Segmental organisation of the head in the embryo of Drosophila melanogaster

Abstract

Embryos of Drosophila melanogaster were irradiated in the presumptive head region with a UV-laser microbeam of 20 μm diameter at two developmental stages, the cellular blastoderm and the extended germ band. The ensuing defects were scored in the cuticle pattern of the head of the first-instar larva, which is described in detail in this paper. The defects caused by irradiating germ band embryos when morphologically recognisable lobes appear in the head region were used to establish the segmental origin of various head structures. This information enabled us to translate the spatial distribution of blastoderm defects into a fate map of segment anlagen. The gnathal segments derive from a region of the blastoderm between 60% and 70% egg length (EL) dorsally and 60% and 80% ventrally. The area anterior to the mandibular anlage and posterior to the stomodaeum is occupied by the small anlagen of the intercalary and antennal segments ventrally and dorsally, respectively. The labrum, which originates from a paired anlage dorsally at 90% EL, is separated from the remaining head segments by an area for which we did not observe cuticle defects following blastoderm irradiation, presumably because those cells give rise to the brain. The dorsal and lateral parts of the cephalo-pharyngeal skeleton appear to be the only cuticle derivatives of the non-segmental acron. These structures derive from a dorso-lateral area just behind the putative brain anlage and may overlap the latter. In addition to the segment anlagen, the regions of the presumptive dorsal pouch, anterior lobe and post-oral epithelium, whose morphogenetic movements during head involution result in the characteristic acephalic appearance of the larva, have been projected onto the blastoderm fate map. The results suggest that initially the head of the Drosophila embryo does not differ substantially from the generalised insect head as judged by comparison of fate map and segmental organisation.

Number, identity, and sequence of the Drosophila head segments as revealed by neural elements and their deletion patterns in mutants.

“…In the head of a Saltatoria, the camel cricket, seven "morphogenetic units" have been defined by ablation experiments (30 (7). Our analysis of the neural elements and their deletion in various head mutants are consistent with seven morphological units, which can be assigned to the three prominent gnathal and four pregnathal segments, placing the optic region to the second metamer (7) rather than to the nonsegmental acron (5).…”

“…The anterior pregnathal region, however, is poor in diagnostic morphological structures (8). Morphological and evolutionary studies based on epidermal head structures (5,8), coelomic cavities, or brain regions (1-4) have so far failed to unambiguously determine the number and identity of pregnathal segments in the insect head. Due to the evolutionary changes that resulted in the "acephalic" appearance of higher dipteran larvae, the segmental composition of the Drosophila head was analyzed only recently.…”

“…Due to the evolutionary changes that resulted in the "acephalic" appearance of higher dipteran larvae, the segmental composition of the Drosophila head was analyzed only recently. The detailed fate map analysis of a few cuticular specializations of the larval Drosophila head by laser ablation studies suggested six segments-i.e., three pregnathal and three gnathal segments (5). This interpretation was questioned through a detailed analysis of the expression patterns of the segment polarity genes engrailed (en) and wingless (wg), which serve as molecular markers for metamers in the prospective trunk and gnathal regions of the Drosophila embryo.…”

“…Abbreviations refer to antennal nerve (an, SNIM), anterior trunk of supraesophageal neuropile (at), Bolwig nerve (bon, SNII), labial nerve (Ian, SNVII), maxillary nerve (mxn, SNVI), and posterior trunk of supraesophageal neuropile (pt). [Bars represent 7 pm (A and B), 5 ,um (C-E), and 13 ,Am (F and G).] ems mutant embryos.…”

A novel spatial transcription pattern associated with the segmentation gene, giant, of Drosophila.

“…Strong hypomorphic giant larvae also possess salivary glands, but these are attached to the exoskeleton at the anterior edge of the prothorax. Placement of the affected structures on the blastoderm fate map of Jurgens et al (1986) reveals two regions of the anterior blastoderm fated to give rise to defective structures in mutant giant larvae (Figure 2): the labral segment on the dorsolateral side of the blastoderm embryo at -90-95% egg length and the labial segment at -60-65% egg length of the blastoderm. In weaker alleles (e.g.…”