Oxygen-induced plasticity in tracheal morphology and discontinuous gas exchange cycles in cockroaches Nauphoeta cinerea

The function and mechanism underlying discontinuous gas exchange in terrestrial arthropods continues to be debated. Three adaptive hypotheses have been proposed to explain the evolutionary origin or maintenance of discontinuous gas exchange cycles (DGCs), which may have evolved to reduce respiratory water loss, facilitate gas exchange in high CO2 and low O2 micro-environments, or to ameliorate potential damage as a result of oversupply of O2. None of these hypotheses have unequivocal support, and several non-adaptive hypotheses have also been proposed. In the present study, we reared cockroaches Nauphoeta cinerea in selected levels of O2 throughout development, and examined how this affected growth rate, tracheal morphology and patterns of gas exchange. O2 level in the rearing environment caused significant changes in tracheal morphology and the exhibition of DGCs, but the direction of these effects was inconsistent with all three adaptive hypotheses: water loss was not associated with DGC length, cockroaches grew fastest in hyperoxia, and DGCs exhibited by cockroaches reared in normoxia were shorter than those exhibited by cockroaches reared in hypoxia or hyperoxia

Origin of the animal circadian clock: diurnal and light-entrained gene expression in the sponge Amphimedon queenslandica

The circadian clock is a molecular network that coordinates organismal behavior and physiology with daily environmental changes in the day-night cycle. In eumetazoans (bilaterians + cnidarians), this network appears to be largely conserved, yet different from other known eukaryotic circadian networks. To determine if the eumetazoan circadian network has an older origin, we ask here whether orthologs comprising this network are expressed in a manner consistent with a role in regulating circadian patterns in a representative of an earlier-branching animal lineage, the sponge Amphimedon queenslandica. The A. queenslandica genome encodes orthologs of many eumetazoan circadian genes, including two cryptochrome genes that encode flavoproteins, three Timeout genes, and two PAR-bZIP and seven bHLH-PAS transcription factor genes. There is no apparent Cycle ortholog, although we can identify three closely related ARNT genes. Of the putative circadian genes, only AqPARa and AqCry2 have a consistent oscillating diurnal expression profile, and the rhythmic expression of both these genes is partially lost when the animals are exposed to constant light or darkness. Expression of the other putative circadian genes, in particular AqClock, is neither diurnally-oscillating nor light-dependent. AqPARa and AqCry2 are also temporally and spatially co-expressed throughout embryonic and larval development. Transcripts of these genes are enriched first in cells comprising the larval posterior pigment ring, which is a simple photosensory organ that is responsible for the negative phototactic behavior displayed by larvae, and subsequently in the larval epithelial and subepithelial layers. The combined findings of no clear Cycle ortholog and of PAR-bZIP and cryptochrome being the only orthologs expressed in a pattern consistent with a circadian role suggests that either (i) the ancestral metazoan circadian network was simpler than the eumetazoan network, or (ii) that this sponge has lost some components, as has occurred in some other animals such as Hydra