Female sperm storage

Female sperm storage is a biological process and often a type of

• Supporting the sperm by: a.) enabling sperm to undergo biochemical transitions, called capacitation and motility hyperactivation, in which they become physiologically capable of fertilizing an oocyte (e.g. mammals)^[5][6] and b.) maintaining sperm viability until an oocyte is ovulated (e.g. insects and mammals).^[5][7]
• Decreasing the incidence of polyspermy (e.g. some mammals such as pigs).^[5][8]
• Enabling mating, ovulation and/or fertilization to occur at different times or in different environments (e.g. many insects and some amphibians, reptiles, birds and mammals).^[9][10][11]
• Supporting prolonged and sustained female fertility (e.g. some insects).^[12][13]
• Having a role influencing offspring sex ratios among some insects possessing a
  haplodiploid sex-determination system (e.g. ants, bees, wasps and thrips as well as some true bugs and some beetles).^[14][15][16]
• Serving as an arena in which sperm from different mating males compete for access to oocytes, a process called sperm competition, and in which females may preferentially utilize sperm from some males over those of others, called female sperm preference or cryptic female choice (e.g. many invertebrate animals, birds and reptiles).^[17][18][19]

Increased diversity of offspring

One important advantage female insects that store sperm gain is increased genetic diversity in their offspring. There are many ways that females can alter offspring genetics to increase their success and diversity. An example of how this can be accomplished is in female Scathophaga that preferentially fertilize eggs to survive in different environments. Since many environments require different traits for success, females are somehow able to match sperm (acquired from multiple mates) that have the best genes for whichever environment in which they will develop.^[20] Many of the different properties of the environment, including temperature and thermal properties affect the female's sperm choice.^[21] Studies have also shown that ovipositing is nonrandom and females lay eggs with varying PGM(phosphoglucomutase) genotypes in different environments in order to optimize offspring success. Females are acutely aware to their environment and manipulate the genetic diversity of their offspring in appropriate ways to ensure their success.

Another way sperm-storing females can alter the diversity of their offspring is controlling the relatedness to the males that provide them with sperm. Inbreeding depression can have a deleterious impact on populations of organisms and is due to mating of closely related individuals. To combat this effect, female insects appear to be able to sort out the sperm of relatives from the sperm of non-relatives to choose the best option to fertilize their eggs. Female crickets are able to preferentially store sperm of multiple unrelated males to that of their siblings; this results in more of the offspring having unrelated parentage. Being able to choose between sperm after coupling might be advantageous to females because choosing between mates precopulation may be more costly, or it may just be too difficult to tell males apart before mating.^[22] Females possess remarkable abilities to select sperm to store and to fertilize their eggs that will make their offspring diverse. Though it has been shown that a majority of female insect species can store sperm, specific examples that have been studied could include field crickets,^[22] dung flies ^[21] and Mediterranean fruit flies.^[23] Females largely benefit from this mechanism, but males often can experience low fitness because their paternity is not secure.

Sperm stored often determines which male ends up fertilizing the most eggs. An example of this is seen in Red Flour Beetles, where most of the sperm is stored in the first mating. Another male can remove previously stored sperm to better chances of successful mating.

Antagonistic coevolution

Antagonistic coevolution is the relationship between males and females where sexual morphology changes over time to change with the opposite's sex traits in order to achieve the maximum reproductive success.

Like females, males have developed responses to counter evolutionary adaptations of the opposite sex. Responses in insects can vary in both genitalia and sperm structures, along with variations in behavior. Spiny male genitalia help to anchor the male to the female during copulation and remove sperm of previous males from female storage structures.[27] Males have also developed alternative ways to copulate. In the case of the bed bug, males traumatically inseminate females, which allows faster passage of sperm to female sperm storage sites.^[28] The sperm are received by the mesospermalege and eventually reach the spermathecae, also referred to as seminal conceptacles.

At the microscopic level, Drosophila males have differing sperm tail lengths that correspond to various sizes of female seminal receptacles.^[29] Longer male sperm tail length has shown a greater reproductive success with a larger female seminal receptacle while sperm with short tail lengths have been found to be more successful in smaller seminal receptacles.

Cryptic female choice

The ability to store and separate sperm from multiple males enables females to manipulate paternity by choosing which sperm fertilize their eggs, a process known as cryptic female choice. Evidence for this ability exists in many different taxa, including

The female insect nervous system affects many of the processes involved in sperm storage. The nervous system may signal for muscular contractions, fluid absorption, and hormone release, all of which aid in moving the sperm into the storage organs.^[44] When the nervous system of female fruit flies (Drosophila melanogaster) was replaced with a masculinized nervous system through genetic manipulation, sperm storage was affected suggesting that the female nervous system is unique and required to store sperm properly.^[45]

The nervous system is responsible for several fertilization methods. In the