The evolution of approach herkogamy from protandry in New ZealandGentiana (Gentianaceae)

Most mainland New Zealand species ofGentiana are protandrous as are manyGentianaceae elsewhere in the world. Selfing has evolved in some species confined to areas where pollinator availability is limited by weather, for example, inG. lineata. InG. chathamica, endemic to the Chatham Islands, dichogamy has been lost without the evolution of selfing; instead the flowers are approach herkogamous. It is suggested that as herkogamous blossoms can both receive and dispatch pollen during a single insect visit, whereas dichogamous systems usually require two visits, such a system may have evolved inG. chathamica to maintain outcrossing in an area where prospects for insect pollination are poor.     

A stochastic model for evolution of sociality in insects

We study population biology of eusocial insects such as Ropalidia marginata through a stochastic model based on random (matrix) difference equations. This facilitates a study of dynamics of such populations when the survival and other rates vary randomly over time. The worker-brood relatedness, which is a function of the underlying population structure, can be used to explain theories on the evolution of altruism. The effect of demographic parameters and the queen takeover probabilities on the worker-brood relatedness has been studied. Based on the proposed model, we simulate insect colonies where queens are replaced. Simulation results help us to study the effect of various factors on the worker-brood relatedness. Further, we study two estimators of the worker-brood relatedness and suggest procedures for estimating their standard errors. Approximate confidence intervals for the same can be constructed with the help of these results.     

昆虫寄主选择行为的进化机制

昆虫的寄主选择行为受到遗传、可传承的环境因素或学习行为的调控。尽管大量实验已证实昆虫寄主选择中的遗传变异,但在实际中很难将其与温度、营养、条件作用和可传承的环境因素的作用区别开来。一些报道已证实可传承的环境因素会影响昆虫的寄主选择行为。幼虫期和成虫期对寄主的经历可以改变该虫态的取食和产卵寄主偏嗜行为。虽然巴甫洛夫条件反射和神经组织移植实验初步揭示了幼虫经历对成虫气味选择行为的影响,但寄主选择行为中的前印象条件作用尚需进一步验证。在城市垃圾生态系统中,杂食性的蝇类的取食和产卵选择行为较腐食性的蝇类更易受到学习经历的重塑。     

Exaptation and the evolution of dealation in insects

This paper presents the results of an empirical test of the hypothesis that the evolution of dealation, or self-inflicted removal of wings, is favored in some insect species by the pre-existence of a physiological response of enhanced reproduction following dealation. Two Orthopteran species that do not naturally shed their wings are studied, Teleogryllus oceanicus and Gryllus firmus. The hypothesis that enhanced reproduction following dealation is accompanied by histolysis of the wing muscles is also examined. Dealation increases the rate of egg production in both species. However, total fecundity is not increased by dealation. Wing muscle histolysis is increased by dealation and there is a significant correlation between the degree of histolysis and cumulative egg production to days 4 and 7. It is suggested that Orthoptera are “preadapted” for the evolution of dealation and that ecological factors rather than physiological are most important in determining its appearance in various species.     

Models on butterfly protandry: Virgin females are at risk to die

Current models on protandry in butterflies assume that females are mated instantaneously upon eclosion. However, for most butterfly species this assumption is not realistic. In this paper a model is formulated in which the mating rate depends on both male and female density. Given the female presence curve, protandry is an evolutionarily stable strategy (ESS) for males. The evolutionarily stable amount of protandry decreases with increasing death rate and decreasing encounter rate. Given the male presence curve, protandry also is an ESS for females. However, male and female ESS are not identical; moreover, in the present model a simultaneous ESS does not exist. Protandry critically depends on the assumption that females mate only once, whereas males are capable of multiple mating. If females too are capable of multiple mating, absence of protandry is the ESS for males as well as females. The model predicts that protandry depends on population density: protandry should be more pronounced in populations with high density than in populations with low density. Protandry also depends on sex ratio. It becomes more pronounced when the proportion of males among emerging adults increases.     

Genome evolution in bacterial endosymbionts of insects

Many insect species rely on intracellular bacterial symbionts for their viability and fecundity. Large-scale DNA-sequence analyses are revealing the forces that shape the evolution of these bacterial associates and the genetic basis of their specialization to an intracellular lifestyle. The full genome sequences of two obligate mutualists, Buchnera aphidicola of aphids and Wigglesworthia glossinidia of tsetse flies, reveal substantial gene loss and an integration of host and symbiont metabolic functions. Further genomic comparisons should reveal the generality of these features among bacterial mutualists and the extent to which they are shared with other intracellular bacteria, including obligate pathogens.     

Implications of senescence patterns for the evolution of age polyethism in eusocial insects

Division of labor based on age, with workers delaying performanceof risky tasks, evolved in all major lineages of eusocial insects,raising questions about possible selective advantages of thispattern. We present a model showing that patterns of senescencein workers can have profound implications for the evolutionof age polyethism in insect societies. Controversy exists overthe relative importance of programmed versus rate of livingsenescence in determining the longevity of animals. If programmedsenescence occurs in eusocial insects, workers that delay performanceof risky tasks until later in life have greater expected longevityand therefore perform more labor per unit production cost. Ifsenescence does not occur or if the age of senescent death isdetermined solely by rate of living effects, then worker longevityis not increased by delayed performance of risky tasks. Severallines of indirect evidence suggest that high rates of work havenegative effects on worker longevity, supporting the rate ofliving hypothesis. Critical empirical data on patterns of senescencein eusocial insect workers are lacking, however, particularlywith respect to the occurrence of programmed senescence. Furthermore,programmed senescence may operate in conjunction with rate ofliving effects. In this case, a negative relationship betweenwork rate and longevity will be evident in highly active workers,while longevity of inactive workers will reach an upper plateauand will not be related to differences in work rate. If programmedsenescence is demonstrated in eusocial insect workers, thenincreased worker longevity may have favored the evolution ofage polyethism with delayed performance of risky tasks.     

Polyandry and protandry in butterflies

Current models on protandry in butterflies assume that females mate only once, yet for many species this assumption is not realistic. In this paper a model is formulated to study how polyandry, i.e. repeated mating of females, affects protandry. Moreover, the model is elaborated to describe the probability distribution of the number of matings per female. Field data on this distribution are well described by the model, which supports the use of the law of mass action to describe the encounter rate between males and females. Finally, a weight factor is derived, taking into account the decline in oviposition rate with age, as well as the chance that a female is remated. In comparison with the situation that all matings contribute equally to a male's reproductive success, the application of the weight factor enhances protandry. This suggests that mate competition is not the sole cause of protandry.     

Social evolution: community policing in insects

A recent study shows that, in social insects where workers suppress or 'police' the reproduction of nestmate workers, only a subset of workers act as police. This confirms that policing can serve a collective rather than a selfish interest.     

Social immunity and the evolution of group living in insects

The evolution of group living requires that individuals limit the inherent risks of parasite infection. To this end, group living insects have developed a unique capability of mounting collective anti-parasite defences, such as allogrooming and corpse removal from the nest. Over the last 20 years, this phenomenon (called social immunity) was mostly studied in eusocial insects, with results emphasizing its importance in derived social systems. However, the role of social immunity in the early evolution of group living remains unclear. Here, I investigate this topic by first presenting the definitions of social immunity and discussing their applications across social systems. I then provide an up-to-date appraisal of the collective and individual mechanisms of social immunity described in eusocial insects and show that they have counterparts in non-eusocial species and even solitary species. Finally, I review evidence demonstrating that the increased risks of parasite infection in group living species may both decrease and increase the level of personal immunity, and discuss how the expression of social immunity could drive these opposite effects. By highlighting similarities and differences of social immunity across social systems, this review emphasizes the potential importance of this phenomenon in the early evolution of the multiple forms of group living in insects.     

Circadian rhythms and the evolution of photoperiodic timing in insects

This review discusses possible evolutionary trends in insect photoperiodism, mainly from a chronobiological perspective. A crucial step was the forging of a link between the hormones regulating diapause and the systems of biological rhythms, circadian or circannual, which have independently evolved in eukaryotes to synchronize physiology and behaviour to the daily cycles of light and darkness. In many of these responses a central feature is that the circadian system resets to a constant phase at the beginning of the subjective night, and then ‘measures’ the duration of the next scotophase. In ‘external coincidence’, one version of such a clock, light now has a dual role. First, it serves to entrain the circadian system to the stream of pulses making up the light/dark cycle and, second, it regulates the nondiapause/diapause switch in development by illuminating/not illuminating a specific light sensitive phase falling at the end of the critical night length. Important work by A. D. Lees on the aphid Megoura viciae using so‐called ‘night interruption experiments' demonstrates that pulses falling early in the night lead to long‐day effects that are reversible by a subsequent dark period longer than the critical night length and also show maximal sensitivity in the blue–green range of the spectrum. Pulses falling in the latter half of the night, however, produce long‐day effects that are irreversible by a subsequent long‐night and show a spectral sensitivity extending into the red. With movement to higher latitudes, insects develop genetic clines in various parameters, including critical night length, the number of long‐night cycles needed for diapause induction, the strength of the response, and the ‘depth’ or intensity of the diapause thus induced. Evidence for these and other types of photoperiodic response suggests that they provided strong selective advantages for insect survival.     

Utility and evolution of cytochrome b in insects

Cytochrome b (cyt-b) is widely used in molecular phylogenetic studies of vertebrate, but not invertebrate, taxa. To determine whether this situation is an historical accident or reflects the utility of cyt-b, we compared the abilities of cyt-b, COI, and one nuclear ribosomal gene region (D1 of 28S) to recover intergeneric relationships within the tiger moth tribes Ctenuchini and Euchromiini. Additionally, we compared the rate of sequence and amino acid evolution of cyt-b across insects. Cytochrome b had the same level of sequence variation and A/T bias as COI, but was less useful for recovering intergeneric relationships. The total evidence tree casts doubt on the traditional taxonomy of the group. For the class Insecta, we found that functional conservation of amino acids occurs for the same regions as those found in vertebrates with the exception of Mallophaga (lice). Lice have an accelerated rate of nonsynonymous substitutions. Accelerated rate of cyt-b nucleotide and amino acid evolution in Apidae (bees) may be correlated with increased metabolic rates associated with facultative endothermy (= heterothermy).     

Developmental evolution: Axial patterning in insects.

The Drosophila bicoid gene is well known for encoding a protein that forms a morphogenetic gradient with a key role in anterior patterning of the fruitfly embryo. Recent results suggest the evolution of bicoid might have involved dramatic changes in function - essentially the invention of a new regulatory protein.     

Design, function and evolution in the wings of holometabolous insects

The wings of Holometabola are reviewed in functional terms. Two basic types, anteroposteriorly symmetric ('symmetric') and anteroposteriorly asymmetric ('asymmetric'), are recognized and their modes of operation discussed. Most neuropteroids have asymmetric wings, whereas relative anteroposterior symmetry appears plesiomorphic for mecopteroids. Wing coupling has only occurred in lineages with wings of symmetric type, but anteroposterior asymmetry of the wing couple has later developed several times in association with improved, more versatile, flight performance. Coleoptera wings seem to have developed from predecessors of symmetric type, but their morphology is strongly influenced by the need to fold up at rest. Diptera provide an excellent illustration of the evolution of asymmetric, high-performance wings from the symmetric, 'mecopteran' type. Asymmetric and symmetric wings can both be derived from less specialized types. Both are well represented as fossils from the Permian onwards, and these supply further examples of the evolution of asymmetry from the symmetric pattern.