The heterospory is the development of spores of two different sizes and sexes, in esporofitos of land plants with seeds, as well as in certain mosses and ferns. The smallest spore is the microspore and is male, the largest spore is the megaspore and is female.
Heterosporia appears as an evolutionary sign in some plant species, during the Devonian period from isosporia, autonomously. This event happened as one of the pieces of the evolutionary process of sexual differentiation.
Natural selection is the cause of the development of heterosporia, since the pressure exerted by the environment on the species stimulated an increase in the size of the propagule (any asexual or sexual reproduction structure).
This led to an increase in the size of the spores and, subsequently, to the species producing smaller microspores and larger megaspores.
On many occasions, the evolution of heterosporia was from homosexuality, but the species in which this event occurred for the first time, are already extinct.
Among the heterosporic plants, those that produce seeds are the most common and prosperous, in addition to constituting the largest subgroup.
The process of heterosporia
During this process the megaspore evolves into a female gametophyte, which only produces oospheres. In the male gametophyte, the microspore is produced which is smaller and only produces sperm.
Megaspores are produced in small quantities within megasporangia and microspores are produced in large quantities within microsporangia. Heterosporia also influences the sporophyte, which must produce two types of sporangia.
The first extant plants were all homosporic, but there is evidence that heterosporia appeared several times in the first successors of the Rhyniophyta plants.
The fact that heterosporia has appeared on several occasions suggests that it is a characteristic that brings advantages to selection. Subsequently, the plants became increasingly specialized towards heterosporia.
Both vascularized plants (plants that have root, stem and leaves) that do not have seeds, as well as non-vascularized plants require water in one of the key stages of their life cycle, since only through it, the sperm reaches the oosphere.
Microspores and megaspores
Microspores are haploid cells (cells with a single set of chromosomes in the nucleus) and in endosporic species include the male gametophyte, which is transported to megaspores by wind, water currents, and other vectors, such as animals.
Most microspores do not have flagella, which is why they cannot make active movements to move. In their configuration, they have external double-walled structures that surround the cytoplasm and the nucleus, which is central.
Megaspores have female megaphytes in heterospore plant species and develop an archegonia (female sexual organ), which produces ovules that are fertilized by the sperm produced in the male gametophyte, originating from the microspore.
As a consequence of this, the formation of a fertilized diploid egg or zygote occurs, which will then develop into the sporophyte embryo.
When the species are exosporic, the small spores germinate to give rise to the male gametophytes. The largest spores germinate to give rise to the female gametophytes. Both cells are free-living.
In endosporic species, the gametophytes of both sexes are very small and are located on the wall of the spore. Megaspores and megagametophytes are conserved and fed by the sporophyte phase.
In general, endoscopic plant species are dioecious, that is, there are female individuals and male individuals. This condition encourages interbreeding. For this reason microspores and megaspores are produced in separate sporangia (heterangy).
Heterosporia is a determining process for the evolution and development of plants, both extinct and existing today. The maintenance of the megaspores and the dissemination of the microspores favors and stimulates the dispersal and reproduction strategies.
This adaptability of the heterosporia highly enhances the success of reproduction, since it is favorable to have these strategies in any environment or habitat.
Heterosporia does not allow self-fertilization to occur in a gametophyte, but does not stop gametophytes originating from the same mating sporophyte. This type of self-fertilization is called sporophytic self-fertilization and is common in angiosperms.
To understand the origin of heterosporia, the Haig-Westoby model is used, which establishes a relationship between the minimum spore size and the successful reproduction of bisexual gametophytes.
In the case of female function, increasing the minimum spore size increases the probability of successful reproduction. In the male case, the success of the reproduction is not affected by the increase of the minimum size of the spores.
The development of seeds is one of the most important processes for terrestrial plants. It is estimated that the pool of characters that establish the abilities of the seed are directly influenced by the selective pressures that caused those characteristics.
It can be concluded that most of the characters are produced by direct influence of the appearance of heterosporia and the effect of natural selection.
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