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Pregnancy is always is the stage when developing embryos are nurtured in the body after the egg unites with the sperm. Researchers do not use the word pregnancy for non-mammals to recognize the viviparity in eutherians. However, there are similarities between viviparity in mammals and other vertebrates. Syngnathids have an exceptional system of reproduction in which the males incubate developing broods in specialized brood pouches. There is a possibility that the evolution of the brood pouch was partly as a result of the parent and the brood offspring or sexual conflict. This paper presents the evolution of the brood pouches in male seahorses.
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Male pregnancy is a phenomenon found only in Syngnathids, this is made possible due to the detailed brood pouch into which the female drops eggs when copulating. These pouches are not only offspring bags but they also provide airing, security, and nourishment to the developing embryo during pregnancy. Studies have it that nutrients shift from the male and the brood and from the brood to the male thus resulting the unexplained likelihood that the brood pouch may have a responsibility in altering sexual selection or causing sexual conflict after copulation. The conflicts occur as a result of the male withdrawing resources from some broods to keep them for other pregnancies in the future. Another possibility is that the females may be transferring materials during mating that arouse the male to provide more resources in the present pregnancy. Even though the male and female-mediated effects are the possible drivers of sexual conflicts, the function of the brood pouch in such a process would bring a different perception concerning adaptive means accountable for the evolution of male pregnancy and the interaction between the parents and the offspring interactions in Syngnathids.
Evolutionary theory indicates that living things should try to exploit success of reproductive achievement by controlling resources and sex mates and optimizing costs and advantages of parental care. Female parental care is much better than males in most vertebrates but this model is different in fishes where males provide both genetic investment and provide most of the parental care. The hypervariable major histocompatibility complex is the suitable model which best explains the significance of natural selection in determining genetic diversity (Somer et al., 2012). This model is an important part of the adaptive immune system for vertebrate which is responsible for destroying the infected cells and the response of the antibody. The MHC molecules are represented by two main antigens which have different functions, structure and expression patterns. Natural selection of MHC is influenced by pathogens thus resulting in selection balance that operate on the PBR of the MHC genes. The MHC is important as a vital part of the adaptive immune system and its suggested cues of odor are significant in choice of mate, in avoiding recognition and inbreeding.
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Sexual selection can be directly contributed to the difference in MH through disassortative choice of mate. Previous studies have indicated that the structure of MH allelic diversity influences natural and sexual selection in species whose females are the ones that make their sex mate choices, no studies have explored MH variation in sex-role reversed kinds where the male make mating decisions (Paczolt & Jones,2010). Males and females have different abilities to perceive smell indications and difference in sexes in the production, dispensation, and application of mediated signals of MH are thought to manipulate the virtual effectiveness of sexual selection in reversed sex roles and THE species that mate conventionally, thus plummeting the variation of MH level in type with reversed sex-roles (Avise & Nicholson, 2013). The reversal of sex roles has changed severally independently in the Syngnathidae group. Studies of the seahorses that have pot like bellies established proof of female competition and the choice made by the male concerning their sex mates, signifying that natural populations of this kind are reversed sex roles.
Studies indicate that new group come up from physically and hereditarily secluded populations. However, seahorses are said to have diversified in a development known as sympatric speciation in which new types come up from a group without geographical barriers to restrain gene flow. There is a possibility that sympatric speciation may have taken place in seahorse. The idea of sympatric speciation process in the seahorses is evident in the manner in which these species have size-similar mating practice imposed by the male (Sagebakken, Ahnesjo, & Kvarnemo, 2016). These fishes choose similar- size mates to ensure successful reproduction. Male raye of reproduction, the size of the pouch and the quantity of eggs that the female horses produce is dependent on the size of the seahorse. The male seahorses prefer the larger female as this will ensure increase the survival of the brood. First, the amount of nutrients appears to be controlled or selected by the male after the eggs have been deposited. In Paczolt & Jones (2010) experiment when a male was provided with a large female to mate the amount of brood that survives are larger. One problem with this example could be the nutrients of the first possible mating being taken resulting in the second being smaller due to lack of nutrition. However, the experiment results remained the same when the smaller female of first to mate and the larger was second. This indicates an ability to select which brood is more favorable dependent on male preference and female traits.
This process and the kind of pre-copulatory selection are as a result of sympatric speciation. Pregnancies in the seahorses have different variations depending on the quantity of eggs shifted during copulation and the surviving chances of the eggs in the brood pouch thus setting the stage for the sexual selection after copulation. According to the previous studies, it is evident that sexual selection after the copulation strengthens precopulatory sexual selection in the seahorses (Paczolt & Jones, 2010). This indicates that even though the sexual selection in seahorses is determined by the type of the female that deposits the eggs in the males, other factors such as the number of eggs produced also matter. The males that have their pouches filled with the eggs tend to brood more young ones to term compared to those whose pouches are partially filled. It is therefore important for the females to produce more eggs to increase chances of survival.
Investment done by the parents of the sexes in young ones is another aspect that is accountable for sexual selection. The convolution of the structures of brooding indicates the level of parental investment in the Syngnathidae species predicts that sex roles reversal is seahorses indicate several change in the group sex roles. However, this does not depend on the development of the brood pouch (Paczolt & Jones, 2010). The reversed sex roles may be as a result of polygamous mating patterns thus this explains the sex-roles among the seahorses. Sometimes several female seahorses tend to search for the male to mate with but this does not occur in other animals. This may the cause of the reversed sex roles (Sagebakken et al., 2016). In the process, the female may transfer matter during copulation that makes the males to provide supplementary resources in the pregnancy. If this occurs the young ones born may be of the same traits as the females. However, the other young ones got after the first brooding may not inherit the same traits due to the investments on the former brood.
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Sexual selection may of benefit to the brood and the species as there will always healthy brood who can survive and this can help the species to exist for long. However, if there is no sexual selection, the seahorses may brood unhealthy young ones. The broods may also fail to survive thus resulting in reduced population hence extinction. The females may play a role in the sexual selection but the ones that play the biggest role are the males. Their pouches and the manner in which they select the females to mate with indicates that the survival of the broods depend on their selection and the type of pouch they have. The females are just like egg donors but the survival of the brood relies on the males. However, the size of the female also matters since it is said that the largely sized females give more eggs than the small ones. Apart from the size of the female seahorses, there is nothing in them that can determine the survival of the brood.
From the studies, it is evident that the brood pouch in the male seahorses has a role in the sexual selection or causes sexual conflict in post-copulation. Restraints forced by male pregnancy result in a state where seahorse offspring will diverge rapidly if a selective condition arise. This means that the male pregnancy signifies a unique form of parental care with unusual evolutionary results both in regards of reversal in sexual selection and a predisposition for sympatric speciation. Selection in seahorses that have male-pregnancy appears to be an ability created by properties of the brood pouch. I believe this is very important to our understanding of evolution and how natural selection work. Seemingly the only reason for male seahorse to care for the brood is to have control in what traits are selected to be passed on. I think the reversal of selection shows that while not always true the primary parent, as in the parent with the ability to give birth, has natural control on the selection of evolution in that species. This could also be true in other species and this sex-role reversal in seahorses brings that trait to light.
This research is important as it has enlightened me on several issues I was not aware of before. This is a topic that had never crossed my mind to look into but after the research, I have the answers to many of the questions that I kept wondering about. This research can also be of help to those who want to conduct studies on this topic. There is still a need for the researchers to conduct research to explain more on this reverse sex-role phenomenon in the Syngnathids. Several theories and studies have attempted to explain the sex-roles reverse in seahorse however, there is still need to conduct more research to understand why this is so. The reversed sex roles in seahorses is a phenomenon that requires a more understanding since it not much clear as to what is the main cause of the sexual selection among the seahorse. This study of this topic is not just educative but it also raises curiosity. One may wonder why this type of pregnancy only occurs among the in Syngnathids and not other mammals. However, the main explanation for this phenomenon is the type of brood pouch among the male seahorses.
- Avise, J. C., & Nicholson, T. H. (2013). Evolutionary perspectives on pregnancy. New York: Columbia University Press.
- Paczolt, K. A., & Jones, A. G. (2010). Post-copulatory sexual selection and sexual conflict in the evolution of male pregnancy. Nature, 464(7287), 401-404. doi:10.1038/nature08861
- Sagebakken, G., Ahnesjö, I., & Kvarnemo, C. (2016). Costs and Benefits to Pregnant Male Pipefish Caring for Broods of Different Sizes. Plos ONE, 11(5), 1-13. doi:10.1371/journal.pone.0156484
- Sommer, S., Whittington, C. M., & Wilson, A. B. (2012). Standardised classification of pre-release development in male-brooding pipefish, seahorses, and seadragons (Family Syngnathidae). BMC Developmental Biology, 12(1), 39-44.