No abstract

Aphids are typical photoperiodic insects that switch from viviparous parthenogenetic reproduction typical of long day seasons to oviparous sexual reproduction triggered by the shortening of photoperiod in autumn yielding an overwintering egg in which an embryonic diapause takes place. While the involvement of the circadian clock genes in photoperiodism in mammals is well established, there is still some controversy on their participation in insects. The availability of the genome of the pea aphid Acyrthosiphon pisum places this species as an excellent model to investigate the involvement of the circadian system in the aphid seasonal response. In the present report, we have advanced in the characterisation of the circadian clock genes and showed that these genes display extensive alternative splicing. Moreover, the expression of circadian clock genes, analysed at different moments of the day, showed a robust cycling of central clock genes period and timeless. Furthermore, the rhythmic expression of these genes was shown to be rapidly dampened under DD (continuous darkness conditions), thus supporting the model of a seasonal response based on a heavily dampened circadian oscillator. Additionally, increased expression of some of the circadian clock genes under short-day conditions suggest their involvement in the induction of the aphid seasonal response. Finally, in situ localisation of transcripts of genes period and timeless in the aphid brain revealed the site of clock neurons for the first time in aphids. Two groups of clock cells were identified: the Dorsal Neurons (DN) and the Lateral Neurons (LN), both in the protocerebrum.

Most organisms exhibit some kind of rhythmicity in their behaviour and/or physiology as an adaptation to the cyclical movements of the Earth. In addition to circadian rhythms, many organisms have an annual rhythmicity in certain activities, such as reproduction, migration or induction of diapause. Current knowledge of the molecular basis controlling seasonal rhythmicity, especially in insects, is scarce. One element that seems to play an essential role in the maintenance of both circadian and seasonal rhythms in vertebrates is the hormone melatonin. In vertebrates, the limiting enzyme in its synthesis is the arylalkylamine N-acetyltransferase (AANAT). Melatonin is also present in insects but the precise biochemical pathway and the enzymes involved in its synthesis are unknown. Insects possess phylogenetically distant arylalkylamine N-acetyltransferases but their involvement in melatonin synthesis still needs to be fully demonstrated. Aphids have a seasonally rhythmical life cycle, reproducing parthenogenetically by viviparity in favourable seasons but, in unfavourable seasons, they produce a single generation of sexual individuals. The length of the photoperiod is the main environmental factor that controls the mode of reproduction in aphids. Taking advantage of the availability of the genome of the aphid Acyrthosiphon pisum, we searched for genes encoding aphid arylalkylamine N-acetyltransferase homologues that could be candidates for participation in seasonal rhythmicity. We identified four AANAT genes, of which at least two (Ap-AANAT1 and Ap-AANAT3) showed highly significant variation in transcription levels depending on the photoperiod conditions. These results are discussed in the context of how seasonality can be controlled in aphids.