Female sperm storage
Female sperm storage is a biological process in which sperm cells transferred to a female during mating are temporarily retained within a specific part of the reproductive tract before the oocyte
An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...

, or egg, is fertilized. The site of storage is variable among different animal taxa and ranges from structures that appear to function solely for sperm retention, such as insect spermatheca
The spermatheca , also called receptaculum seminis , is an organ of the female reproductive tract in insects, some molluscs, oligochaeta worms and certain other invertebrates and vertebrates...

 and bird sperm storage tubules (bird anatomy
Bird anatomy
Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capable of very high metabolic rates and oxygen supply,...

), to more general regions of the reproductive tract enriched with receptors to which sperm associate before fertilization, such as the caudal portion of the cow oviduct containing sperm-associating annexin
Annexin is a common name for a group of cellular proteins. They are found in all kingdoms with the exception of the bacteria....

s. Female sperm storage is an integral stage in the reproductive process for many animals with internal fertilization. It has several documented biological functions including:
  • Supporting the sperm by: a.) enabling sperm to undergo biochemical transitions, called capacitation
    Capacitation is the penultimate step in the maturation of mammalian spermatozoa and is required to render them competent to fertilize an oocyte.This step is a biochemical event; the sperm move normally and look mature prior to capacitation....

     and motility hyperactivation
    Hyperactivation is a type of sperm motility. Hyperactivated sperm motility is characterised by a high amplitude, asymmetrical beating pattern of the sperm tail...

    , in which they become physiologically capable of fertilizing an oocyte (e.g. mammals) and b.) maintaining sperm viability until an oocyte is ovulated (e.g. insects and mammals).
  • Decreasing the incidence of polyspermy
    In biology, polyspermy describes an egg that has been fertilized by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome -- one from the...

     (e.g. some mammals such as pigs).
  • 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).
  • Supporting prolonged and sustained female fertility (e.g. some insects).
  • 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).
  • Serving as an arena in which sperm from different mating males compete for access to oocytes, a process called sperm competition
    Sperm competition
    Sperm competition is a term used to refer to the competitive process between spermatozoa of two different males to fertilize an egg of a lone female. Competition occurs whenever females engage in promiscuous mating to increase their chances in producing more viable offspring...

    , 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).

Increased diversity of offspring

One important advantage female insects that store sperm gain is increased genetic diversity
Genetic diversity
Genetic diversity, the level of biodiversity, refers to the total number of genetic characteristics in the genetic makeup of a species. It is distinguished from genetic variability, which describes the tendency of genetic characteristics to vary....

 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
The genus Scathophaga are small to medium sized predatory flies that for the most part, have larvae that feed on other insect larva within animal dung or decaying vegetable matter. Many are highly variable, sometimes producing small, infertile males that superficially resemble females...

 that preferentially fertilizing 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. Many of the different properties of the environment, including temperature and thermal properties affect the female's sperm choice. Studies have also shown that ovipositing is nonrandom and females lay eggs with varying PGM(phosphoglucomutase
Phosphoglucomutase is an enzyme that transfers a phosphate group on an α-D-glucose monomer from the 1' to the 6' position in the forward direction or the 6' to the 1' position in the reverse direction....

) 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
Inbreeding depression
Inbreeding depression is the reduced fitness in a given population as a result of breeding of related individuals. It is often the result of a population bottleneck...

 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. 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, dung flies and Mediterranean fruit flies. Females largely benefit from this mechanism, but males often can experience low fitness because their paternity is not secure.

Antagonistic coevolution

Antagonistic co-evolution is the relationship between males and females where sexual morphology changes over time to counteract the opposite's sex traits in order to achieve the maximum reproductive success. This has been compared to an arms race between sexes. In many cases, male mating behavior is detrimental to the female's fitness. For example, when insects reproduce by means of traumatic insemination
Traumatic insemination
Traumatic insemination, also known as hypodermic insemination, is the mating practice in some species of invertebrates in which the male pierces the female's abdomen with his penis and injects his sperm through the wound into her abdominal cavity . The sperm diffuse through the female's hemolymph,...

, it is very disadvantageous to the female's health. During mating, males will try to inseminate as many females as possible. However, the more times a female's abdomen is punctured, the less likely she is to survive. Females that possess traits to avoid multiple matings will be more likely to survive, resulting in those morphologies being retained in future generations. In males, genitalia are relatively simple and more likely to vary among generations than female genitalia. This results in a new trait that females have to counter in order to survive.

Females who possess traits where they can lessen the impacts of male behavior will be more likely to survive and reproduce. There are many methods that females have evolved over time to "defend" themselves against the onslaught of potential mates. One such development is alternative sperm storage sites, such as seminal receptacles, spermathecae, and pseudospermathecae, that are complex and extremely variable to allow for more choice in sperm selection. In some cases, sperm storage sites can produce proteases that break down various proteins in male seminal fluid resulting in female selection in sperm.

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. 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. In addition, shorter male developmental times allow them to emerge before females, eliminating females' mating choice. At the microscopic level, Drosophila males have differing sperm tail lengths that correspond to various sizes of female seminal receptacles. 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 birds, reptiles, gastropods, arachnids, and perhaps most-notably, insects.

Cryptic choice allows females to preferentially choose sperm. Females are thus able to mate multiple times and allocate sperm to their eggs according to paternal phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

, or according to other characteristics. In some cases, such as in the yellow dung fly, certain male traits will affect the fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...

 of eggs laid in particular environmental conditions. Females can choose sperm based on male quality as a function of its interaction with the environment. In other species, such as the fly Dryomyza anilis, females preferentially choose sperm from one storage location over another. Males of this species have developed behaviors, such as abdominal tapping, to increase their number of sperm stored in the favored storage site. Evidence for this pattern of storage, cryptic choice, and male behavior also exists in the flour beetle Tribolium castaneum
Red flour beetle
The red flour beetle is a tenebrionid beetle. It is a worldwide stored product pest.Red flour beetles attack stored grain products causing loss and damage...


Female muscular contractions

Muscle contraction as a means of moving spermatozoa through the reproductive system into and out of the storage structures has been examined in Diptera
Diptera , or true flies, is the order of insects possessing only a single pair of wings on the mesothorax; the metathorax bears a pair of drumstick like structures called the halteres, the remnants of the hind wings. It is a large order, containing an estimated 240,000 species, although under half...

, Orthoptera
Orthoptera is an order of insects with paurometabolous or incomplete metamorphosis, including the grasshoppers, crickets and locusts.Many insects in this order produce sound by rubbing their wings against each other or their legs, the wings or legs containing rows of corrugated bumps...

, and Lepidoptera
Lepidoptera is a large order of insects that includes moths and butterflies . It is one of the most widespread and widely recognizable insect orders in the world, encompassing moths and the three superfamilies of butterflies, skipper butterflies, and moth-butterflies...

 as well as in the species Rhodnius prolixus
Rhodnius prolixus
Rhodnius prolixus is the second most important triatomine vector of the Chagas parasite due to its efficient adaptation to the human domicile in northern South America, where sylvatic populations also exist, and in Central America where it is exclusively domestic...

and the boll weevil
Boll weevil
The boll weevil is a beetle measuring an average length of six millimeters, which feeds on cotton buds and flowers. Thought to be native to Central America, it migrated into the United States from Mexico in the late 19th century and had infested all U.S. cotton-growing areas by the 1920s,...

. In R. prolixus, rhythmic peristaltic contractions of the oviduct cause contractions of the bursa copulatrix and spermatheca
The spermatheca , also called receptaculum seminis , is an organ of the female reproductive tract in insects, some molluscs, oligochaeta worms and certain other invertebrates and vertebrates...

 movement. This movement of the spermatheca results in spermatozoa migration into the spermathecal duct and into the spermatheca reservoir. In the boll weevil, contractions are also used to move sperm from the spermathecae so they can fertilize the egg. It has been observed in locusts, that the nervous system initiates female reproductive muscular contractions. In some species, such as R. prolixus, the contractions that move spermatozoa into sperm storage are initiated by a male secretion in the ejaculate. Male secretions, such as the glycoprotein
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...

 ACP36D in Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

, can also play a role in preparing the female reproductive system for sperm storage. It causes changes in uterine shape allowing spermatozoa access to the sperm storage organs.

Female insect nervous system

The female insect nervous system affects many of the processes involved in sperm storage. The nervous system may signal for muscular contractions
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...

, fluid absorption, and hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

 release, all of which aid in moving the sperm into the storage organs. 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.

The nervous system is responsible for several fertilization methods. In the migratory locust
Migratory locust
The migratory locust is the most widespread locust species, and the only species in the genus Locusta. It occurs throughout Africa, Asia, Australia and New Zealand. It used to be common in Europe but has now become rare there...

 (Locusta migratoria), the presence of an egg in the genital chamber results in an increase of spermathecal contractions. As a result, sperm is released to fertilize the egg. A neural loop (from the VIIIth ganglion
In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....

 through the N2B nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...

 to N2B2, N2B3, N2B4, and N2B6b nerves) is then activated to direct the sperm to fertilize the egg via muscular contractions. In the Caribbean fruit fly (Anastrepha suspensa), both the spermathecae and their ducts are innervated by an abdominal ganglion located under the first abdominal sternite. This location suggests that the sperm receptacle can compress and influence the amount of sperm taken in or released by the female fly.
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