Maternal age as a source of variation in the ability of an aphid to produce dispersing forms

Apterous parthenogentic females of the pea aphid, Acyrthosiphon pisum (Harris ), begin to produce alate offspring soon after they have been subjected to crowding. Females which were born early in their own parent's reproductive period respond most strongly to crowding, producing much larger numbers of alatae than their late-born sisters. In contrast, the early-born daughters of most alate females do not produce winged offspring after being crowded. Some of their later-born sisters may produce a few winged individuals, resembling in this respect the late-born daughters of the apterous females. Control of the production of alatae thus begins in the grandparental generation. Risk-spreading by means of differential dispersal becomes a less uncertain venture when local populations can modify their responses to environmental changes by utilizing past as well as present signals from their surroundings.     

The effects of food and density on the nocturnal behaviour ofArion ater andAriolimax columbianus (Pulmonata: Stylommatophora)

The effects of food supply and population density on the nocturnal behaviour of Arion ater and Ariolimax columbianus were investigated. Density did not significantly affect A. ater's level of activity or short-term movement, resting, or feeding. A. columbianus was more active and moved, rested, and fed more frequently when slug density was high. A. ater foraged and rested more often, but fed less when good food was unavailable. Ariolimax's only response to the food regime was to feed more when good food was available. Seasonal changes in the level of activity and behaviour of Arion were evident, whereas Ariolimax's activity and behavioural repertoire were not similarly affected. Arion ater's nightly activity appeared to be mainly food oriented, while Ariolimax columbianus seemed most responsive to slug density during its nocturnal activity periods.     

The effects of food and density of the movement ofArion ater andAriolimax columbianus (Pulmonata: Stylommatophora) between habitats

Experiments were conducted to determine if slug density, or food quantity and quality could cause dispersal by slugs. Arion ater dispersed quickly from overcrowded areas, where high mortality was incurred by this species. Poor food conditions, caused more dispersal than good food conditions, but food had no effect on the mortality of A. ater. Neither food nor density factors had significant effects on the dispersal of Ariolimax columbianus. However, more A. columbianus died in the high- than low-density treatments. A. columbianus mortality was unaffected by the food factor. Dispersing Arion were heavier than non-dispersers, but there was no difference between the weights of dispersing and non-dispersing Ariolimax. There were significant seasonal changes in the dispersal rate, mortality and weight of dispersing Arion, whereas only the mortality of Ariolimax changed significantly during the experiments. Since Arion ater did not markedly reduce its activity during unsuitable weather it was able to leave areas of poor food and high slug density fairly rapidly. Ariolimax columbianus' large size allowed it to remain inactive during unfavourable weather and permitted this species to be relatively unresponsive to the experimental treatments.     

Temperature and development in host-parasite relationships

Summary The effects of temperature on growth and development of the cabbage butterfly, Pieris rapae, and three wasp parasites: Apanteles rubecula, Apanteles glomeratus and Pteromalus puparum in Vancouver, Canada, and Canberra, Australia, are examined. We compare the estimates of temperature threshold for development and the number of degree-days above this threshold required to complete development for the immature stages of all species in both localities. Developmental patterns of both the host and its parasites differ between localities. Within the range of temperatures likely to be experienced during the host's breeding season, Australian parasites have longer generation times than their host at low temperatures and shorter generation times at high temperatures. Canadian parasites have shorter generation times, relative to the host, at all temperatures. This may be necessitated by the shorter breeding season available to the Canadian parasites.Besides temperature, parasite development is affected by host size and, in the gregarious species, parasite density. Host larval development is retarded by both Apanteles.All parasites are smaller at higher temperatures and males are smaller than females, but size is also affected by host size and parasite density.Although parasite size, and consequently fecundity, varies greatly, parasites experiencing similar temperatures will have closely similar developmental periods. The ecological significance of these developmental responses is discussed.     

The dynamics of outbreaks: Further simulation experiments with the western tent caterpillar

A stochastic model of western tent-caterpillar populations on southern Vancouver Island was used to assess conditions leading to outbreaks. Three qualitatively different populations-declining, minimal, and recovering-were exposed to six-year climatic sequences incorporating various combinations of good, bad, and mediocre spring weather. Starting populations were either distributed randomly or concentrated in and around small-, medium-, or large refuges. The results showed that virtually any type of population could temporarily increase in numbers during two highly favorable springs. Only a very few vigorous populations, however, could sustain the momentum required to surge to outbreak proportions. No low-quality population could do so. And even vigorous populations could not achieve outbreak status unless they were initially concentrated in a sufficiently large refuge that was close enough to potentially suitable habitats to permit rapid immigration into such places as soon as their local climates improved. The concept of “climatic release” of insect populations is discussed in the light of these findings. The results demonstrate that climatic release is inextricably linked with a population's qualitative profile and spatial distribution.     

The insert within the catalytic domain of tripeptidyl-peptidase II is important for the formation of the active complex.

Tripeptidyl-peptidase II (TPP II) is a large (Mr>10(6)) tripeptide-releasing enzyme with an active site of the subtilisin-type. Compared with other subtilases, TPP II has a 200 amino-acid insertion between the catalytic Asp44 and His264 residues, and is active as an oligomeric complex. This study demonstrates that the insert is important for the formation of the active high-molecular mass complex. A recombinant human TPP II and a murine TPP II were found to display different complex-forming characteristics when over-expressed in human 293-cells; the human enzyme was mainly in a nonassociated, inactive state whereas the murine enzyme formed active oligomers. This was surprising because native human TPP II is purified from erythrocytes as an active oligomeric complex, and the amino-acid sequences of the human and murine enzymes were 96% identical. Using a combination of chimeras and a single point mutant, the amino acid responsible for this difference was identified as Arg252 in the recombinant human sequence, which corresponds to a glycine in the murine sequence. As Gly252 is conserved in all sequenced variants of TPP II, the recombinant enzyme with Arg252 is atypical. Nevertheless, as Arg252 evidently interferes with complex formation, and this residue is close to the catalytic His264, it may also explain why oligomerization influences enzyme activity. The exact mechanism for how the G252R substitution interferes with complex formation remains to be determined, but will be of importance for the understanding of the unique properties of TPP II.     

Variation due to maternal age in the onion root maggot,Hylemya antiqua (Meigen)

A variety of characteristics were studied in populations of the onion root maggot, Hylemya antiqua, to determine the extent to which the age of the female producing a batch of eggs might affect heterogeneity in her offspring. Early-born offspring were found to be reproductively most successful. They had the longest mean expectation of life, a relatively low mortality rate into mid-life, the highest average fecundity, the highest mean rate of egg production, and they produced the highest percentage of female offspring. On the other hand, mid-born offspring were hardiest. They also had a long mean expectation of life, comparable to that for early-born offspring but, in addition, they had the lowest sustained mortality rate and the greatest ability to survive food stress. Their robustness may have been acquired at the expense of certain reproductive capabilities however, in that their average fecundity was lower than their early-born siblings', and their rate of egg production was more variable. Late-born offspring were the most mortality-prone and the least fecund, but they had the shortest mean generation time. These differences must be evaluated in terms of their effects on the rate of natural increase and on population growth. Maternal age was found to influence pupal size, although the provenance groups differed significantly from each other and, in fact, showed opposite trends within their respective cohort groups. Size itself, and the ability of a species to alter the size at which it pupates, may be characteristics which have evolved to fit a specific set of environmental conditions.