Megoura Viciae Buckton Research Articles

SEXUAL FORMS OF THE PEA APHID, ACYRTHOSIPHON PISUM, PRODUCED ON AN ARTIFICIAL DIET

In recent years a number of aphid species have been successfully reared on chemically defined diets (Dadd & Mittler, 1966; Auclair, 1965; Dadd & Krieger, 1967; Ehrhardt, 1968). The development of such diets has offered an opportunity to study in detail several aspects of the physiology of these insects. In particular, the composition of the artificial diet has been shown to influence the production of winged forms by Myzus persicae (Sulzer) (Mittler & Dadd, 1966; Dadd, 1968). In all studies thus far, the aphids have been maintained in the parthenogenetic condition in order to accumulate data concerning the growth and fecundity of females of successive generations. However, it was clearly of interest to determine whether sexual forms could be produced by diet‐reared aphids. If so, further information regarding the sexual reproduction of these insects would be forthcoming.Attempts to achieve this using local strains of Myzus persicae, Aphis fabae Scopoli and Acyrthosiphon pisum (Harris) were unsuccessful, presumably because these strains were anholocyclie (Kunkel, unpubl.). A clone of a green strain of A. pisum, previously found to be holocyclic (Sutherland, unpubl.), was then obtained from England and was cultured at 20° C in a photoperiodic regime of 16 hr light: 8 hr dark per day on young bean plants (Vicia faba). The artificial diet on which the experimental aphids were maintained was formulated according to Dadd (1967) and enclosed in sachets of stretched parafilm (Dadd, Krieger & Mittler, 1967). The aphids were provided with fresh sachets every 3–4 days.Adult apterous females taken from the plant culture were caged in groups of five, and exposed to a photoperiodic regime of 10 hr light: 14 hr dark per day at 15°. Successive batches of larvae deposited by these aphids were maintained in this environment until they were adult and had produced offspring for several days. These developed into oviparae and males. Compared to the sexual forms produced on host plants, these diet‐reared individuals grew relatively poorly and mortality was high. Nevertheless, the surviving oviparae deposited numerous eggs, but these remained pale green whereas viable, fertilized eggs deposited by plant‐reared oviparae rapidly become black.In the case of aphids reared on host plants in a short‐day environment the possibility exists that the photoperiod may act indirectly via the plants and not directly on the insects themselves — a doubt which has been raised concerning Megoura viciae Buckton (von Dehn, 1967). However, our results with A. pisum reared on artificial diet demonstrate, as Lees did for M. viciae (Lees, 1960, 1967), that it is unnecessary to implicate the host plant in the determination of males and oviparae. This does not, of course, exclude the possibility that photoperiod‐induced changes in a host plant may play a role as well. Nor have we ruled out the possibility that dietary composition may influence the aphids directly or their response to other environmental stimuli leading to the production of sexual forms.

Direct and indirect effects of day length on the aphid Megoura viciae Buckton

Although it is known that the production of the sexual and parthenogenetic female morphs in Megoura viciae can be controlled by the direct action of photoperiod on the parent aphid, von Dehn has recently advanced the view that there is an additional, if subsidiary, photoperiodic effect which is exerted through the medium of the plant. However, an examination of the experimental methods employed by von Dehn suggests that the evidence for the indirect action of photoperiod may be far from conclusive. In particular, it is pointed out that no account has been taken of the very early (prenatal) onset of photoperiodic sensitivity in this insect. Very slight differences in the day-length régime during this period of ontogeny greatly influence the response pattern.

The production of the apterous and alate forms in the aphid Megoura viciae Buckton, with special reference to the rôle of crowding

Although high temperature suppresses the production of alate forms in Megoura, the most significant controlling factor is the degree of crowding. This has been studied by confining aphids in tubes before returning them to the host plant for progeny testing. Adult apterae crowded together in groups of ten for 24 hr produce successive batches of progeny containing alatae and apterae in varying proportions, the number of alatae tending to fall as parturition proceeds. Control aphids isolated in tubes for the same length of time produce only apterous daughters. Crowding effects are therefore due to interactions between the aphids themselves and are not caused by starvation. The response is maternal and begins to appear in the fourth instar. Younger larvae are insensitive. The development of the crowding response is related to the frequency of the encounters between aphids. This was shown by varying either the population density or the size of the container. However, two individuals are adequate to constitute a ‘crowd’, provided the volume of the ‘arena’ is not too great. The contacts must extend over several hours. Overcrowding tends to reduce the response intensity. The sensory mechanisms of the crowding response have been examined. Visual and olfactory stimulation, as well as the exchange of pheromones, can be excuded. Since contacts are required, the relevant sense is probably that of touch. The mechanoreceptors are doubtless the thick-walled articulated bristles on the legs, antennae, and body. Antennal amputation, which removes all the known chemosensilla, fails to abolish the response. Artificial tactile stimulation has so far failed to elicit alata-production. But a variety of alien insects, such as small cockroaches or the beetle Ptinus tectus, are efficient ‘aphid substitutes’. Insects that are too sluggish to provide many encounters are ineffective. Wing dimorphism is apparently controlled by a diffusible ‘alata-determiner’ liberated from the head of the parent. This agent seems partially to inhibit the secretory activity of the corpus allatum during the late prenatal and early postnatal periods of larval development. But the determination process, which decides the level of secretion, occupies only a few hours prior to birth.

The role of photoperiod and temperature in the determination of parthenogenetic and sexual forms in the aphid Megoura viciae Buckton—III. Further properties of the maternal switching mechanism in apterous aphids

In the aphid Megoura the maternal photoperiodic process is initiated about 2 days before the birth of the apterous parent, whereas her first embryos are determined as virginoparae or oviparae several days after her birth. The prenatal light régime nevertheless influences the progeny sequence in several ways. First, a reversal from long to short day photostimulation at birth greatly accelerates the response to a second reversal of photoperiod (that is, from short to long days) during the adult life of the same individual. This suggests that the controlling (endocrine?) system is ‘switched on’ by long photoperiods and that full functional activity is hastened by a previous partial activation. Secondly, the effect of a brief long day treatment during the prenatal period is seen in the progeny sequence, provided the subsequent short photoperiod is intrinsically ‘weak’. The ease of reversal of a previous photoperiodic treatment can be used for evaluating the intensity of photoperiodic stimuli. Photoperiods 30 min longer or shorter than the critical length exert weak long and short day effects. Short photoperiods within the 4–14 hr range are ‘strong’, but permanent darkness again exerts a weak short day effect. At 15°C the critical daily photoperiod is approximately 14 hr 55 min. Near this point a difference of only 15 min produces a significant change in the response. Within the 10–20°C range the response is virtually temperature compensated, the only overt effect being that the critical photoperiod shortens by about 15 min/5°C rise of temperature. Above 23°C temperature compensation is progressively lost and virginoparae are produced even under strong short day conditions. It is suggested that the photoperiodic mechanism involves a light receptor and a humoral component, both possibly located in the neuroendocrine system. Under near-neutral photoperiodic conditions the endocrine moiety acts autonomously and ‘drifts’ irregularly between activity and inactivity.

The role of photoperiod and temperature in the determination of parthogenetic and sexual forms in the aphid Megoura viciae Buckton—II. The operation of the ‘interval timer’ in young clones

The ability of young clones of Magoura to form sexuales (males and oviparae) under short day conditions is regulated by a timing mechanism which at first prevents the appearance of these morphs. The operation of this ‘interval timer’ is unaffected by the number of generations through which the lineage has passed. It is, however, influenced by temperature, though not by the type of parents (apterous or alate virginoparae) used in the intermediate generations. Under permanent long day conditions the ‘interval timer’ operates at the appropriate moment, but the potentiality for forming oviparae remains latent until evoked by short days. The timing mechanism is probably located in the cytoplasm. Since the number of individual- and cell-generations does not influence its rate of operation, it is suggested that the elements involved may be capable of self-duplication while undergoing continuous transformation. The latter change must culminate in the restitution of the maternal control system which permits the egg or embryo to develop along alternative pathways in response to the appropriate environmental signals.

The role of photoperiod and temperature in the determination of parthenogenetic and sexual forms in the aphid Megoura viciae Buckton—I : The influence of these factors on apterous virginoparae and their progeny

Apterous virginoparae from long-established parthenogenetic clones of Megoura are capable of producing three types of daughters (winged and wingless virginoparae and sexual oviparae) in addition to males. This polymorphism is regulated by the environment which, by acting through a series of maternal “switch” mechanisms, directs the growing oöcytes and embryos into different developmental pathways. Although there is little reason to doubt that sex is determined genetically (presumably at the oöcyte stage), the sex ratio of the progeny is also under physiological control. In certain environments, notably at high or low temperatures (although not in long photoperiods), male production is inhibited. But even under the most favourable conditions males are relatively scarce and are born predominantly in the middle of the reproductive life of the parent. The later differentiation of part-grown female embryos is also governed environmentally. Long photoperiods and high temperatures induce them to become virginoparae, the reverse conditions, oviparae. The action of photoperiods of intermediate or “critical” length is particularly instructive as providing evidence of the maternal, and perhaps humoral, control of these forms. Their differentiation begins when the parent is still in the early larval instars and is completed in the adult stage. The progression of determination can be explained in terms of the competence of embryos in different stages of growth to respond to alternative maternal stimuli.