Cyprinodontiformes vivíparos e ovovivíparos

Livebearer Cyprinodontiformes

Biologia > Reprodução ( conceitos básicos )

Biology > Reproduction ( basic concepts )

 Reproduction ( basic concepts )

To science reproduction is the functionality by which living organisms can originate new individuals like, or identical, to them ( descendants ).

Reproduction is the fundament of live because each particular organism only exists as the result of this process.

The known methods of procreation are broadly grouped in two main forms; asexual ( hermaphrodite ) and sexual ( heterosexual ).

Hermaphrodite or monoecious ( monoicous ) are species who opt by a reproductive organization on which an individual can reproduce by himself alone ( without cooperation with another compatible ). This is possible when sexual female and male organs are present in the same organism.

Nevertheless, the majority of fishes are heterosexual or dioecious ( diocious ) organisms.

As a rule this requires the involvement of two individuals, typically one of each sex ( unisexual ones ).

Sometimes hermaphroditism can take place at hetrosexual fish species, however this eventuality is quite rare and results from a teratologic process or, in other words, it happens usually as result from a genetic embryonic malformation or from early fetal stages irregularity.

At hermafroditism the asexual method is predominant. This practice allow an organism to be capable of produce “ copies “ genetically like him without needing other members of his kind cooperation.

Easily we can conclude from this strategy that genetic material combination ( recombination ) is impossible or very atypical, therefore many species had evolutes to get a genetically richer new generation, consequently they have opt for sexual reproduction.

As a rule, sexual reproduction works precisely on the opposite direction of asexual reproduction. More often than not, or to be precisely, almost constantly, this method involve genetic material exchange ( usually DNA ), concerning two independent living beings throughout two cells ( gametes ), one from each one.

Sexual cells are generically known by gametes and are produced by sexual organs called gonads.

These gametes come together during conception in organisms that reproduce sexually. Male sexual cell is recognized by spermatozoon ( sperm cell ), while the female one is the ovum ( egg ).

The process of fertilisation involves a sperm fusing with an ovum, which eventually leads to the development of an embryo, when at least one spermatozoon can win the extra cellular layer surrounding the ovum and penetrates inside. Immediately after this, some alterations take place. The egg external cell membrane will change, preventing other penetrations, as well as further fusions.

Union between the sperm and egg plasma membranes is right next step. This will allow the entry of the sperm nucleus, into the ovum.

The process of each gamete chromosomes fusion ultimately leads to the formation of a cell called a zygote, ( or zygocyte ). This new cell resulting from fertilisation ( fusion of the two gamete nuclei ), has twice the chromosome number from the original gametes, a complete set and therefore considered a diploid one.

Resulting from the above described, from now on we will consider the zygote as a fertilized egg, who contains the both parents genetic heritage. After multiple cell divisions and cellular differentiation, a zygote develops trough meiosis into an embryo, and ultimately into a mature individual capable of producing gametes later.

In meiosis, we have a process of cell division and genetic recombination. The result is that the offspring produced by this way will have slightly different genetic information which has instructions for the cells to work, contained in the DNA.

Fallowing this idea, a hypothetical question about two different species gametes union is quite expected.

Fecundation, as a rule, is an intra-specific matter or, by other words, it can take place only when individuals from the same specie are implicated in sexual production of new organisms of the same kind.

Reproductive independence is even reasonably important at species classification.

Even so, there are a few rare cases when fecundation between two different but relatively close species is possible.

Offspring resulting from these inter-specific fecundation processes are called hybrids.

This is not a common occurrence because, as explained above, the egg ( ovum ) is surrounded by barriers against sperm cells penetration.

When in contact with these obstacles the specie spermatozoon relists various enzymes to penetrate on the egg, so some compatibility among gametes is crucial in order to allow the right access combination. These enzymes work like a kind of “ key “ or “ password “.

Despite that, there is an exclusively inter-specific method of reproduction.

However, some sexual species also demonstrate to be able to take advantage from an asexual practice. This strategy makes use of a method identified as parthenogenesis.

This term is composed from classic Greek original words - παρθενος “ virgin “ + γενεσις “ birth “ “. From these words it’s easy to conclude that we are talking about some way of conception as well as the growth and development of an embryo without fertilization. By other words during parthenogenesis parental gametes from two distinct sexes are not an essential condition to generate new individuals.

A special type of this reproduction approach is gynogenesis.

Species like Poecilia formosa that opt for it are almost exclusively composed from females.

However, these female needs at all times some very close species male’s sperm, required to egg stimulation in order to start the development of it in to an embryo.

Nonetheless, unlike other reproductive process, the sperm cell does not contribute with any genetic material to the offspring. This means that sperm fusing with an ovum in theory never takes place ( usually genetic material exchange never occurs ).

In Poecilia formosa, however extraordinarily and once in a time a true fertilization takes place. This way new gene pool is introduced in this species heritage.