Sperm Competition

Multiple matings are common in many insect species. The occurrence of remating under natural conditions can be determined by observing the mating behavior of individual females or by dissection to establish the amount of ejaculate or the number of spermatophores present in the female's sperm storage organs. Some of the best documentation of remating comes from studies of many Lepidoptera, in which part of each spermatophore persists in the bursa copulatrix of the female throughout her life (Fig. 5.6). These studies show that remating occurs, to some extent, in almost all species of Lepidoptera for which adequate field data are available.

The combination of internal fertilization, sperm storage, multiple mating by females, and the overlap within a female of ejaculates from different males leads to a phenomenon known as sperm competition. This occurs within the reproductive tract of the female at the time of oviposition when sperm from two or more males compete to fertilize the eggs. Both physiological and behavioral mechanisms determine the outcome of sperm competition. Thus, events inside the female's reproductive tract, combined with various attributes of mating behavior, determine which sperm will succeed in reaching the eggs. It is important to realize that male reproductive fitness is measured in terms of the number of eggs fertilized or offspring fathered and not simply the number of copulations achieved, although these measures sometimes are correlated. Often there may be a trade-off between the number of copulations that a male can secure and the number of eggs that he will fertilize at each mating. A high copulation frequency is generally associated with low time or energy investment per copulation but also with low certainty of paternity. At the other extreme, males that exhibit substantial parental investment, such as feeding their mates (Boxes 5.1 & 5.2), and other adaptations that more directly increase certainty of paternity, will inseminate fewer females over a given period.

There are two main types of sexually selected adaptation in males that increase certainty of paternity. The first strategy involves mechanisms by which males can ensure that females use their sperm preferentially. Such sperm precedence is achieved usually by displacing the ejaculate of males that have mated previously with the female (Box 5.3). The second strategy is to reduce the effectiveness or occurrence of subsequent inseminations by other males. Various mechanisms appear to achieve this result, including mating plugs, use of male-derived secretions that "switch off" female receptivity (Box 5.4), prolonged copulation (Fig. 5.8), guarding of females, and improved structures for gripping the female during copulation to prevent "take-over" by other males. A significant selective advantage would accrue to any male that could both achieve sperm precedence and prevent other males from successfully

Box 5.4 Control of mating and oviposition in a blow fly

Number of prior matings by male Number of prior matings by male

Number of prior matings by male Number of prior matings by male

The sheep blow fly, Lucilia cuprina (Diptera: Calliphoridae), costs the Australian sheep industry many millions of dollars annually through losses caused by myiases or "strikes". This pestiferous fly may have been introduced to Australia from Africa in the late 19th century. The reproductive behavior of L. cuprina has been studied in some detail because of its relevance to a control program for this pest. Ovarian development and reproductive behavior of the adult female are highly stereotyped and readily manipulated via precise feeding of protein. Most females are anautogenous, i.e. they require a protein meal in order to develop their eggs, and usually mate after feeding and before their oocytes have reached early vitellogenesis. After their first mating, females reject further mating attempts for several days. The "switch-off" is activated by a peptide produced in the accessory glands of the male and transferred to the female during mating. Mating also stimulates oviposition; virgin females rarely lay eggs, whereas mated females readily do so. The eggs of each fly are laid in a single mass of a few hundred (illustration at top right) and then a new ovarian cycle commences with another batch of synchronously developing oocytes. Females may lay one to four egg masses before remating.

Unreceptive females respond to male mating attempts by curling their abdomen under their body (illustration at top left), by kicking at the males (illustration at top centre), or by actively avoiding them. Receptivity gradually returns to previously mated females, in contrast to their gradually diminishing tendency to lay. If remated, such non-laying females resume laying. Neither the size of the female's sperm store nor the mechanical stimulation of copulation can explain these changes in female behavior. Experimentally, it has been demonstrated that the female mating refractory period and readiness to lay are related to the amount of male accessory gland substance deposited in the female's bursa copulatrix during copulation. If a male repeatedly mates during one day (a multiply-mated male), less gland material is transferred at each successive copulation. Thus, if one male is mated, during one day, to a succession of females, which are later tested at intervals for their receptivity and readiness to lay, then the proportion of females either unreceptive or laying is inversely related to the number of females with which the male had previously mated. The graph on the left shows the percentage of females unreceptive to further mating when tested 1 day (o) or 8 days (•) after having mated with multiply-mated males. The percentage unreceptive values are based on 1-29 tests of different females. The graph on the right shows the percentage of females that laid eggs during 6 hours of access to oviposition substrate presented 1 day (o) or 8 days (•) after mating with multiply-mated males. The percentage laid values are based on tests of 1-15 females. These two plots represent data from different groups of 30 males; samples of female flies numbering less than five are represented by smaller symbols. (After Bartell et al. 1969; Barton Browne et al. 1990; Smith et al. 1990.)

inseminating the female until his sperm had fertilized at least some of her eggs.

The factors that determine the outcome of sperm competition are not totally under male control. Female choice is a complicating influence, as shown in the above discussions on sexual selection and on morphology of genitalic structures. Female choice of sexual partners may be two-fold. First, there is good evidence that the females of many species choose among potential mating partners. For example, females of many mecopteran species mate selectively with males that provide food of a certain minimum size and quality (Box 5.1). In some insects, such as a few beetles and some moth and katydid species, females have been shown to prefer larger males as mating partners. Second, subsequent to copulation, the female might discriminate between partners as to which sperm will be used. One idea is that variation in the stimuli of the male genitalia induces the female to use one male's sperm in preference to those of another, based upon an "internal courtship". Differential sperm use is possible because females have control over sperm transport to storage, maintenance, and use at oviposition.

5.8 OVIPARITY (EGG-LAYING)

The vast majority of female insects are oviparous: they lay eggs. Generally, ovulation - expulsion of eggs from the ovary into the oviducts - is followed rapidly by fertilization and then oviposition. Ovulation is controlled by hormones released from the brain, whereas oviposition appears to be under both hormonal and neural control. Oviposition, the process of the egg passing from the external genital opening or vulva to the outside of the female (Fig. 5.9), is often associated with behaviors such as digging or probing into an egg-laying site, but often the eggs are simply dropped to the ground or into water. Usually the eggs are deposited on or near the food required by the offspring upon hatching. Care of eggs after laying often is lacking or minimal, but social insects (Chapter 12) have highly developed care, and certain aquatic insects show very unusual paternal care (Box 5.5).

An insect egg within the female's ovary is complete when an oocyte becomes covered with an outer protective coating, the eggshell, formed of the vitelline membrane and the chorion. The chorion may be composed of any or all of the following layers: wax layer, innermost chorion, endochorion, and exochorion (Fig. 5.10). Ovarian follicle cells produce the eggshell and the surface sculpturing of the chorion usually reflects the outline of these cells. Typically, the eggs are yolk-rich and thus large relative to the size of the adult insect; egg cells range in length from 0.2 mm to about 13 mm. Embryonic development within the egg begins after egg activation (section 6.2.1).

The eggshell has a number of important functions. Its design allows selective entry of the sperm at the time of fertilization (section 5.6). Its elasticity facilitates oviposition, especially for species in which the eggs are compressed during passage down a narrow egg-laying tube, as described below. Its structure and composition afford the embryo protection from deleterious conditions such as unfavorable humidity and temperature, and microbial infection, while also allowing the

Fig. 5.8 A copulating pair of stink or shield bugs of the genus Poecilometis (Hemiptera: Pentatomidae). Many heteropteran bugs engage in prolonged copulation, which prevents other males from inseminating the female until either she becomes non-receptive to further males or she lays the eggs fertilized by the "guarding" male.

Fig. 5.8 A copulating pair of stink or shield bugs of the genus Poecilometis (Hemiptera: Pentatomidae). Many heteropteran bugs engage in prolonged copulation, which prevents other males from inseminating the female until either she becomes non-receptive to further males or she lays the eggs fertilized by the "guarding" male.

exchange of oxygen and carbon dioxide between the inside and outside of the egg.

The differences seen in composition and complexity of layering of the eggshell in different insect groups generally are correlated with the environmental conditions

Fig. 5.9 Oviposition by a South African ladybird beetle, Chilomenes lunulata (Coleoptera: Coccinellidae). The eggs adhere to the leaf surface because of a sticky secretion applied to each egg. (After Blaney 1976.)

encountered at the site of oviposition. In parasitic wasps the eggshell is usually thin and relatively homogeneous, allowing flexibility during passage down the narrow ovipositor, but, because the embryo develops within host tissues where desiccation is not a hazard, the wax layer of the eggshell is absent. In contrast, many insects lay their eggs in dry places and here the problem of avoiding water loss while obtaining oxygen is often acute because of the high surface-area-to-volume ratio of most eggs. The majority of terrestrial eggs have a hydrofuge waxy chorion that contains a meshwork holding a layer of gas in contact with the outside atmosphere via narrow holes, or aeropyles.

The females of many insects (e.g. Zygentoma, many Odonata, Orthoptera, some Hemiptera, some Thysanoptera, and Hymenoptera) have appendages of the eighth and ninth abdominal segments modified to form an egg-laying organ or ovipositor (section 2.5.1). In other insects (e.g. many Lepidoptera, Coleoptera, and Diptera) it is the posterior segments rather than appendages of the female's abdomen that function as an ovipositor (a "substitutional" ovipositor). Often these segments can be protracted into a telescopic tube in which the opening of the egg passage is close to the distal end. The ovipositor or the modified end of the abdomen enables the insect to insert its eggs into particular sites, such as into crevices, soil, plant tissues, or, in the case of many parasitic species, into an arthropod host. Other insects, such as termites, parasitic lice, and many Plecoptera, lack an egg-laying organ and eggs are deposited simply on a surface.

In certain Hymenoptera (some wasps, bees, and ants) the ovipositor has lost its egg-laying function and is used as a poison-injecting sting. The stinging

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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