sloppy paired 1

DEVELOPMENTAL BIOLOGY

Embryonic

See the embryonic expression pattern of slp1 at the Berkeley Drosophila Genome Project Patterns of Gene Expression Site.

Transcription of slp is first detected in the head, in a gap-like pattern during the syncytial blastoderm. Seven primary segmental stripes appear during the cellular blastoderm followed by seven additional secondary stripes between the primary stripes. By germ band expansion [Images], expression in the head is more complex, due to the development of the different head primordia. The metameric pattern fades at germ band retraction and transcript appears in the border of the dorsal epidermis, during dorsal closure. slp2 lags slp1 in many of these expression features (Grossnicklaus, 1992) .

The Drosophila brain develops from the procephalic neurogenic region of the ectoderm. About 100 neural precursor cells (neuroblasts) delaminate from this region on either side in a reproducible spatiotemporal pattern. Neuroblast maps have been prepared from different stages of the early embryo (stages 9, 10 and 11, when the entire population of neuroblasts has formed), in which about 40 molecular markers representing the expression patterns of 34 different genes are linked to individual neuroblasts. In particular, a detailed description is presented of the spatiotemporal patterns of expression in the procephalic neuroectoderm and in the neuroblast layer of the gap genes empty spiracles, hunchback, huckebein, sloppy paired 1 and tailless; the homeotic gene labial; the early eye genes dachshund, eyeless and twin of eyeless; and several other marker genes (including castor, pdm1, fasciclin 2, klumpfuss, ladybird, runt and unplugged). Based on the combination of genes expressed, each brain neuroblast acquires a unique identity, and it is possible to follow the fate of individual neuroblasts through early neurogenesis. Furthermore, despite the highly derived patterns of expression in the procephalic segments, the co-expression of specific molecular markers discloses the existence of serially homologous neuroblasts in neuromeres of the ventral nerve cord and the brain. Taking into consideration that all brain neuroblasts are now assigned to particular neuromeres and individually identified by their unique gene expression, and that the genes found to be expressed are likely candidates for controlling the development of the respective neuroblasts, these data provide a basic framework for studying the mechanisms leading to pattern and cell diversity in the Drosophila brain, and for addressing those mechanisms that make the brain different from the truncal CNS (Urbach, 2003).

The cephalic gap genes are expressed in large domains of the procephalon and play a crucial role not only in the patterning of the peripheral ectoderm, but also in regionalizing the brain primordium. The segmental organization of the Drosophila brain is based on the expression pattern of segment polarity and DV patterning genes. To see whether the cephalic gap genes respect the neuromeric boundaries segment polarity and DV patterning genes, and to provide a basis for studying their potential role in the formation or specification of brain precursor cells, the expression was studied of orthodenticle, empty spiracles, sloppy paired 1, tailless, huckebein, and hunchback in the developing head ectoderm, as well as in the entire population of identified NBs during stages 9-11 (Urbach, 2003).

The sloppy paired (slp) locus contains the two related genes slp1 and slp2. slp1, which acts as a head gap gene, plays a predominant role in head formation, while slp2 is largely dispensable. In the trunk neuroectoderm, where slp1 has a function as a pair-rule and segment polarity gene, it is segmentally expressed in neuroectodermal stripes as well as in NBs of row 4 and 5. This segmental appearance of slp1 expression is found to be conserved in parts of the procephalon. In the blastoderm, Slp1 protein is detected in a large domain of the procephalon anlage, which subsequently diminishes in its anterior/ventral part. As a result, only the posterior half of the original slp1 domain remains as a circumferential ring ('head stripe') and gets separated from the anterodorsal part ('head cap'). To follow the dynamics in the Slp1 expression pattern, Slp1/En double labelling was examined during stages 8-11. The 'head stripe' corresponds to the slp1 stripe of the prospective mandibular segment, and the posterior part of the 'head cap' to the Slp1-positive stripe in the prospective antennal segment (slp1 as). At the beginning of gastrulation, a new Slp1 ectodermal spot in the anterodorsal procephalon is observed; this spot later becomes part of the labral ectoderm. In addition, at stage 9, three new ectodermal domains become detectable: one stripe anterior to the en intercalary stripe belonging to the intercalary segment, and two small spots in the region of the ocular segment (anterior to the en head spot). Except for the labral domain, the slp1 domains contribute NBs to the brain. Thus, slp1 is segmentally expressed in the procephalic neuroectoderm and subsets of brain NBs, resembling the situation in the trunk. At stage 11 patchy expression of Slp1 becomes detectable within the ocular and labral ectoderm and in some underlying ocular and labral NBs. Some of these NBs initiate slp1 expression after delamination; e.g. Pcv6 and Pcd2 delaminate at stage 9 and do not express slp1 before stage 11. Slp1 expression is observed in the brain until the end of embryogenesis (Urbach, 2003).

Larval

Very little information is available about gene expression during the larval period, a developmental interval critical to the formation of the adult. To what extent does gene expression during this period resemble that in the embryonic stages, and how does gene expression during the larval period contribute to segment polarity in the adult? In fact, all the genes expressed during embryonic segment polarity also play a similar role in the formation of the adult. Cells destined to form the cuticle of the adult abdomen are present as clusters of small, non-dividing diploid cells (the anterior dorsal, posterior dorsal and ventral histoblast nests) located at stereotyped postions in the larval epidermis. These cells, just as do their embryonic counterparts, express engrailed, hedgehog, wingless, patched, cubitus interruptus and sloppy paired in a stereotyped manner dependent on their positions within each segment. Each segment is subdivided into an anterior (A) and posterior (P) compartment, distinguished by activity of the selector gene engrailed (en) in P but not A compartment cells. The ventral epidermis of each abdominal segment forms a flexible cuticle, the pleura, with a small plate of sclerotised cuticle, the sternite, centered on the ventral midline. The pleura is covered with a uniform lawn of hairs, all pointed posteriorly, whereas the sternite contains a stereotyped pattern of bristles. Posterior compartments are to a large degree devoid of hairs and bristles, while the sternite cuticle of the A compartment consists of an anterior-to posterior progression of six types of cuticle distinguished by ornamentation and pigmentation. Just anterior to the posterior compartment, A6 is unpigmented, with hairs and none of the larger ornaments called bristles. A5 is darkly pigmented with hairs and bristles of large size. A4 and A3 are darkly and lightly pigmented respectively with moderately sized hairs and bristles. A2 is lightly pigmented with hairs, and A1, adjacent to the next more anteriorly located "posterior" compartment is unpigmented without hairs (Struhl, 1997a).

Hedgehog (Hh), a protein secreted by engrailed expressing P compartment cells, spreads into each A compartment across the anterior and the posterior boundaries to form opposing concentration gradients that organize cell pattern and polarity. Anteriorly and posteriorly situated cells within the A compartment respond in distinct ways to Hh: they express different combinations of genes and form different cell types. patched is expressed at both boundaries. patched is expressed in a graded fashion within each stripe, just anterior to each P compartment. ci peaks at high level in those cells abutting Hh- secreting cells of the P compartment and declines progressively in cells further away. wingless is also expressed in this domain and sloppy paired is expressed in the same region as wingless. decapentaplegic is expressed only in the ventral pleura in those A compartment cells neighboring P compartment cells within the same segment. dpp is not expressed immediately behind posterior compartments (Struhl, 1997).

Effects of Mutation or Deletion

A severe segmentation phenotype is obtained only when both slp genes are deleted. The phenotypes of embryos containing various combinations of functional slp genes suggest that for early slp function, at least until gastrulation, only slp1 is required. At later times, there is still a greater requirement for slp1, but in many respects the two slp genes are completely redundant. Both slp genes produce similar phenotypes when ubiquitously expressed via a heat shock promoter. It has been suggested that the SLP proteins are biochemically equivalent and that the greater requirement for slp1 in some functions can be explained in large part by its earlier expression (Cadigan, 1994b).

Structures derived from the mandibular and labial segments are missing in slp mutants. These deficiencies are due to partial and defective expression of wingless and engrailed in slp mutants (Grossnicklaus, 1994).

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date revised: 15 July 2008

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