Phenotypic comparison between maternal and zygotic genes controlling the segmental pattern of the Drosophila embryo

Abstract

lntroduction The longitudinal pattern of the Drosophila embryo is controlled by the concerted activity of gene products provided duritrg oogenesis (maternally active genes) and embryogenesis (zygotically active genes). An initially relatively coarse system of positional information laid down by maternal gene products becomes successively refined towards the repeating pattern of segments first by the division into domains by the products of the zygotic gap genes and subsequently by the action of pair-rule and segment-pol arity genes (Ni.isslein-Volhard & Wieschaus, 1980; Ingham & Martinez-Arias, 1986; Ingham, 1988; for review see Akam , 1987). Maternal genes affecting anteroposterior pattern have been classified into three groups according to their phenotype: the terminal group, the anterior group and the posterior group (NiissleinVolhard et al. I9B7). Together the three maternal gene groups control the establishment of the entire segmental pattern. Embryos that lack all maternal information show no anteroposterior pattern (Ntisslein-Volhard et al. 1987, R.L. unpublished data). The information provided by the maternal genes is interpreted by tygotic genes. The best candidates for genes that may directly respond to the maternal signals are the zygotic gap genes (Table 1; NtissleinVolhard & Wieschaus, 1980). The number of genes with a 'gap' phenotype is small and each gap gene has a distinct phenotype. Similar to mutations in the maternal genes , gap mutations cause large continuous deletions including several consecutive segments while the remaining structures are relatively normal. In this article, I would like to summarize and discuss some of the results concerning the establishment of positional information in the egg cell and its interpretation by differential activation of. zygotic genes. The first part deals with the properties of the posterior group genes as they have been characterized genetically as well as by fate-map analysis. This description should provide some idea about the methods used to characterize phenotypic groups. In the second part, these results will be compared to similar studies on the anterior and terminal group. The goal of this article is to point to the relative roles maternal anteroposterior genes and zygotic gap genes play in the generation of the segmented pattern.