Female reproduction has been a major focus of entomological research for the past century, driven by the need to control populations of insect pests. The central process in female reproduction in insects, the production of eggs, is hormonally regulated. To reproduce successfully, females must coordinate egg production with other aspects of reproduction such as dispersal, the availability of resources, and selection of mates and oviposition sites. Environmental signals are effectively translated into physiological processes by networks of hormonal signals.
Egg development in insects has become a model experimental system studied to understand the general principles of stage-, sex-, and tissue-specific responses to hormones. In insects, juvenile hormone (JH) and ecdysone typically play important roles in orchestrating egg development. The development of improved analytical methods has led to the elucidation of the roles of other key hormones, particularly a variety of neurosecretory hormones. Common themes in the hormonal control of egg production are becoming clearer, as are important differences between insect groups.
PATTERNS OF FEMALE REPRODUCTION
To reproduce successfully, female insects must coordinate feeding, mating, and locating places to lay their eggs. Mating and egg laying almost always take place in the adult stage, but female insects vary widely in the manner in which they feed and accumulate nutrients for egg production. The accumulation of nutrients for eggs takes place during larval or nymphal development, as well as during adult life.
Eggs are generally filled with bulk nutrients to support embryonic development, and so it is not surprising that the timing of egg production is often tightly linked to the timing of feeding. The earlier the nutrients required for egg production are eaten, the earlier the egg production can begin. Starting egg production during the immature stage and completing it before eclosion allows female insects having very short adult lives to focus on mating and laying eggs. In fact, adults in many insects do not feed at all and even lack mouthparts. Mayflies are an example. At the other end of the spectrum, insects may take only sufficient nutrients during the larval stage to support their own larval or nymphal development. As adults, they must compensate for the lack of stored reserves by additional feeding. Female mosquitoes that emerge ready to find a blood meal are one example of this pattern. The relative advantages of these contrasting strategies depend on the opportunities for acquiring nutrients, as well as other ecological factors that exist in both the immature and adult habitats. When egg development is telescoped into preadult stages, hormonal signals that control larval or nymphal development and metamorphosis must be coordinated with those of ovarian and reproductive development. Not surprisingly, setting the timing of egg development to different points relative to preadult development requires different hormonal controls.
Female insects that feed as adults differ in the size and number of their meals. At one extreme, females feed fairly continuously. At the other extreme, females take very large meals and have long periods of fasting between them. In blood-feeding insects particularly, a single blood meal can supply the bulk of nutrients necessary for a batch of eggs. Hormones signal the results of feeding to ensure that egg development is matched to an adequate supply of nutrients.
Egg development begins in the germarium, when stem germ cells produce daughter cells that become oocytes. Depending on the type of egg development, oocytes occur alone or in association with sister nurse cells, and follicle cells surround either the oocyte or the oocyte-nurse cells complex. Both nurse cells and follicle cells make and transfer materials important for future embryonic development to developing oocytes. In addition, vitellogenins, fats, and carbohydrates are all taken up into the egg, mostly from the blood. Most or all of the vitellogenin is made by the fat body, a complex organ analogous to the vertebrate liver. Fat body cells release yolk proteins into the blood from which they are taken up by rapidly growing eggs.
When uptake of nutrients is complete, the follicle cells secrete egg coverings in preparation for oviposition.
Hormonal control of egg production is well understood in only a few species. Nevertheless, enough is known about a wide variety of species to infer some general patterns. Four checkpoints in ovarian development are commonly regulated by hormonal signals: (1) the formation of new oocytes by stem cells in the germarium, (2) initial growth of the oocyte, (3) vitellogenin synthesis by the fat body, and (4) vitellogenin uptake by the oocyte. The hormonal signals that break these checkpoints reflect various aspects of the external environment and internal conditions.
HORMONES
The two major hormones that control female reproduction, ecdys-one and JH, also control preadult development and metamorphosis. In adult females, ecdysone is produced by ovaries, rather than by the prothoracic glands as in nymphs and larvae. JH is produced by a pair of glands called the corpora allata in both preadult and adult stages. The corpora allata are located near the brain and are connected to it by tracts of neurosecretory cells. Neurosecretory hormones made in the brain and in other parts of the central nervous system are also important controlling factors. Nerves are also part of control networks, especially in relaying sensory information to the brain cells, which affect the hormone-producing organs.
EXAMPLES OF HORMONAL CONTROL OF EGG PRODUCTION
Hemimetabolous Insects That Feed Continuously
Migratory locusts, Locusta migratoria, feed almost continuously if they can, and females produce and lay eggs in batches. The importance of JH in controlling egg production in this species has long been recognized. Shortly after eclosion, JH levels rise and stimulate synthesis of vitellogenin by the fat body. Mating and plant odors also affect JH level through neurosecretory cells in the brain. JH alone, however, is insufficient to complete a batch of eggs. An ovary-maturing parsin (OMP), which is a neurohormone, is required, in addition to JH, to stimulate sufficient vitellogenin synthesis and uptake.
In the viviparous Pacific beetle cockroach, Diploptera punctata, JH also regulates vitellogenin synthesis and uptake. Signals associated with mating and pregnancy result in increased and decreased levels of JH, respectively. Mating stimulates the release of JH through mechanical stretch of the reproductive tract. The signal is transmitted to the brain by nerves. After mating, females produce a set of eggs and retain them in a pouch off the oviduct called the brood chamber until embryonic development is complete. After the hatching and deposition of nymphs, the female can reproduce again. As in many insects, lack of sufficient nutrients causes JH levels to fall, which delays production of the next batch of eggs.
Hemimetabolous, Blood
Feeding Insects
The bloodsucking bug Rhodnius prolixus takes blood meals throughout its life. For nymphs, blood meals are required for growth and molting; for adults they are needed for egg production. In adult females, abdominal stretching associated with feeding stimulates the release of a peptide hormone from the thoracic ganglion. As a result, the corpora allata release JH, which causes vitellogenin synthesis by the fat body and vitellogenin uptake by growing oocytes. Only the terminal, largest oocyte in each ovariole develops, however. Nerves that stretch as the ovaries grow secrete a neurohormone, called an oostatic hormone, that prevents smaller oocytes from taking up vitellogenin. The result is a synchronously produced batch of eggs. As the eggs mature, the ovary secretes ecdysone, which triggers the release from the brain of a neurohormone that stimulates contractions for laying the eggs.
Lepidoptera: Holometabolous Insects
That Vary in Timing of Egg Production Lepidoptera are holometabolous insects, which means that pread-ults (larvae) and adults differ greatly in form, function, and ecology. Larvae are specialized for feeding, and adults are specialized for dispersal and reproduction. Despite the tidy appearance of such discrete stages, an extensive part of egg production can occur before the adult stage. Enough Lepidoptera species have been studied to allow some appreciation of the variation in egg production patterns across the group. Development and egg production are orchestrated by the same toolbox of hormones, so it is not surprising to find that Lepidoptera in which development and egg production occur simultaneously have control networks different from those in which these processes are sequential.
At one end of the spectrum, females complete egg production before eclosion. Adults of these species do not feed; they mate immediately after eclosion and are short lived. Examples are silkworms (Bombyx mori) and gypsy moths (Lymantria dispar). JH does not stimulate egg production and can even inhibit some aspects of it. Pulses of ecdysone associated with pupal development apparently stimulate vitellogenin synthesis and uptake.
In some moths, only part of egg production is completed before eclosion. Examples include pyralid moths such as the Indian meal moth (Plodia interpunctella) and the southwestern corn borer (Diatraea gran-diosella). Synthesis and uptake of vitellogenin take place before the molt to adult, and egg coverings are added afterward by the follicle cells. The portion of egg development that occurs after eclosion can be controlled by JH.
Finally, in many Lepidoptera, such as the monarch butterfly (Danaus plexippus), development and egg production are sequential and nonoverlapping, and hormones regulate egg development during the adult stage. In these insects, JH typically stimulates both vitellogenin synthesis and uptake.
Holometabolous, Blood-Feeding Insects
Hormonal control of egg production has been studied extensively in Aedes aegypti, the yellow fever mosquito, spurred by the ability of these and other mosquitoes to transmit diseases. Most mosquitoes must drink blood to obtain sufficient amino acids to make eggs. Females in a few species of mosquitoes have the ability to carry over reserves from the larval stage to make part or all of entire first batch without blood feeding.
Eclosion of female mosquitoes is usually followed closely by an increase in JH titer. The rise in JH induces target tissues to become responsive to hormonal signals that occur later. Specifically, the ovaries become sensitive to ovarian ecdysiotropic hormone (OEH), the primary oocyte grows slightly, and the fat body becomes responsive to ecdysone. The posteclosion peak of JH also causes behavioral changes: females search first for mates and then for hosts. Egg development is blocked until a blood meal can be taken. This checkpoint ensures that the intensive metabolic activity required for making eggs will not occur unless enough nutrients are available. When the mosquito has found a sufficiently large blood meal or series of meals, OEH is released into the blood by neurosecretory cells in the brain. OEH stimulates the ovary to produce ecdysone, which then stimulates the fat body to synthesize vitellogenin and the ovary to take it up. The ecdysone peak also causes new, secondary oocytes to separate from germaria, the first step in preparing for the next cycle of egg production. Growth of the primary oocyte ends at about 36 h after blood feeding. JH begins to rise again, and this stimulates the secondary oocytes to grow slightly and become ready for the next round of egg production. Finally, the eggshell is produced, the eggs are laid, and the mosquito is then ready to seek another blood meal.
Stimulation of vitellogenin synthesis by ecdysone has been studied at the molecular level. Analyses of the nucleotide sequence upstream of the vitellogenin gene show several broad regions of control. First, closest to the gene, is a binding site for ecdysone bound to its receptor complex. Binding to this site is necessary for expression. Second, more distally, are binding sites for two transcription factors, E74 and E75, that are known to be expressed quickly in response to an ecdysone signal. Therefore, the regulatory region of the gene has both direct and indirect interactions with ecdysone.
Holometabolous, Feeding Continuously
In contrast to mosquitoes, the fruit fly Drosophila melanogaster feeds continuously, and eggs develop serially as resources become available. The chain of oocytes in each ovariole includes a range of developmental stages. A balance between JH and ecdysone levels controls how many eggs produced.
In D. melanogaster, yolk proteins are made not only by the fat body but also by ovarian follicle cells. JH and ecdysone stimulate synthesis in the fat body, but only JH stimulates follicle cells.
JH appears to enhance egg maturation, whereas ecdysone inhibits it. The antagonism in the effects of the two hormones can be seen by manipulating the amount of food and mating. When food is withheld, JH level drops relative to ecdysone levels and oocytes on the verge of taking up yolk proteins die. In addition, formation of new oocytes is reduced as more die than progress. Providing sufficient food again increases JH levels and reduces cell death. Mating enhances oocyte progression. Sex peptide, a hormone-like substance in seminal fluid, stimulates a rise in JH. Therefore, the increase in egg production seen after mating seems to act through the same hormonal mechanism used in nutrition. In this way, a sexually mature virgin female will not produce new eggs if she lacks either food or a mate. The two points of cell death, at separation of oocytes from the germarium and at the initiation of yolk protein uptake, correspond with similar checkpoints in mosquitoes.
HORMONAL LINKS BETWEEN EGG PRODUCTION, RECEPTIVITY, OVIPOSITION, AND PARENTAL CARE
Hormonal control of egg production in insects can be linked to other activities or states related to reproduction such as receptivity to mating, feeding, oviposition, and migration. In mosquitoes and D. melanogaster, receptivity and feeding are associated with an increase in JH level, which triggers early events in the production of eggs. In other insects, however, JH can be inhibitory or have no effect.
Oviposition, too, can be under hormonal control. Oviposition by L. migratoria requires extreme extension of the abdomen. Muscles in the appropriate segments respond to JH by altering their contractile properties so that they can function when greatly extended.
Adult insects are often specialized for migration in addition to reproduction. These two functions can create a conflict between protein needs for flight muscles and eggs, and between lipid and carbohydrate needs for flight fuel and eggs. Hormonal coordination can minimize such physiological conflicts. Both the large milkweed bug, Oncopeltus fasciatus, and the boll weevil, Anthonomus grandis, use JH to stimulate egg production and long-duration flights. Stimulation of flight apparently requires low levels of hormone, whereas higher levels stimulate oogenesis but inhibit flight. High levels of JH in some insect species lead to both yolk uptake and breakdown of flight muscle.
Burying beetles (Nicrophorus orbicollis) use carcasses of small vertebrates for their own food, for an oviposition site, and as food for their offspring. Immediately after females have found a suitable dead body, their JH levels rise and egg production is stimulated. JH cannot, however, stimulate yolk uptake in the absence of a carcass, indicating that other factors are important in integrating the discovery of the food source and oviposition site with egg production. Juvenile hormone surges even higher in females when the larvae hatch. Egg production is not stimulated by this rise, which is necessary to stimulate and maintain parental care of developing larvae.
SUMMARY
Female reproduction in insects is controlled in diverse ways. The most fundamental process, egg production, is linked to other aspects of reproduction and reproductive behaviors by hormonal controls. Common themes are the central roles of JH and ecdysone, with control points modulated by neurohormones. Hormonal control of egg production is relatively well understood in a few insects because of its use as a model system for studying hormone action. The integration of hormonal controls with other aspects of reproduction is more complex and is less well understood.
