The selfing It is the union of the male and female gametes of the same individual. It occurs in organisms that are hermaphrodites - beings that combine male and female functions in a single individual, either sequentially or simultaneously.
When the production of the gametes of both types overlap in time (at least in time), the hermaphrodites are simultaneous. This modality offers the possibility of self-fertilization.
In multicellular organisms, especially plants and animals, being hermaphrodite seems to be a widely distributed phenomenon..
Self-fertilization is an optimal strategy for constant environments with little partner availability. However, it brings some negative consequences, such as depression due to consanguinity.
In this phenomenon, the genetic variability of the population is reduced, which reduces its ability to adapt to environmental changes, resistance to pathogens or herbivores. These aspects seem to be important for the lineage of plants and animals..
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In plants it is common for the same individual to be "the father and mother" of their seeds. Although the main role of flowers is - most likely - to promote cross-fertilization, self-fertilization can exist in hermaphroditic species.
Some examples of plants where this phenomenon occurs are peas (the organism used by Gregor Mendel to develop the basic laws of inheritance, where the self-fertilization event was crucial for the process) and some legumes..
In the case of soybean flowers, for example, the flowers can open to allow cross-pollination by insects or they can remain closed and self-pollinate..
According to Jarne et al. (2006), excluding insects, approximately one third of animal species present the phenomenon of hermaphroditism. This fact has facilitated the evolution of self-fertilization in numerous animal species.
The distribution of self-fertilization rates are similar to those in plants, suggesting that similar processes have operated in both lineages in favor of the evolution of self-fertilization..
For Jarne et al. (2006), hermaphroditism is rare in the phyla of larger animals, primarily arthropods. It is a common phenomenon in smaller phyla, including sea sponges, jellyfish, flatworms, mollusks, sea squirts, and annelids..
These authors found that the self-fertilization event occurs in taxa where gametes (both male and female) are produced in a single site or gland, as occurs in lung snails..
It can also occur in situations where gametes are produced in different places, or when they are expelled into the water, as occurs in marine species..
In some flukes and oligochaetes, self-fertilization occurs after a necessary copulation in the same individual..
There are some advantages of self-fertilization in the short term. First, both the female and male gametes come from the same parental individual..
For this reason, organisms benefit an extra 50% from the transmission of their genes - compared to only the typical 50% contribution of sexual reproduction, since the remaining 50% corresponds to that contributed by the sexual partner..
Self-fertilization can also be favored when the region inhabited by the species in question is characterized by low numbers of potential mates or, in the case of plants, in areas where there is little availability of pollinators..
In addition, in plant species, self-fertilization would lead to energy savings, since the flowers of these plants can be small (they no longer have to be large and visible to attract pollinators) with a limited amount of pollen.
Thus, self-fertilization ensures reproduction and increases the colonization of the area. The most accepted ecological hypothesis to explain the evolution of self-fertilization is related to guaranteeing reproduction.
The main disadvantage of self-fertilization is considered inbreeding depression. This phenomenon implies the reduction of fitness o biological attitude of consanguineous progeny in relation to cross progeny.
For this reason, there are species that, although they are hermaphrodites, have mechanisms to avoid self-fertilization. The main mechanisms will be discussed in the next section..
The current view of the evolution of self-fertilization involves ecological and evolutionary forces. From Fisher's perspective, an interaction between the obvious advantages of self-fertilization and depression due to consanguinity is assumed..
This model predicts the formation of self-fertilization or pure crosses, as a result of disruptive selection (when the extremes of a trait are favored), which does not favor an increase in the frequency of intermediate variants..
In this way, the models propose the evolution of this system as the interaction of its benefits versus its disadvantages..
Ecological models, for their part, propose that intermediate rates of self-fertilization.
It is widely known that sexual reproduction provides immense benefits. Sex increases the genetic diversity of the descendants, which translates into a greater probability that the successors can face greater challenges, such as environmental changes, pathogenic organisms, among others.
In contrast, self-fertilization occurs in certain crop plants and animals. It is suggested that this process ensures that the new individual will develop fully, and is also a viable strategy - although it depends on the species and environmental conditions..
It has been found that in various angiosperms there are mechanisms that prevent self-fertilization in hermaphroditic organisms, complicating in various ways that the flower can fertilize itself.
These barriers increase the genetic variety of the population, since they seek to ensure that the male and female gametes come from different parents..
Plants that present flowers with functional stamens and carpels avoid self-fertilization with the discrepancy of the maturation time of the structures. Another modality is a structural arrangement that prevents the transfer of pollen..
The most common mechanism is self-incompatibility. In this case, the plants tend to reject their own pollen..
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