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Pattern specification in the insect embryo.

Specification of developmental pathways by specific determining substances prelocalized in the egg cytoplasm is duscussed using the so-called germ cell determinants as an example. Some theoretical considerations speak against the assumption that in insects the various elements of the basic body plan are specified by a prelocalized mosaic of specific determinants. Experimental evidence also points towards a largely epigenetic mode of pattern specification. The process of axial pattern specification can be altered drastically by experiment: in some insects, tail ends may be formed in place of head parts and identical wequences of bocy segments may be specified two or even three times along the longitudinal egg axis. The experimental results indicate that polarity and regional character of pattern elements formed are specified by one and the same influence, and that this influence can be shifted to or simulated in various other egg regions by transposition or elimination of egg components, or by UV irradiation. Evidence obtained from several types of experiment in the chironomid midge Smittia points towards a key role for local metabolsim or energy charge in determination of cell polarity and in pattern specification. A model for embryonic pattern specification involving differential reaction of cells to a system of longitudinal gradients, which was proposed in 1960, can in principle formally account for all results described. Some striking coincidences of model and experimental results with Wolpert's concept of positional information are noted. Finally it is pointed out that universality of mechansims for pattern specification is much more likely with respect to formal principles than at the level of their physiological realization.