Entomology 3rd edition - C.Gillott - Chapter 19 ppsx

Gillott, 2002, Insect accessory reproductive glands: Key players in production and protection of eggs, in: Chemoecology of Insect Eggs and Egg Deposition M.. Gillott, 2002, Insect access

Reproduction and Development

ff orable conditions As reproduction is almost always sexual in insects, there arise withininsect populations large numbers of genetic combinations, as well as mutations, which can

be tested out in the prevailing environmental conditions As these conditions change withtime, insects are able to adapt readily, through natural selection, to a new situation Over theshort term their high reproductive capacity enables insects to exploit temporarily favorableconditions, for example, availability of suitable food plants The latter requires that both thetiming of mating, egg production, and hatching, and the location of a suitable egg-layingsite must be carefully “assessed” by an insect

Like other terrestrial animals insects have had to solve two major problems in nection with their reproductive biology, namely, the bringing together of sperm and egg

con-in the absence of surroundcon-ing water and the provision of a suitable watery environment con-inwhich an embryo can develop The solution to these problems has been the evolution ofinternal fertilization and an egg surrounded by a waterproof cover (chorion), respectively.The latter has itself created two secondary problems First, because of the generally im-permeable nature of the chorion, structural modifications have had to evolve to ensure thatadequate gaseous exchange can occur during embryonic development Second, the chorion

is formed while an egg is still within the ovarian follicle, that is, prior to fertilization,which has necessitated the development of special pores (micropyles) to permit entry ofsperm

2 Structure and Function of the Reproductive System

The external structure of male and female reproductive systems has been dealt with inChapter 3 (Section 5.2.1), so that only the structure of internal reproductive organs will bedescribed here

561

Though details vary, the female system (Figure 19.1) essentially includes a pair ofovaries from each of which runs a lateral oviduct The lateral oviducts fuse in the midline,and the common oviduct typically enters a saclike structure, the vagina In some speciesthe vaginal wall evaginates to form a pouchlike structure, the bursa copulatrix, in which

FIGURE 19.1. Representative female reproductive systems (not to scale) (A) Melanoplus sanguinipes (Orthoptera); (B) Rhodnius prolixus (Hemiptera); (C) Periplaneta americana (Dictyoptera); and (D) Nasonia

vitripennis (Hymenoptera) Abbreviations: BC, bursa copulatrix; CA, calyx; CG, collateral (accessory) glands;

CO, common oviduct; DG, Dufour’s gland; LCG, left colleterial gland; LO, lateral oviduct; OV, ovariole; PCG, pseudocolleterial gland; RCG, right colleterial gland; SP, spermatheca; SPG, spermathecal gland; VG, venom gland; VGR, venom gland reservoir [A, C, D, from C Gillott, 2002, Insect accessory reproductive glands: Key

players in production and protection of eggs, in: Chemoecology of Insect Eggs and Egg Deposition (M Hilker

and T Meiners, eds.), By permission of Blackwell Verlag, Berlin; B, from R P Ruegg, 1981, Factors influencing

reproduction in Rhodnius prolixus (Insecta: Hemiptera), Ph.D Thesis, York University, Canada.]

REPRODUCTION

spermatophores and/or seminal fluid is deposited during copulation Also connected with the

vagina are the spermatheca in which sperm are stored and various accessory glands In some

species part of the spermatheca takes the form of a diverticulum, the spermathecal gland

It is noteworthy that the ovaries themselves lack innervation though the ductal components

of the system receive nerves from the terminal abdominal ganglion (Sections 5 and 7.2)

The ovaries are usually dorsolateral to the gut, and each comprises a number of tubular

ovarioles ensheathed by a network of connective tissue in which numerous tracheoles and

muscles are embedded The number of ovarioles per ovary, though approximately constant

within a species, varies widely among species For example, in some viviparous aphids and

in dung beetles there is one ovariole per ovary in contrast to the more than 2000 ovarioles per

ovary in some higher termite queens The wall of each ovariole includes an outer epithelial

sheath and an inner acellular, elastic layer, the tunica propria Each ovariole (Figure 19.2)

FIGURE 19.2. Types of ovarioles The upper portion of each figure is enlarged to a greater extent than the lower

in order to make details of germarial structure clear [After A P Mahowald, 1972, Oogenesis, in: Developmental

Systems: Insects, Vol I (S J Counce and C H Waddington, eds) By permission of Academic Press Ltd., and theV

author.]

CHAPTER 19

consists of a terminal filament, germarium, vitellarium, and pedicel (ovariole stalk) Theterminal filaments may fuse to form a sheet of tissue attached to the dorsal body wall ordorsal diaphragm by which an ovary is suspended within the abdominal cavity Withinthe germarium, oogonia, derived from primary germ cells, give rise to oocytes and, insome types of ovarioles, also to nutritive cells (see below) As oocytes mature and enterthe vitellarium they tend in most insects to become arranged in a linear sequence alongthe ovariole Each oocyte also becomes enclosed in a one-cell-thick layer of follicularepithelium derived from mesodermal prefollicular tissue located at the junction of thegermarium and vitellarium As its name indicates, the vitellarium is the region in which

an oocyte accumulates yolk, a process known as vitellogenesis (Section 3.1.1) Normallyvitellogenesis occurs only in the terminal oocyte, that is, the oocyte closest to the lateraloviduct, and during the process the oocyte’s volume may increase enormously, for example,

by as much as 105times in Drosophila Each ovariole is connected to a lateral oviduct by

a thin-walled tube, the pedicel, whose lumen is initially occluded by epithelial tissue Thisplug of tissue is lost during ovulation (movement of a mature oocyte into a lateral oviduct)and replaced by the remains of the follicular epithelium that originally covered the oocyte.Ovarioles may join a lateral oviduct linearly, as in some apterygotes, Ephemeroptera andOrthoptera, or, more often, open confluently into the distal expanded portion of the oviduct,the calyx

Three types of ovarioles can be distinguished (Figure 19.2) The most primitivetype, found in Thysanura, Paleoptera, most orthopteroid insects, Siphonaptera, and someMecoptera, is the panoistic ovariole in which specialized nutritive cells (trophocytes) areabsent Trophocytes occur in the two remaining types, the polytrophic and telotrophic ovari-oles, which are sometimes grouped together as meroistic ovarioles In polytrophic ovarioles,several trophocytes (nurse cells) are enclosed in each follicle along with an oocyte Thetrophocytes and oocyte originate from the same oogonium Polytrophic ovarioles are found

in most endopterygotes, and in Dermaptera, Psocoptera, and Phthiraptera In Hemipteraand Coleoptera telotrophic (acrotrophic) ovarioles occur in which the trophocytes form asyncytium in the proximal part of the germarium and connect with each oocyte by means

of a trophic cord

The lateral oviducts are thin-walled tubes that consist of an inner epithelial layer set on

a basal lamina and an outer sheath of muscle In many species they include both mesodermaland ectodermal components In almost all insects they join the common oviduct mediallybeneath the gut, but in Ephemeroptera the lateral oviducts remain separate and open tothe exterior independently The common oviduct, which is lined with cuticle, is usuallymore muscular than the lateral oviducts Posteriorly, the common oviduct is confluent withthe vagina that, as noted above, may evaginate to form the bursa copulatrix In some speciesthe bursa forms a diverticulum off the oviduct In nearly all Lepidoptera the bursa is physi-cally distinct from the oviduct and opens to the outside via the vulva (Figure 19.3) A narrowsperm duct connects the bursa with the oviduct and forms the route along which the spermmigrate to the spermatheca

Usually a single spermatheca is present in which sperm are stored, though in somehigher Diptera up to three such structures occur The spermatheca and the duct with which

it joins the bursa are lined with cuticle The cuticle overlays a one-cell-thick layer ofepithelium whose cells are glandular and assumed to secrete nutrients for use by the storedsperm Typically, also, the cells have a much folded apical plasmalemma, adjacent to whichare many mitochondria, features characteristic of cells involved in ion exchange (comparethe structure of Malpighian tubule and rectal epithelial cells, Chapter 18, Sections 2.1

REPRODUCTION

FIGURE 19.3. Reproductive system of female Lepidoptera-Ditrysia [After A D Imms, 1957, A General

Textbook of Entomology ee , 9th ed (revised by O W Richards and R G Davies), Methuen and Co.]

and 4.1) These features may indicate that the sperm stored in the spermatheca require an

ionic milieu different from that of the surrounding hemolymph

Various accessory glands (also called collateral or colleterial glands) may be present and

usually open into the bursa However, in Acrididae (Orthoptera) the glands, known as

pseu-docolleterial glands, are anterior extensions of the lateral oviducts (Figure 19.1A) Normally,

there is one pair of glands, which secrete materials that form a protective coating around

the eggs or stick the eggs to the substrate during oviposition Less commonly the glands

produce antibacterial substances that coat the eggs, toxic egg protectants, and

oviposition-stimulating or oviposition-deterring pheromones (Gillott, 2002) In some species the glands

may be structurally distinct bi- or multipaired structures, each pair presumed to have

dis-crete functions In Hymenoptera, the glands are single, not paired, and produce the venom

used in the sting, secrete trail- or oviposition site-marking pheromones, or lubricate the

ovipositor valves (Figure 19.1D)

2.2 Male

Functions of the male reproductive system include production, storage, and, finally,

delivery to the female of sperm In some species, the system produces substances

trans-ferred during copulation that regulate female receptivity and fecundity (Gillott, 1995, 2003;

Wolfner, 1997, 2002) An additional function may be to supply the female with nutrients

that can be incorporated into developing oocytes, thereby increasing the rate and number

of eggs produced

The male system includes paired testes (in Lepidoptera these fuse to form a single

median organ), paired vasa deferentia and seminal vesicles, a median ejaculatory duct, and

various accessory glands (Figure 19.4) The testes, which lie either above or below the gut,

comprise a varied number of tubular follicles bound together by a connective tissue sheath

The follicles may open into the vas deferens either confluently or in a linear sequence

The wall of each follicle is a layer of epithelium set on a basal lamina Within the follicles

several zones of development can be readily distinguished (Figure 19.5) The distal zone

is the germarium in which spermatogonia are produced from germ cells In Orthoptera,

CHAPTER 19

FIGURE 19.4. Examples of male reproductive systems (not to scale) (A) Melanoplus sanguinipes (Orthoptera); (B) Lytta nuttalli (Coleoptera); (C) Anagasta kuhniella ¨ (Lepidoptera); and (D) Drosophila melanogaster (Diptera).

In M sanguinipes 16 pairs of tubules make up each collateral gland (CG) There are 4 white tubules (WT),

10 short hyaline tubules (SHT), and a long hyaline tubule (LHT) The 16th tubule serves as a seminal vesicle

(SV) In L nuttalli there are three tubules in each collateral gland; a spiral tubule (SpT), short tubule (ST), and

a long tubule (LT) Other abbreviations: CED, cuticular ejaculatory duct; D, duplex; EB, ejaculatory bulb; ED,

ejaculatory duct; LVD, lower vas deferens; MED, mesodermal ejaculatory duct; T, testis; TF, testis follicle; UVD, upper vas deferens; VD, vas deferens (A, from C Gillott, 2002, Insect accessory reproductive glands: Key players

in production and protection of eggs, in: Chemoecology of Insect Eggs and Egg Deposition (M Hilker and T.

Meiners, eds.) By permission of Blackwell Verlag, Berlin; B, from G H Gerber, N S Church, and J G Rempel,

1971, The anatomy, histology, and physiology of the reproductive systems of Lytta nuttalli Say (Coleoptera:

Meloidae) I The internal genitalia, Can J Zool 49:523–533 By permission of the National Research Council of

Canada; C, from a diagram supplied by Dr J G Riemann; D, from E Kubli, 1996, The Drosophila sex-peptide:

A peptide pheromone involved in reproduction, Adv Dev Biochem 4:99–128 By Permission of JAI Press,

Inc.)

REPRODUCTION

FIGURE 19.5. (A) Section through testis to show arrangement of follicles; and (B) zones of maturation in testis

follicle [A, from R E Snodgrass, Principles of Insect Morphology Copyright 1935 by McGraw-Hill, Inc Used

with permission of McGraw-Hill Book Company B, after V B Wigglesworth, 1965, The Principles of Insect

Physiology, 6th ed., Methuen and Co By permission of the author.]

Dictyoptera, Hemiptera, and Lepidoptera a prominent apical cell is also present whose

pre-sumed function is to supply nutrients to the spermatogonia As each spermatogonium moves

proximally into the zone of growth, it becomes enclosed within a layer of somatic cells,

forming a “cyst.” Within the cyst, the cell divides mitotically to form a varied number

(usu-ally 64–256) of spermatocytes In the zone of maturation, the spermatocytes undergo two

maturation divisions, so that from each spermatocyte four haploid spermatids are formed

In the proximal part of the follicle, the zone of transformation, spermatids differentiate into

flagellated spermatozoa At this time the cyst wall normally has ruptured, though often the

sperm within a bundle (spermatodesm) remain held together by a gelatinous cap that covers

their anterior end This cap may be lost as the sperm enter the vas deferens or persist until

the sperm have been transferred to the female

In Lepidoptera two types of sperm occur Pyrene (nucleate) sperm are those that fertilize

eggs, while apyrene (anucleate) sperm are speculated to have several functions, including

assisting in the movement of pyrene sperm from the testes to the seminal vesicles, providing

nourishment to the pyrene sperm, and destroying sperm from previous matings (Silberglied

et al., 1984) In each species within the Drosophila obscura complex two size classes of

CHAPTER 19

nucleated sperm are produced, which differ in head and tail lengths Snook and Karr (1998)confirmed that only the long-sperm type fertilize eggs, though the function(s) of the shortsperm remain unidentified

Sperm are moved from the testes to their site of storage (normally, the seminal vesicles)

by peristaltic contractions of the vas deferens The seminal vesicles are dilations of the vasadeferentia Their walls are well tracheated and frequently glandular, which may indicate apossible nutritive function In Acrididae (Orthoptera) sperm are stored in a pair of highlymodified accessory gland tubules (Figure 19.4A) In many Lepidoptera the migration of

sperm follows a circadian rhythm (Giebultowicz et al., 1989) Typically sperm are released

from the testes into the upper vasa deferentia shortly before or just after dark, and thenare moved into the seminal vesicles during the next light phase However, they quicklyleave this site, being moved into the duplex region of the reproductive tract (Figure 19.4C),where they remain until the next copulation occurs The timing of sperm movement is suchthat the sperm produced each day move into the duplex a few hours after the male’s dailyperiod of receptivity to female pheromone This ensures that when the male next has anopportunity to mate, a substantial amount of new sperm will be available for insemination.The vasa deferentia enter the anterior tip of the ejaculatory duct, an ectodermallyderived tube lined with cuticle whose walls normally are heavily muscularized Posteriorly,the ejaculatory duct may run through an evagination of the body wall, which thus forms

an intromittent organ In insects that form a complex spermatophore, subdivision of theejaculatory duct into specialized regions may occur In Ephemeroptera no ejaculatory duct

is present, and each vas deferens opens directly to the exterior

The accessory glands may be either mesodermal (mesadenia) or ectodermal (ectadenia)

in origin and are connected with either the lower part of the vasa deferentia or the upperend of the ejaculatory duct In some species considerable morphological and functionaldifferentiation of the glands occurs Essentially, however, their secretions may contribute

to the seminal fluid and/or form the spermatophore In some species the glands producesubstances that, when transferred to the female during insemination, cause increased eggproduction (Section 3.1.3) and/or decreased receptivity (willingness to mate subsequently)(Gillott, 1988, 2003)

3 Sexual Maturation

Most male insects eclose (emerge as adults) with mature sperm in their seminal vesicles.Indeed, in a few insects, for example, some Lepidoptera, Plecoptera, and Ephemeroptera,both egg and sperm production occur in the final larval or pupal instar to enable mating andegg laying to take place within a few hours of eclosion Generally, however, after eclosion,

a period of sexual maturity is required in each sex during which important structural,physiological, and behavioral changes occur This period may extend from only a few days

up to several months in species that have a reproductive diapause (Section 3.1.3)

3.1 Female

Among the processes that occur as a female insect becomes sexually mature are genesis, development of characteristic body coloration, maturation of pheromone-producingglands, growth of the reproductive tract, including accessory glands, and an increase in

REPRODUCTION

receptivity These processes are controlled by the endocrine system whose activity, in turn,

is influenced by various environmental stimuli

3.1.1 Vitellogenesis

As noted above, vitellogenesis occurs, by and large, only in the terminal oocyte within

an ovariole, yet in many species the process is highly synchronized among ovarioles and

between ovaries; that is, the eggs are produced in batches Why vitellogenesis does not occur

to any great extent in the more distal oocytes is unclear, though various suggestions have been

made One suggestion is that the terminal oocyte, as the first to mature, that is, to become

capable of vitellogenesis, simply outruns the competition In other words, once the oocyte

begins vitellogenesis and growth, its increasing surface area enables it to capture virtually

all of the available nutrients This, however, cannot be the complete answer because in many

female insects vitellogenesis in the penultimate oocyte appears to be inhibited even after the

terminal oocyte has completed its yolk deposition and become chorionated, provided that the

mature egg is not laid Two explanations have been proposed Adams and co-workers (see

Adams, 1970, 1981) showed, in Musca domestica at least, an ovary containing mature eggs

produces an oostatic hormone that prevents release of the ovarian ecdysiotropic hormone

necessary for vitellogenesis (Section 3.1.3) In contrast, in Rhodnius prolixus and Locusta

migratoria an antigonadotropic hormone is produced by the abdominal perisympathetic

organs and thoracic ganglia, respectively, when the ovariole contains a mature egg The

function of this hormone, it is proposed, is to block the action of juvenile hormone on the

follicle cells (Section 3.1.3), again preventing vitellogenesis (Huebner and Davey, 1973;

Daveyaa et al., 1993) Remarkably, the metacestode stage of the rat tapeworm, Hymenolepis

diminuta, which infects female mealworm beetles (Tenebrio molitor), also produces an

antigonadotropin This acts similarly to that of Rhodnius, thus allowing the parasite to

make use of resources originally intended for egg production (Hurd, 1998)

As yolk appears, it is seen to be made up almost entirely of roundish granules or

vacuoles known as yolk spheres Within the yolk spheres, protein, lipid, or carbohydrate

can be detected The membrane-bound protein yolk spheres are most abundant, followed

by lipid droplets that are not membrane-bound Relatively few glycogen-containing yolk

spheres are usually present Small amounts of nucleic acids are normally detectable, but

these are not within the yolk spheres The source of some of these materials is different in

the various types of ovarioles

In all ovarioles, however, almost all yolk protein is extraovarian in origin In most insects

the proteins are accumulated from the hemolymph The source of these proteins, as was noted

in Chapter 16, Section 5.4, is the fat body, which, during vitellogenesis, synthesizes∗and

releases large quantities of a few specific proteins (vitellogenins or female-specific proteins)

that are selectively accumulated by the terminal oocytes The higher Diptera are a notable

exception in that the follicular epithelium is also a major source of yolk proteins; indeed,

in the stable fly, Stomoxys calcitrans, all yolk protein production occurs here (Kelly, 1994).

Shortly before vitellogenesis, a space appears between the follicle cells and the terminal

oocytes, and intercellular spaces develop in the follicular epithelium (patency), so that the

oocytes become bathed in hemolymph The tunica propria appears to be freely permeable

to all solutes within the hemolymph Electron microscopic and other studies have shown

∗ In insects that have fully developed eggs at eclosion, the proteins are synthesized (and stored) by the fat body

during larval development, to be released during the pupal stage when vitellogenesis occurs.

CHAPTER 19

that extensive pinocytosis occurs in the oocyte plasma membrane, resulting in the mulation of yolk protein in membrane-bound vacuoles Vitellogenins are large (molecularweight 200,000–650,000), conjugated proteins containing both lipid and carbohydrate com-ponents The latter are oligosaccharides and probably important as the agents by which theoocyte plasmalemma recognizes the vitellogenins prior to pinocytosis Though most yolkprotein is produced in the fat body, small contributions may be made by follicular epithelialcells, nurse cells where present, or the terminal oocyte Studies have shown, for example,that in some species active RNA synthesis occurs in the follicular epithelium during vitello-genesis, especially the early stages, and that isotopically labeled amino acids are first taken

accu-up by follicle cells to appear later in protein spheres within the oocytes In telotrophic andpolytrophic ovarioles some protein may be transferred during early vitellogenesis to theoocyte from the nurse cells However, the latter appear to be more important as suppliers

of nucleic acids to the developing oocyte, and several autoradiographic and electron scopic studies have shown the movement of labeled RNA or ribosomes down the trophiccord in telotrophic ovarioles or across adjacent nurse cells into the oocyte in polytrophicovarioles It is presumed that this RNA is then associated with protein synthesis within theoocyte In addition to the RNA derived from nurse cells, RNA may also be produced bythe oocyte nucleus for use in protein synthesis In several species, active incorporation oflabeled RNA precursors into the oocyte nucleus has been observed to occur early in vitel-logenesis, concomitant with the accumulation of protein (non-membrane-bound) adjacent

micro-to the nuclear envelope

Studies on the accumulation of lipid, carbohydrate, and other components of yolk arefew Though lipid may make up a considerable proportion of the yolk, its source remainsdoubtful in most species An apparent association between the Golgi apparatus and theaccumulation of lipid led early authors to suggest that the lipid was synthesized by the

oocyte per se, though this has not been confirmed Another suggestion that requires further

study is that the follicle cells contribute lipid to the oocyte In the polytrophic ovariole of

Drosophila, the nurse cells supply lipids to the oocyte, though this apparently is not the case

in Culex In telotrophic ovarioles, lipid may be derived both from the nurse cells (early in

vitellogenesis) and from the follicle cells

Glycogen usually can be detected only in small amounts and, in meroistic ovarioles,

after degeneration of the nurse cells In Apis and Musca, labeled glucose injected into

the hemolymph is rapidly accumulated by oocytes in late vitellogenesis and apparentlyconverted to glycogen (Engelmann, 1970)

In the German cockroach, Blattella germanica, the terminal oocytes accumulate large

amounts of hydrocarbons during vitellogenesis The hydrocarbons apparently are sized by the epidermis of the abdominal sternites, then transported to the ovaries bound to a

synthe-hemolymph lipophorin Later, they become incorporated into the ootheca (Fan et al., 2002).

3.1.2 Vitelline Membrane and Chorion Formation

When vitellogenesis is completed, the vitelline membrane and, later, the chorion(eggshell) are formed Though some early observations suggested that the vitelline mem-brane was produced by the oocyte itself, perhaps as a modification of the existing plas-malemma, recent studies covering a range of insect orders have confirmed that the vitellinemembrane is secreted by the follicle cells The nature of the membrane varies both amongspecies (correlated with the egg’s environment) and regionally over the egg surface For

example, the vitelline membrane of Drosophila melanogaster is perforated in the “collar”

REPRODUCTION

FIGURE 19.6. Egg of Locusta (A) General view; (B) enlargement of posterior end; (C) section through chorion

along micropylar axis; and (D) details of chorion structure [A, after R F Chapman, 1971, The Insects: Structure

and Function By permission of Elsevier/North-Holland, Inc., and the author B, C, after M L Roonwal, 1954,

The egg-wall of the African migratory locust, Locusta m migratorioides R&F (Orthoptera: Acrididae), Proc.

Natl Inst Sci India 20:361–370 By permission of the Indian National Science Academy D, after J C Hartley,

1961, The shell of acridid eggs, Q J Microsc Sci 102:249–255 By permission of Cambridge University Press.]

region which ruptures to facilitate hatching, while in the eggs of parasitic Hymenoptera the

membrane is extremely thin to permit uptake of nutrients from the host’s hemolymph In

D melanogaster the follicle cells secrete wax immediately after production of the vitelline

membrane The wax layer is found in eggs of many species and, like that of the cuticle,

prevents desiccation A wax layer is not found, however, in eggs that are normally laid in

humid or wet microclimates or that take up water from the environment prior to embryonic

development

The chorion is usually secreted entirely by the follicle cells and comprises two main

layers, an endochorion adjacent to the vitelline membrane and an exochorion (Figure 19.6)

In some insects, for example, Acrididae, the shell takes on a third layer, the extrachorion,

CHAPTER 19

FIGURE 19.7. Diagrammatic sagittal section through an egg at oviposition [After R F Chapman, 1971, The

Insects: Structure and Function By permission of Elsevier/North-Holland, Inc., and the author.]

as an oocyte moves through the common oviduct Interestingly, though the follicle cells aremesodermal derivatives, the chorion is cuticlelike in nature and contains layers of proteinand lipoprotein (but not chitin), some of which are tanned by polyphenolic substancesreleased by the cells

The chorion is not produced as a uniform layer over the oocyte For example, in somespecies a ring of follicle cells near the anterior end of the oocyte secrete no exochorion, sothat a line of weakness is created at this point for ease of hatching Also, certain folliclecells appear to have larger than normal microvilli which, when withdrawn after chorionformation, leave channels (micropyles) to permit entry of sperm (Figure 19.6C,D) Theaeropyles (air canals) appear to be formed in a similar way In eggs of many species,both terrestrial and aquatic, the aeropyles connect with a network of minute air spaceswithin the endochorion (the plastron), which improves gas exchange between the oocyteand the atmosphere The plastron in eggs of terrestrial species usually serves to preventdesiccation caused by evaporation; for aquatic insects (or terrestrial species whose eggsbecome immersed temporarily in water, for example, after rain), the plastron preventswaterlogging of the egg while facilitating gas exchange (see Chapter 15, Section 4.2) Likethe vitelline membrane, the chorion varies in thickness among species In the cecropia moth,

Hyalophora cecropia, it is 55–60µm, providing a rigid protective coat, while in parasiticµµ

Hymenoptera it is very thin (e.g., Habrobracon juglandis, 0.17µm) and flexible to permitµµstretching of the egg during its passage along the very narrow ovipositor

The internal structure of a mature (i.e., chorionated) egg is shown diagrammatically inFigure 19.7

3.1.3 Factors Affecting Sexual Maturity in the Female

Environmental. A number of environmental factors have been observed to influencethe rate at which a female becomes sexually mature, for example, quantity and quality offood consumed, population density and structure, mating, photoperiod, temperature, andhumidity For most factors there is evidence that they exert their influence by modifyingthe activity of the endocrine system, though availability of food and temperature also haveobvious direct effects on the rate of egg development

Many reports indicate that the qualitative nature of food may have a marked effect onthe number of eggs matured However, it is often not clear whether the observed differences

REPRODUCTION

in egg production are related to differences in palatability (more palatable foods might be

eaten in larger quantities) or to differences in the nutritive value of the food For most

insects, dietary proteins are essential for maturation of eggs Many Diptera, for example,

may survive for several weeks on a diet that contains carbohydrate but no protein, yet will

mature no eggs Equally, different proteins may have different nutritional values related to

their amino acid composition and, possibly, to their digestibility Carbohydrates, too, are

important, especially in the provision of energy for synthesis of yolk components As noted

above, lipids may be a significant component of the yolk, and therefore, an essential part of

the diet, though some may be formed from carbohydrates Water, vitamins, minerals, and,

for some species, specific growth substances are also necessary constituents of the diet if

maximum egg production is to be achieved

Quantitative influences of feeding are much more easily documented Anautogenous

mosquitoes and many other bloodsucking insects, for example, do not mature eggs until

they have fed Furthermore, the number of eggs produced is, within limits, proportional

to the quantity of blood ingested However, it must be noted that in endopterygotes many

of the materials to be used in egg production are laid down during larval development, and

therefore the nutrition of the larva must also be considered when attempting comparisons

In addition to its direct function of providing raw materials for egg maturation, feeding

also has an important indirect effect, namely, to stimulate endocrine activity In continuous

feeders (those that take a series of small meals) stretching of the foregut as food is ingested

results in information being sent via the stomatogastric nervous system to the brain/corpora

cardiaca complex where it stimulates synthesis and release of neurosecretion In occasional

feeders such as Rhodnius and anautogenous mosquitoes, which require a single, large meal

in order to mature a batch of eggs, stretching of the abdominal wall is the stimulus, sent to

the brain via the ventral nerve cord, which triggers endocrine activity In both arrangements

the degree of endocrine activity and, in turn, the number of eggs developed, is proportional

(other things being equal) to the amount of food ingested

In some insects, especially gregarious species, population density and structure may

influence sexual maturation In some species of Drosophila, Locusta migratoria, and

No-madacris septemfasciata (the red locust), for example, the greater the population density,

the slower is the rate of egg maturation Though there are some obvious potential reasons

for this, such as interference with feeding, other effects of this stress, perhaps manifest

through a decrease in endocrine activity, may also be important By contrast, in the desert

locust, Schistocerca gregaria, crowded females mature eggs faster than isolated

individu-als, probably as a result of increased endocrine activity in the former group In addition,

in S gregaria and S paranensis (the Central American locust) there is evidence that egg

development in females is promoted in the presence of older males, which are believed

to secrete a maturation-accelerating pheromone (Chapter 13, Section 4.1.1) Conversely,

in colonies of social insects, secretion of a maturation-inhibiting pheromone by the queen

prevents development of the reproductive system of other females, the workers Again, the

effects of these pheromones are probably mediated through the endocrine system, though

the evidence for this proposal is mostly circumstantial

Mating is, for many species, a most important factor in sexual maturation For example,

in some bloodsucking Hemiptera and some cockroaches almost no eggs develop in virgin

females In other insects eggs of virgin females mature more slowly than those of mated

females, and many are eventually resorbed if mating does not take place The stimulus given

to a female may be physical or chemical in nature, but in either case its effect may be to

enhance endocrine activity In some cockroaches, for example, mechanical stimulation of

CHAPTER 19

the genitalia or the presence of a spermatophore in the bursa results in information beingsent to the brain via the ventral nerve cord, followed by activation of the corpora allata Inrepresentatives of many insect orders male accessory gland secretions are transferred to thefemale during mating These chemicals, which move to a range of sites within the female’s

body (Lay et al., 2004), may serve either as signals, triggering subsequent steps in the

reproductive process, or as nutrient contributions enabling females to produce more eggs

For example, in Drosophila, many species of mosquitoes, and some Lepidoptera peptidic materials from the male accessory glands stimulate egg development Thus, in Drosophila

one of the actions of the so-called “sex peptide” is to promote juvenile hormone synthesisand hence increased vitellogenesis (Wolfner, 1997, 2002; Kubli, 2003) In tettigoniids,

some butterflies, and Photinus fireflies the spermatophore, either eaten or digested within

the reproductive tract, provides nutrients that increase the number of eggs produced (seealso Section 4.3.1) (Boggs, 1995; Gwynne, 2001, Rooney and Lewis, 2002)

Like all metabolic processes, egg development is affected by temperature and occurs

at the maximum rate at a specific, optimum temperature, whose value presumably reflectsthe normal temperature conditions experienced by a species during reproduction On eachside of this optimum egg maturation is decreased, in the normal temperature-dependentmanner of enzymatically controlled reactions Sometimes superimposed on this basic effect,however, are more subtle effects of temperature, of both a direct and an indirect nature For

example, in Locusta migratoria regular temperature fluctuations (provided these are not too

extreme) appear to stress the insect, causing release of neurosecretion and enhanced rates ofdevelopment In some other species mating occurs only within a certain temperature range,yet, as noted earlier, may have an important influence on egg development It follows that,

in this situation, temperature can have an important indirect effect on maturation

Few direct observations have been made on the effects of humidity on egg maturation,though it is known that humidity may determine whether or not oviposition occurs In manyspecies eggs are laid only when the relative humidity is high (80% to 90%), and oviposition

is increasingly retarded as the environment becomes drier Engelmann (1970) suggested apossible explanation for this is that, as increasing amounts of water are lost from the body

by evaporation, insufficient remains for use in egg development whose rate is thereforedecreased

Photoperiod, the earth’s naturally recurring alternation of light and darkness, is ably the best-studied environmental factor that influences egg maturation The effect ofphotoperiod is long-term (seasonal) and serves to correlate egg development with the avail-ability of food, suitable egg-laying conditions, and/or suitable conditions for the eventualdevelopment of the larvae Implicit in this statement is the idea that an insect, by hav-ing its reproductive activity seasonal in nature, is able to overcome adverse environmentalconditions Commonly, an insect survives these adverse conditions by entering a specificphysiological condition known as diapause, whose onset and termination are induced bychanges in daylength (sometimes acting in conjunction with temperature) (For a generaldiscussion of diapause, see Chapter 22, Section 3.2.3.) Essentially diapause is a phase ofarrested development, and, in the context of adult (reproductive) diapause, this means thatthe eggs do not mature In different species diapause may be induced by increasing daylength (number of hours of light in a 24-hour period), which enables an insect to overcomehot and/or dry summer conditions (estivation), or by decreasing day length prior to the onset

prob-of cold winter conditions (hibernation)

For example, in the Egyptian tree locust, Anacridium aegyptium, reproductive diapause

is induced by the decreasing day lengths experienced in fall and is maintained for about

REPRODUCTION

4 months during which no eggs are produced Termination of diapause, brought about by

increasing day lengths in spring, is correlated with renewed availability of oviposition sites

and food for the juvenile stages Likewise, hibernation in newly eclosed adult Colorado

potato beetles (Leptinotarsa decemlineata(( ) is induced by the short day lengths of fall (and

also by a lack of food at this time) At the onset of diapause, the beetles become negatively

phototactic and bury themselves under several inches of soil Termination of diapause

in L decemlineata is unlikely to be induced by increasing day length (given the beetles’

position); rather, diapause may have terminated simply as a result of the passage of time or by

a temperature cue as soil warms in the spring (Hodek, 2002, and personal communication)

Estivation is seen in Schistocerca gregaria and Hypera postica, the alfalfa weevil.

Sexual maturation in S gregaria is retarded by long and promoted by short day lengths.

This observation can be correlated with the availability of food and oviposition sites in its

natural habitat, arid areas of Africa and Asia, where in the summer the weather is hot and

dry, but rain falls intermittently during the winter H postica adults emerge in late spring

and undergo reproductive diapause before laying eggs in late summer and fall Low winter

temperatures prevent the eggs from hatching until the following spring when new alfalfa

foliage on which the larvae feed has begun to appear

The effects of photoperiod on egg maturation are mediated via the endocrine system,

though for many species the evidence for this statement is largely circumstantial, for

ex-ample, differences in the histological appearance of the endocrine glands in diapausing

and non-diapausing insects In diapausing adult insects the corpora allata are small, and the

neurosecretory system is typically full of stainable material, which are taken to indicate

inac-tivity of these glands When diapause is terminated and egg development begins, the corpora

allata increase in volume and the amount of stainable material in the neurosecretory system

decreases In the beetle Galeruca tanaceti autoradiographic studies have shown that in

post-diapause beetles the rate of incorporation of labeled cystine into neurosecretory cells (taken

to be a measure of their synthetic activity) is high compared to that of estivating insects

A limited amount of experimental work supports these histological correlations For

exam-ple, in L decemlineata maintained under long-day (non-diapause) conditions, allatectomy,

treatment with precocene (which destroys the corpora allata), or cautery of the brain

neurose-cretory cells mimics the diapause-inducing effects of short days, that is, stimulates digging

behavior, arrests yolk deposition, and causes oxygen consumption to decrease Conversely,

treatment of diapausing beetles with juvenile hormone or its mimics causes the beetles to

leave the soil, and stimulates feeding activity and egg maturation (Denlinger, 1985)

Endocrine. The endocrine control of sexual maturation in female insects, especially

the control of oocyte development, continues to be among the most intensely studied areas

of insect physiology Yet, despite the wealth of literature that has resulted, some major

aspects of hormonal control remain unclear It is apparent, however, that among the Insecta

the relative importance of the various endocrine centers in reproduction may differ, as might

be anticipated in a group of such diverse habits (Belles, 1998) The following account is

therefore generalized, though the major points of contention and differences among insects

will also be outlined (see also Figure 19.8)

The two principal hormonal components involved are juvenile hormone (JH) and

cere-bral neurosecretory factors, though in some insects ecdysone, oostatic hormone, or

antigo-nadotropic hormone also are important

The importance of the corpora allata in egg development first became apparent in 1936

when Wigglesworth and Weed-Pfeiffer (cited in de Wilde and de Loof, 1974b) demonstrated

independently that in Rhodnius and Melanoplus, respectively, allatectomy (removal of the

CHAPTER 19

FIGURE 19.8. Endocrine control of egg development (A) Schistocerca gregaria and other locusts; (B) Rhodnius

prolixus; (C) Aedes aegypti and other mosquitoes; and (D) Sarcophaga bullata [After K C Highnam and L Hill,

1977, The Comparative Endocrinology of the Invertebrates, 2nd ed By permission of Edward Arnold Publishers

Ltd.]

corpora allata) prevented vitellogenesis Since this date, many authors, using allatectomy,followed by replacement therapy (implantation of “active” glands from other insects, ortreatment with JH or its mimics), have confirmed the importance of JH as a gonadotropic

factor in most insects However, in the flesh fly, Sarcophaga bullata, vitellogenesis occurs

only when the median neurosecretory cells are present and is, apparently, independent of JH(Figure 19.8D) In mosquitoes, JH controls only the previtellogenic growth of the primaryfollicles and is not required for vitellogenesis (Figure 19.8C) (Dhadialla and Raikhel, 1994;Klowden, 1997)

Originally, it was believed that JH probably triggered the synthesis of yolk precursors

in the follicle cells, which then passed these materials on to developing oocytes However,

REPRODUCTION

FIGURE 19.8. (Continued)

subsequent studies have shown that in most species yolk is mainly of extraovarian origin,

especially, the fat body (see below) In the ovary, JH has two effects: early in vitellogenesis

it “primes” the follicular epithelial cells, while later it regulates the permeability of the

follicular epithelial layer In Rhodnius and Locusta, for example, JH causes the follicular

epithelial cells to shrink slightly, resulting in the development of prominent intercellular

channels (patency) (Davey et al., 1993; Davey, 1996) Vitellogenins have been shown to

move along these channels, to be accumulated pinocytotically by the oocyte during

vitello-genesis The precise way in which JH stimulates the follicle cells to shrink remains unclear,

though an effect on the arrangement of microtubules within the cells has been demonstrated

(Evidence from other non-insectan systems strongly implicates the involvement of these

organelles in the regulation of cell shape.)

As was noted in Section 3.1.1, for most insects the source of most yolk components

is extraovarian, specifically the hemolymph, which serves as a reservoir for materials

syn-thesized in the fat body In the early to mid-1960s, evidence was collected that suggested a