Atypical Modes Of Reproduction

Sexual reproduction (amphimixis) with separate male and female individuals (gonochorism) is the usual mode of reproduction in insects, and diplodiploidy, in which males as well as females are diploid, occurs as the ancestral system in almost all insect orders. However, other modes are not uncommon. Various types of asexual reproduction occur in many insect groups; development from unfertilized eggs is a widespread phenomenon, whereas the production of multiple embryos from a single egg is rare. Some species exhibit alternating sexual and asexual reproduction, depending on season or food availability. A few species possess both male and female reproductive systems in one individual (hermaphroditism) but self-fertilization has been established for species in just one genus.

5.10.1 Parthenogenesis, pedogenesis (paedogenesis), and neoteny

Some or a few representatives of virtually every insect order have dispensed with mating, with females producing viable eggs even though unfertilized. In other groups, notably the Hymenoptera, mating occurs but the sperm need not be used in fertilizing all the eggs. Development from unfertilized eggs is called parthenogenesis, which in some species may be obligatory, but in many others is facultative. The female may produce parthenogenetically only female eggs (thelytokous parthenogenesis), only male eggs (arrhenotokous parthenogenesis), or eggs of both sexes (amphitokous or deuterotokous parthenogenesis). The largest insect group showing arrhenotoky is the Hymenoptera. Within the Hemiptera, aphids display thelytoky and most whiteflies are arrheno-tokous. Certain Diptera and a few Coleoptera are thelytokous, and Thysanoptera display all three types of parthenogenesis. Facultative parthenogenesis, and variation in sex of egg produced, may be a response to fluctuations in environmental conditions, as occurs in aphids that vary the sex of their offspring and mix parthenogenetic and sexual cycles according to season.

Some insects abbreviate their life cycles by loss of the adult stage, or even both adult and pupal stages. In this precocious stage, reproduction is almost exclusively by parthenogenesis. Larval pedogenesis, the production of young by the larval insect, has arisen at least three times in the gall midges (Diptera: Cecidomyiidae) and once in the Coleoptera (Macromalthus debilis). In some gall midges, in an extreme case of hemocoelous viviparity, the precocially developed eggs hatch internally and the larvae may consume the body of the mother-larva before leaving to feed on the surrounding fungal medium. In the well-studied gall midge Heteropeza pygmaea, eggs develop into female larvae, which may metamorphose to female adults or produce more larvae pedogenetically. These larvae, in turn, may be males, females, or a mixture of both sexes. Female larvae may become adult females or repeat the larval pedogenetic cycle, whereas male larvae must develop to adulthood.

In pupal pedogenesis, which sporadically occurs in gall midges, embryos are formed in the hemocoel of a pedogenetic mother-pupa, termed a hemipupa as it differs morphologically from the "normal" pupa. This production of live young in pupal pedogenetic insects also destroys the mother-pupa from within, either by larval perforation of the cuticle or by the eating of the mother by the offspring. Pedogenesis appears to have evolved to allow maximum use of locally abundant but ephemeral larval habitats, such as a mushroom fruiting body. When a gravid female detects an oviposi-tion site, eggs are deposited, and the larval population builds up rapidly through pedogenetic development. Adults are developed only in response to conditions adverse to larvae, such as food depletion and overcrowding. Adults may be female only, or males may occur in some species under specific conditions.

In true pedogenetic taxa there are no reproductive adaptations beyond precocious egg development. In contrast, in neoteny a non-terminal instar develops reproductive features of the adult, including the ability to locate a mate, copulate, and deposit eggs (or larvae) in a conventional manner. For example, the scale insects (Hemiptera: Coccoidea) appear to have neotenous females. Whereas a molt to the winged adult male follows the final immature instar, development of the reproductive female involves omission of one or more instars relative to the male. In appearance the female is a sedentary nymph-like or larviform instar, resembling a larger version of the previous (second or third) instar in all but the presence of a vulva and developing eggs. Neoteny also occurs in all members of the order Strepsiptera; in these insects female development ceases at the puparium stage. In some other insects (e.g. marine midges; Chironomidae), the adult appears larva-like, but this is evidently not due to neoteny because complete metamorphic development is retained, including a pupal instar. Their larviform appearance therefore results from suppression of adult features, rather than the pedogenetic acquisition of reproductive ability in the larval stage.

5.10.2 Hermaphroditism

Several of the species of Icerya (Hemiptera: Mono-phlebidae) that have been studied cytologically are gynomonoecious hermaphrodites, as they are femalelike but possess an ovotestis (a gonad that is part testis, part ovary). In these species, occasional males arise from unfertilized eggs and are apparently functional, but normally self-fertilization is assured by production of male gametes prior to female gametes in the body of one individual (protandry of the hermaphrodite). Without doubt, hermaphroditism greatly assists the spread of the pestiferous cottony-cushion scale, Icerya purchasi (Box 16.3), as single nymphs of this and other hermaphroditic Icerya species can initiate new infestations if dispersed or accidentally transported to new plants. Furthermore, all iceryine margarodids are arrhenotokous, with unfertilized eggs developing into males and fertilized eggs into females.

5.10.3 Polyembryony

This form of asexual reproduction involves the production of two or more embryos from one egg by subdivision (fission). It is restricted predominantly to parasitic insects; it occurs in at least one strepsipteran and representatives of four wasp families, especially the Encyrtidae. It appears to have arisen independently within each wasp family. In these parasitic wasps, the number of larvae produced from a single egg varies in different genera but is influenced by the size of the host, with from fewer than 10 to several hundred, and in Copidosoma (Encyrtidae) up to 3000 embryos, arising from one small, yolkless egg. Nutrition for a large number of developing embryos obviously cannot be supplied by the original egg and is acquired from the host's hemolymph through a specialized enveloping membrane called the trophamnion. Typically, the embryos develop into larvae when the host molts to its final instar, and these larvae consume the host insect before pupating and emerging as adult wasps.

5.10.4 Reproductive effects of endosymbionts

Wolbachia, an intracellular bacterium (Proteobacteria: Rickettsiales) discovered first infecting the ovaries of Culex pipiens mosquitoes, causes some inter-populational (intraspecific) matings to produce inviable embryos. Such crosses, in which embryos abort before hatching, could be returned to viability after treatment of the parents with antibiotic, thus implicating the microorganism with the sterility. This phenomenon, termed cytoplasmic or reproductive incompatibility, now has been demonstrated in a very wide range of invertebrates that host many "strains" of Wolbachia. Surveys have suggested that up to 76% of insect species may be infected. Wolbachia is transferred vertically (inherited by offspring from the mother via the egg), and causes several different but related effects. Specific effects include the following.

• Cytoplasmic (reproductive) incompatibility, with directionality varying according to whether one, the other, or both sexes of partners are infected, and with which strain. Unidirectional incompatibility typically involves an infected male and uninfected female, with the reciprocal cross (uninfected male with infected female) being compatible (producing viable offspring). Bidirectional incompatibility usually involves both partners being infected with different strains of Wolbachia and no viable offspring are produced from any mating.

• Parthenogenesis, or sex-ratio bias to the diploid sex (usually female) in insects with haplodiploid genetic systems (sections 5.6, 12.2, & 12.4.1). In the parasitic wasps (Trichogramma) studied this involves infected females that produce only fertile female offspring. The mechanism is usually gamete duplication, involving disruption of meiotic chromosomal segregation such that the nucleus of an unfertilized, Wolbachia-infected egg contains two sets of identical chromosomes (diploidy), producing a female. Normal sex ratios are restored by treatment of parents with antibiotics, or by development at elevated temperature, to which Wolbachia is sensitive.

• Feminization, the conversion of genetic males into functional females, perhaps caused by specific inhibitions of male-determiner genes. This effect has been studied in terrestrial isopods and a few insects (one species each from the Lepidoptera and Hemiptera), but may be more widespread in other arthropods. In the butterfly Eurema hecabe (Pieridae), feminizing Wolbachia endosymbionts have been shown to act continuously on genetic males during larval development, leading to female phenotypic expression; antibiotic treatment of larvae leads to intersexual development. Lepidopteran sex determination is thought to be complete in early embryogenesis, and thus the Wolbachia-induced feminization apparently does not target embryonic sex determination.

• Male killing, usually during early embryogenesis and possibly as a result of lethal feminization (see below).

The strategy of Wolbachia can be viewed as reproductive parasitism (section 3.6.5), in which the bacterium manipulates its host into producing an imbalance of female offspring (this being the sex responsible for the vertical transmission of the infection) compared with uninfected hosts. Only in a very few cases have infections been shown to benefit the insect host, primarily via enhanced fecundity. Certainly, with evidence derived from phylogenies of Wolbachia and their host, Wolbachia often has been transferred horizontally between unrelated hosts, and no coevolution is apparent.

Although Wolbachia is now the best-studied system of a sex-ratio-modifying organism, there are other somewhat similar cytoplasm-dwelling organisms (such as Cardinium bacteria in the Bacteroidetes), with the most extreme sex-ratio distorters known as malekillers. This phenomenon of male lethality is known across at least five orders of insects, associated with a range of maternally inherited, symbiotic-infectious causative organisms, from bacteria to viruses, and microsporidia. Each acquisition seems to be independent, and others are suspected to exist. Certainly, if parthenogenesis often involves such associations, many such interactions remain to be discovered. Furthermore, much remains to be learned about the effects of insect age, remating frequency, and temperature on the expression and transmission of Wolbachia. Even less is known about Cardinium, which occurs most commonly in haplodiploid insects (for example, certain parasitic wasps and some armored scale insects) and has been implicated in induction of parthenogenesis. There is an intriguing case involving the parasitic wasp Asobara tabida (Braconidae) in which the elimination of Wolbachia by antibiotics causes the inhibition of egg production, rendering the wasps infertile. Such obligatory infection with Wolbachia also occurs in filarial nematodes (section 15.5.6).

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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