Gradients and insect segmentation

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Abstract
`Morphogen' gradients have long been invoked as a means of specifying spatial patterns of developmental fate, and it has now been demonstrated that they are indeed involved in the early steps of insect segmentation. In many insects, including Drosophila, ligature and transplantation experiments have shown that the segment pattern develops through interactions between the ends of the egg. These results, plus those from irradiation and centrifugation of chironomid eggs, suggest that specific maternally synthesized RNAs are localized at the ends of the oocyte, and act as sources of opposing anterior and posterior gradients in the early egg. In Drosophila, different groups of maternal `segmentation' genes are required for depositing within the oocyte terminal, anterior and posterior spatial cues. Injection of wild-type cytoplasm into mutant eggs which lack the anterior (bicoid or posterior (oskar) cue suggests that these are normally distributed as gradients from strictly localized sources. It has now been shown directly that bicoid RNA passes into the oocyte from the nurse cells, remains localized in the anterior tip, and is later translated into protein which forms an exponential concentration gradient down the early egg. Genes required for posterior spatial information have not yet been cloned, so a posterior gradient (most likely to consist of nanos product) has yet to be directly demonstrated. Analysis of zygotic `segmentation' genes has shown that the different segment primordia are not directly specified by small changes in the anterior or (postulated) posterior gradient. It seems likely that the maternal cues specify a few bands of expression of zygotic gap genes such as hunchback, Krüppel and knirps, and that the pattern is then elaborated through interactions between these. The anterior gradient seems to form by diffusion of bicoid protein, but the posterior signal seems to be capable of reorganization in some injection experiments. This could imply a diffusion/reaction mechanism, or could result simply from the way in which the terminal, anterior and posterior cues act via gap gene activity. Hence the segment pattern formed after injection (and after irradiation of chironomid eggs) will not always correspond to the gradient profile. Other types of insect egg develop with no nurse cells or external anterior source of RNA and, in these, there is some evidence of a posterior gradient but not of a similar signal from the anterior end. It is now clear from the analysis of segmentation in Drosophila that the determinants and gradients inferred from earlier studies do provide a positional framework within which the segment pattern is gradually elaborated. Investigation of segmentation in other eggs will be greatly assisted if the molecular techniques can be transferred from Drosophila.