterospory, known as the Haig-Westoby model, 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
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Heterospory evolved due to natural selection that favoured an increase in propagule size compared with the smaller spores of homosporous plants. Heterosporous plants, similar to anisosporic plants, produce two different sized spores in separate sporangia that develop into separate male and female
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species, the gametophytes of both sexes are very highly reduced and contained within the spore wall. The microspores of both exosporic and endosporic species are free-sporing, distributed by wind, water or animal vectors, but in endosporic species the megaspores and the megagametophyte contained
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Heterospory is advantageous in that having two different types of spores increases the likeliness that plants would successfully produce offspring. Heterosporous spores can respond independently to selection by ecological conditions in order to strengthen male and female reproductive function.
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Heterospory was a key event in the evolution 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
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spores that in endosporic species contain the male gametophyte, which is carried to the megaspores by wind, water currents or animal vectors. Microspores are not flagellated, and are therefore not capable of active movement. The morphology of the microspore consists of an outer double walled
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contain the female gametophytes in heterosporic plant species. They develop archegonia that produce egg cells that are fertilized by sperm of the male gametophyte originating from the microspore. This results in the formation of a fertilized diploid
131:, also known as the scale tree, has been shown in fossils to have been heterosporous; The scale tree had separate cones containing either male or female spores on the same plant. Modern heterosporous plants such as many ferns exhibit
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seem to be one of the many precursors to seed plants and the ovary. Heterosporic plants that produce seeds are their most successful and widespread descendants. Seed plants constitute the largest subsection of heterosporic
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135:, in which a megagametophyte is fertilized by a microgametophyte all while still inside the spore wall, gaining nutrients from the inside of spore. Both heterospory and
201:, 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
169:, that develops into the sporophyte embryo. While heterosporous plants produce fewer megaspores, they are significantly larger than their male counterparts.
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218:. While heterospory stops extreme inbreeding from occurring, it does not prevent inbreeding altogether as sporophytic selfing can still occur.
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Ancient noeggerathialean reveals the seed plant sister group diversified alongside the primary seed plant radiation
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273:"Heterospory - the most iterative key innovation in the evolutionary history of the plant kingdom"
692:"Structure of Microspores and the Development of Male Gametophyte (= germination of Microspores)"
369:"Heterospory: The Most Iterative Key Innovation in the Evolutionary History of the Plant Kingdom"
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Haig, David; Westoby, Mark (1989-11-01). "Selective forces in the emergence of the seed habit".
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Sussex, I.M. (1966) The origin and development of heterospory in vascular plants. Chapter 9 in
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within are retained and nurtured by the sporophyte phase. Endosporic species are thus usually
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The production of spores of two different sizes and sexes by several groups of land plants
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male function, reproductive success does not change as the minimum spore size increases.
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617:"Origins of Heterospory and the Seed Habit: The Role of Heterochrony"
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197:. Some exosporic species produce micro- and megaspores in the same
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structures surrounding the dense cytoplasm and central nucleus.
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sporophyte or on different sporophytes in dioicous species.
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species, the smaller spores germinate into free-living male
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Biological
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39:) produces the microspores of this heterosporic plant.
27:) produces the megaspores of this heterosporic plant.
672:Raven, Peter H.; Evert, Ray F.; Eichhorn, Susan E.
367:Bateman, Richard M.; DiMICHELE, William A. (1994).
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98:. Four extant groups of plants are heterosporous;
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180:and the larger spores germinate into free-living
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74:independently in several plant groups: the
511:: CS1 maint: location missing publisher (
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90:. This occurred as part of the process of
66:is female. Heterospory evolved during the
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579:Biological Journal of the Linnean Society
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427:Ingrouille, Martin; Eddie, Bill (2006).
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50:of two different sizes and sexes by the
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271:Bateman, R.M.; Dimichele, W.A. (1994).
244:Paleobotany and the evolution of plants
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430:Plants : evolution and diversity
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330:, ed. by E.G. Cutter, Longmans.
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477:Cruzan, Mitchell B. (2018).
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58:. The smaller of these, the
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246:(2nd ed.). Cambridge:
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248:Cambridge University Press
146:Microspores and megaspores
62:, is male and the larger
84:arborescent horsetails
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118:Origin of heterospory
46:is the production of
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779:1988JThBi.134..257H
96:sex differentiation
23:A female pinecone (
807:Plant reproduction
724:Biological Reviews
698:on 8 December 2018
531:Biological Reviews
373:Biological Reviews
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736:10.1111/brv.12304
674:Biology of Plants
543:10.1111/brv.12304
490:978-0-19-088268-6
440:978-0-511-64852-6
257:978-0-521-38294-6
184:gametophytes. In
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35:A male pinecone (
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151:Microspores
112:seed plants
108:Salviniales
100:Selaginella
56:land plants
52:sporophytes
44:Heterospory
499:1050360688
313:2010-12-30
226:References
203:monoecious
199:sporangium
186:endosporic
162:Megaspores
76:clubmosses
60:microspore
744:1469-185X
651:1996-8175
599:1095-8312
551:1469-185X
507:cite book
449:667094262
393:1469-185X
191:dioecious
174:exosporic
137:endospory
133:endospory
92:evolution
64:megaspore
37:Pinophyta
25:Pinophyta
801:Category
752:27730728
702:13 April
559:27730728
401:29709953
300:29709953
72:isospory
68:Devonian
775:Bibcode
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155:haploid
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104:Isoetes
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621:Taxon
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307:(PDF)
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276:(PDF)
80:ferns
748:PMID
740:ISSN
704:2017
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555:PMID
547:ISSN
513:link
495:OCLC
485:ISBN
445:OCLC
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252:ISBN
153:are
110:and
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