Regulation of Numb during planar cell polarity establishment in the Drosophila eye

ABSTRACTThe establishment of planar cell polarity (PCP) in the Drosophila eye requires correct specification of the R3/R4 pair of photoreceptor cells, determined by a Frizzled mediated signaling event that specifies R3 and induces Delta to activate Notch signaling in the neighboring cell, specifying it as R4. Here, we investigated the role of the Notch signaling negative regulator Numb in the specification of R3/R4 fates and PCP establishment in the Drosophila eye. We observed that Numb is transiently upregulated in R3 at the time of R3/R4 specification. This regulation of Numb levels in developing photoreceptors occurs at the post-transcriptional level and is dependent on Dishevelled, an effector of Frizzled signaling, and Lethal Giant Larva. We detected PCP defects in cells homozygous for numb15, but these defects were due to a loss of function mutation in fat (fatQ805*) being present in the numb15 chromosome. However, mosaic overexpression of Numb in R4 precursors (only) caused PCP defects and numb loss-of-function had a modifying effect on the defects found in a hypomorphic dishevelled mutation. Our results suggest that Numb levels are upregulated to reinforce the bias of Notch signaling activation in the R3/R4 pair, two post-mitotic cells that are not specified by asymmetric cell division.

A possible 150 million years old cirripede crustacean nauplius and the phenomenon of giant larvae

The larval phase of metazoans can be interpreted as a discrete post-embryonic period. Larvae have been usually considered to be small, yet some metazoans possess unusually large larvae, or giant larvae. Here, we report a possible case of such a giant larva from the Upper Jurassic Solnhofen Lithographic limestones (150 million years old, southern Germany), most likely representing an immature cirripede crustacean (barnacles and their relatives). The single specimen was documented with up-to-date imaging methods (macro-photography, stereo-photography, fluorescence photography, composite imaging) and compared with modern cirripede larvae. The identification is based on two conspicuous spine-like extensions in the anterior region of the specimen strongly resembling the so-called fronto-lateral horns, structures exclusively known from cirripede nauplius larvae. Notably, at 5 mm in length the specimen is unusually large for a cirripede nauplius. We therefore consider it to be a giant larva and discuss possible ecological and physiological mechanisms leading to the appearance of giant larvae in other lineages. Further findings of fossil larvae and especially nauplii might give new insights into larval evolution and plankton composition in the past.

Branchiostoma lanceolatum larvae: Some experiments on the effect of thiouracil on metamorphosis

Larvae of Branchiostoma lanceolatum (Pallas), of a late pre-metamorphosis stage and characterized by a bright fluorescent green around the ilio-colonic ring, were taken in the Plymouth area plankton in August 1965.Adults were kept in 0.04% thiouracil to determine the general effect at this concentration. It was lethal over a period of weeks.The larvae were kept in the dark at a constant temperature of 10° C, individu-ally or in pairs, in 0.04%, 0.02%, 0.01%, 0.005% and 0.0025% thiouracil, in saturated solutions and suspensions of thyroxine and triiodothyronine, and in ordinary sea water. Solutions were changed weekly. The flagellate Dunaliella primolecta Butcher was supplied as food.All the larvae metamorphosed at approximately the same time and rate, two exceptions being that 0.04% thiouracil was a more obviously toxic concentration and the thyroxine and triiodothyronine larvae were rather more precocious in their metamorphosis.It is suggested that metamorphosis is a ‘triggered’ process and is self-sustaining. Thus once it has begun it cannot be delayed or accelerated significantly. All the larvae caught were at a stage when this ‘triggering’ had been released.It is considered possible that the metabolism of the larva and the initiation of metamorphosis is controlled by a secretion of the club-shaped gland.INTRODUCTIONThe position of the amphioxides larva has already been discussed (Wick-stead, 1964). No clear demarcation could be made between the usual type of acraniate larva and amphioxides. It was suggested that the amphioxides was a giant larva, a phenomenon not uncommon in oceanic plankton.IntroductionThe position of the amphioxides larva has already been discussed (Wickstead, 1964).