Two-tiered regulation of spatially patterned engrailed gene expression during Drosophila embryogenesis

Abstract

A regulatory cascade, initiated during the syncytial stage of embryogenesis, culminates in the striped pattern of engrailed gene expression at the cellular blastoderm stage. The early regulatory genes, for example the pair-rule genes, are expressed transiently and as their products decay a distinct regulatory programme involving segment polarity genes takes over. This late programme maintains and perhaps modifies the striped pattern of engrailed expression through interactions that may involve cell communication.An early step in the generation of the Drosophila body pattern is the determination of the segmental subdivisions of the embryo. This occurs quite rapidly. By the cellular blastoderm stage (2-2.5 h after fertilization) the segmental outlines of the embryo, although not morphologically visible, are represented by the precise expression patterns of the segmentation (or segment polarity) genes, such as engrailed 1,2 and wingless 3 . The subsequent activity of genes such as these directs the morphological events leading to segmentation 4,5 .The segmentation genes are expressed in their localized patterns as a consequence of an early regulatory cascade. Three gene classes (maternal genes 6 , and the zygotically acting gap and pair-rule genes 4 ) act in sequence, each establishing the more refined spatial expression pattern of the next 7-9 . This takes place in a syncytial cytoplasm and is thought to rely on the diffusion and interaction of regulatory products encoded by these genes. The subdivision of the embryonic field by this cascade is rapid, occurring during the first thirteen nuclear divisions. After the thirteenth division, the nuclei are cellularized and the cellular blastoderm is formed.One outcome of this regulatory hierarchy is the establishment of engrailed (en) expression at the cellular blastoderm stage in 14 single-cell-wide stripes transecting the antero-posterior axis of the embryo 1,2,10,22 . Previous work has shown that the activity of separate sets of pair-rule genes controls establishment of en expression in even-and in odd-numbered stripes 11,12 . The data presented here suggests further that the separate control of even-and odd-numbered stripes is reflected in the organization of the cis-regulatory sequences at en.During the post-cellular blastoderm phase of development the expression pattern of en is maintained and modified 1,10 . It is unlikely that the pair-rule genes are involved in the postblastoderm regulation of en, since the pair-rule genes characterized to date are only expressed transiently in their pair-rule blastoderm patterns [13][14][15][16]31 . Here we address the mechanism by which the en gene is regulated in later embryonic development.

Multiple modes of engrailed regulation in the progression towards cell fate determination

“…2b). Loss of en parallels that seen in wg null mutants 11,12 . (We do not consider the regions of the embryo where en expression does not involve wg control: the head, the first three en stripes and the central nervous system (CNS) in each segment.)…”

“…a,wg IL114ts /wg CX4 embryo grown at nonpermissive temperature (29 °C) from ∼3.5 to 7.5 h AEL. Expression of en is indistinguishable from that seen in a wg null embryo at this stage 11,12 . Only expression that is not under wg control persists: in the head, the first four ectodermal stripes (1)(2)(3)(4), and a segmentally repeated subset of CNS cells.…”

“…These regulatory programmes act in an overlapping progression. The pair-rule regulatory programme initiates striped en expression at cellular blastoderm 18,19 , and we infer that it remains active until about 4.5 h, as en expression is maintained until this time in wg mutants 11,12 . Even before 4.5 h, wg provides an activating signal from the neighbouring cell.…”

“…To discover whether there is a later stage of autoregulation that is wg-independent, we heat-shocked wg mutant embryos aged beyond the wg-dependent period. Recall that, after about 5 h, wg mutant embryos show no en expression in the ectoderm at any position posterior to the fourth en stripe 11,12 (Fig. 5a).…”

“…neighbouring cells 11,12 . Here we show that en expression becomes independent of this extracellular influence and comes to rely on positive autoregulation.…”