Photoperiodic control of larval development in the semivoltine cockroach Periplaneta japonica (Blattidae: Dictyoptera)

Diapause in Univoltine and Semivoltine Life Cycles

Although it is generally more adaptive for insects to produce additional generations than to have longer life cycles, some insects produce one or fewer generations per year (univoltine or semivoltine life cycles, respectively). Some insects with the potential to produce multiple generations per year produce a univoltine life cycle in response to environmental conditions. Obligatory univoltine insects have a single long diapause or multiple diapauses in different seasons. Semivoltine insects have multiple diapauses in different years, a prolonged diapause for more than a year, or diapause controlled by a circannual rhythm. Diapause in these insects greatly varies among species both in the physiological mechanism and in the evolutionary background, and there is no general rule defining it. In this review, we survey the physiological control of univoltine and semivoltine insects' diapause and discuss the adaptive significance of the long life cycles. Although constraints such as slow development are sometimes responsible for these life cycles, the benefits of these life cycles can be explained by bet-hedging in many cases. We also discuss the effect of climate warming on these life cycles as a future area of research.

Life-History Traits from Embryonic Development to Reproduction in the American Cockroach

Simple Summary

The American cockroach is a widely distributed sanitary pest closely related to human life. The American cockroach is not only a hygienic pest that we all know but also beneficial to humans as its extract can be used medicinally and could be a model organism for physiology and neuroscience studies. In this study, we provide a life table of the American cockroach in a stable environment, including embryonic development, nymphal instars, and adult reproduction. Newly laid eggs hatch into nymphs after about 35 days of embryonic development. Under sufficient materials and space, gregarious nymphs undergo 14 molts before transforming into adults. Adult females can produce fertile offspring whether they have mated or not. On average, mated females produce an ootheca every 4 days, while unmated females produce an ootheca every 10 days. Each ootheca contains 12–16 eggs. Additionally, group living seems to improve the survival rate of offspring of unmated females.

Abstract

The American cockroach, Periplaneta americana (Insecta: Blattodea: Solumblattodea: Blattidae), is an urban hygiene pest but also a model organism for physiology and neuroscience study. However, the current description of the developmental process of the American cockroach is insufficient. In this study, we provide a life table of the American cockroach in a stable environment, including embryonic development, nymphal instars and adult reproduction. Our results show that there are 14 nymphal instars of the American cockroach in groups with sufficient living materials and space. The secondary sexual characteristics are evident in last-instar nymphs and adults, namely, the complete absence of the anal stylus in females. The entire embryonic development process was divided into 20 stages on the basis of lateral-view observations of the embryos. The formation of the embryo involves the fusion of paired blastoderm regions with higher cellular density, similar to that in other insects of Polyneoptera. With respect to reproduction, the gamogenetic females produced their first ootheca earlier than the parthenogenic females, and the frequency of oviposition was higher for the former throughout adulthood. Interestingly, group living seems to improve the parthenogenesis success rate in the American cockroach.

Watching the days go by: Asymmetric regulation of caterpillar development by changes in photoperiod

Many insects possess the plastic ability to either develop directly to adulthood, or enter diapause and postpone reproduction until the next year, depending on environmental cues (primarily photoperiod) that signal the amount of time remaining until the end of the growth season. These two alternative pathways often differ in co-adapted life-history traits, for example, with slower development and larger size in individuals headed for diapause. The developmental timing of these differences may be of adaptive importance: If traits diverge early, the potential for phenotypic differences between the pathways is greater, whereas if traits diverge late, the risk may be lower of expressing a maladaptive phenotype if the selective environment changes during development. Here, we explore the effects of changes in photoperiodic information during life on pupal diapause and associated life-history traits in the butterfly Pararge aegeria. We find that both pupal diapause and larval development rate are asymmetrically regulated: While exposure to long days late in life (regardless of earlier experiences) was sufficient to produce nondiapause development and accelerate larval development accordingly, more prolonged exposure to short days was required to induce diapause and slow down prediapause larval development. While the two developmental pathways diverged early in development, development rates could be partially reversed by altered environmental cues. Meanwhile, pathway differences in body size were more inflexible, despite emerging late in development. These results show how several traits may be shaped by the same environmental cue (photoperiod), but along subtly different ontogenies, into an integrated phenotype.

Local adaptation of photoperiodic plasticity maintains life cycle variation within latitudes in a butterfly

The seasonal cycle varies geographically and organisms are under selection to express life cycles that optimally exploit their spatiotemporal habitats. In insects, this often means producing an annual number of generations (voltinism) appropriate to the local season length. Variation in voltinism may arise from variation in environmental factors (e.g., temperature or photoperiod) acting on a single reaction norm shared across populations, but it may also result from local adaptation of reaction norms. However, such local adaptation is poorly explored at short geographic distances, especially within latitudes. Using a combination of common-garden rearing and life cycle modeling, we have investigated the causal factors behind voltinism variation in Swedish populations of the butterfly Pararge aegeria, focusing on a set of populations that lie within a single degree of latitude but nonetheless differ in season length and voltinism. Despite considerable differences in ambient temperature between populations, modeling suggested that the key determinant of local voltinism was in fact interpopulation differences in photoperiodic response. These include differences in the induction thresholds for winter diapause, as well as differences in photoperiodic regulation of larval development, a widespread but poorly studied phenomenon. Our results demonstrate previously neglected ways that photoperiodism may mediate insect phenological responses to temperature, and emphasize the importance of local adaptation in shaping phenological patterns in general, as well as for predicting the responses of populations to changes in climate.

Development time and body size in Eupolyphaga sinensis along a latitudinal gradient from China

The responses of organisms to temperature variations may be via short term responses of the phenotype (phenotypic plasticity), or they could involve long-term evolutionary change and adaptation (via selection) to the genotype. These could involve changes to the mean size of the animal or to the thermal reaction norm. We examined the effects of various temperatures (of 22, 25, 28, and 31 °C) on development time, adult body size and preadult survivorship in three populations of the cockroach, Eupolyphaga sinensis (Walker), collected at different latitudes. We found substantial temperature-induced plasticity in development time, body size, and preadult survivorship, indicating that developmental temperatures have strong impacts on growth and life history traits of E. sinensis. Genetic differences for development time, body size, and preadult survivorship were detected among populations, and the three traits exhibited highly significant variations in the responses of different populations to various temperature conditions, indicating genetic differences among populations in terms of thermal reaction norms. We also found that two populations seem to support the beneficial acclimation hypothesis whereas the third mid-latitude population does not. The results are likely because of differences in season length and voltinism, indicating that not only temperature regime but also its interactions with generation time (and development time), voltinism, and season length are likely to have considerable effects on insect development time and body size. Overall, changes in development time, body size, and preadult survivorship in E. sinensis can all be regarded as adaptations to changing thermal regimes.

Eco-physiological phases of insect diapause

Insect diapause is a dynamic process consisting of several successive phases. The conception and naming of the phases is unsettled and, sometimes, ambiguous in the literature. In this paper, the ontogeny of diapause was reviewed and the most often used terms and the best substantiated phases were highlighted, explained and re-defined. The aim was to propose relatively simple and generally applicable terminological system. The phases of diapause induction, preparation, initiation, maintenance, termination and post-diapause quiescence were distinguished. The specific progression through diapause phases in each species, population (genotype), or even individual, is based on (thus far largely unknown) physiological processes, the actual expression of which is significantly modified by diverse environmental factors. Thus, such phases are eco-physiological in their nature.

Temperature acclimation in overwintering nymphs of a cockroach, Periplaneta japonica: walking on ice

Nymphs of the Yamato cockroach, Periplaneta japonica, showed a seasonal change in the ability to move at low temperature. Laboratory-reared nymphs buried in ice for 15 min-3 h took about 600 s to get up at 25 degrees C after being placed with their dorsal surface on the floor of a Petri dish. The mean time to get up at 25 degrees C (recovering time) after 15 or 60-min burial in ice was relatively long in early autumn, decreased rapidly in autumn, reached a minimal level in winter (<100 s) and increased in spring. Temperature was responsible for this seasonal change in physiology, and neither photoperiod nor diapause status was important. Recovering time depended on the acclimation temperature and its duration. For a 10-day exposure in a range of 0-25 degrees C, the lower the temperature the shorter the recovering. Longer periods of acclimation at these temperatures tended to shorten recovering time. The acquisition of the ability to move on ice is a seasonal phenomenon unique to P. japonica. No comparable response was found in three other cockroaches including two sub-tropical species (Periplaneta americana and Opisthoplatia orientalis) and a temperate species (Blattela nipponica).