Insect dormancy responses, in the broad sense of modifications of development, are examined from a general perspective. The range of responses is extraordinarily wide because environments are diverse, different taxa have different evolutionary histories, adaptations are needed for both seasonal timing and resistance to adversity, and not only development but also many other aspects of the life-cycle must be coordinated. Developmental options are illustrated by examining the wide range of ways in which development can be modified, the fact that each individual response consists of several components, and the different potential durations of the life-cycle. The concepts of alternative life-cycle pathways (chosen according to current and likely future environmental conditions) and of active and passive default responses are treated. Also introduced are aspects of variation and trade-offs.
Some general conclusions that help in understanding dormancy responses emerge from such an examination. Many options are available (cf. Table 1). The nature of the habitat, especially its predictability, determines the potential effectiveness of many of the developmental options. Any particular set of responses reflects evolutionary history and hence depends on past as well as current environments. It is not necessarily obvious what kinds of selection, especially requirements for timing versus resistance to adversity, explain a particular life cycle. Life-cycle pathways have multiple components, so that components cannot be analyzed in isolation. A given feature, such as delayed development, can have multiple roles. Default responses can be either active (development continues unless signalled otherwise) or passive (development stops unless signalled otherwise), making necessary a broad approach to understanding the action of environmental cues. Even relatively minor effects that fine-tune dormancy responses enhance survival, but may be difficult to detect or measure. Trade-offs are not inevitable, not only when certain resources are surplus, but also because resources in very short supply (constraints) cannot be traded off. Life-cycle components are widely, but not universally, coordinated. These conclusions confirm that the range of dormancy responses is wider, more complex and more integrated than has often been recognized.
Potential involvement of circadian clock genes in so far unknown mechanism of photoperiodic time measurement is an important question of insect life-cycle regulation science. Here we report about the cloning of full-length cDNA of the structural homologue of the Drosophila's timeless gene in Chymomyza costata. Its expression was compared in two strains: a wild-type strain, responding to short days by entering larval diapause and a npd-mutant strain, showing no photoperiodic response. The timeless mRNA transcripts were not detectable by Northern blot analysis in the fly heads of npd-mutants, while they were detectable and showed typical daily oscillations in the wild-type strain. After disrupting the normal process of timeless transcription in the wild-type strain by injection of timeless double-strandRNA into early embryos of wild-type (RNAi method: Kennerdell & Carthew 1998, 2000), a certain proportion of the individuals adopted a npd-mutant phenotype, showing no-diapause in response to short-daylength. Cloning of genomic DNA fragments revealed that npd-mutants carry a different allele, timelessnpd, with a 13-bp insertion in an intron positioned within the 5'-leader sequence. Genetic linkage analysis showed that the 13-bp insertion (a marker for timelessnpd) and the absence of response to short days (a marker for npd-phenotype) are strictly co-inherited in the F2 progeny of the reciprocal crosses between wild-type and npd-mutant flies. Such results indicated that the locus npd could code for the timeless gene in C. costata and its product might thus represent a molecular link between circadian and photoperiodic clock systems in this fly.