odd-skipped genes and lines organize the notum anterior-posterior axis using autonomous and non-autonomous mechanisms

The growth and patterning of Drosophila wing and notum primordia depend on their subdivision into progressively smaller domains by secreted signals that emanate from localized sources termed organizers. While the mechanisms that organize the wing primordium have been studied extensively, those that organize the notum are incompletely understood. The genes odd-skipped (odd), drumstick (drm), sob, and bowl comprise the odd-skipped family of C(2)H(2) zinc finger genes, which has been implicated in notum growth and patterning. Here we show that drm, Bowl, and eyegone (eyg), a gene required for notum patterning, accumulate in nested domains in the anterior notum. Ectopic drm organized the nested expression of these anterior notum genes and downregulated the expression of posterior notum genes. The cell-autonomous induction of Bowl and Eyg required bowl, while the non-autonomous effects were independent of bowl. The homeodomain protein Bar is expressed along the anterior border of the notum adjacent to cells expressing the Notch (N) ligand Delta (Dl). bowl was required to promote Bar and repress Dl expression to pattern the anterior notum in a cell-autonomous manner, while lines acted antagonistically to bowl posterior to the Bowl domain. Our data suggest that the odd-skipped genes act at the anterior notum border to organize the notum anterior-posterior (AP) axis using both autonomous and non-autonomous mechanisms.     

odd-skipped is expressed in multiple tissues during Drosophila embryogenesis

The odd-skipped (odd) gene encodes a zinc finger protein that represses other segmentation genes in the early Drosophila embryo. Though odd is initially expressed in a striped pattern that reflects its function within the segmentation hierarchy, it is also expressed in a variety of patterns during later stages of embryogenesis. To identify the cells and tissues that correspond to these latter patterns, we examined the distribution of the Odd protein at all embryonic stages. Our results indicate that Odd is a specific and persistent marker for subsets of cells in developing mesoderm, ectoderm, and neural tissue. We conclude that Odd is a useful tool for studying cell specification, cell migrations and morphogenetic movements during organogenesis of the heart, gut and central nervous system.     

The odd-skipped family of zinc finger genes promotes Drosophila leg segmentation

Notch signaling controls formation of joints at leg segment borders and growth of the developing Drosophila leg. Here, we identify the odd-skipped gene family as a key group of genes that function downstream of the Notch receptor to promote morphological changes associated with joint formation during leg development. odd, sob, drm, and bowl are expressed in a segmental pattern in the developing leg, and their expression is regulated by Notch signaling. Ectopic expression of odd, sob, or drm can induce invaginations in the leg disc epithelium and morphological changes in the adult leg that are characteristic of endogenous invaginating joint cells. These effects are not due to an alteration in the expression of other genes of the developing joint. While odd or drm mutant clones do not affect leg segmentation, and thus appear to act redundantly, bowl mutant clones do perturb leg development. Specifically, bowl mutant clones result in a failure of joint formation from the distal tibia to tarsal segment 5, while more proximal clones cause melanotic protrusions from the leg cuticle. Together, these results indicate that the odd-skipped family of genes mediates Notch function during leg development by promoting a specific aspect of joint formation, an epithelial invagination. As the odd-skipped family genes are involved in regulating cellular morphogenesis during both embryonic segmentation and hindgut development, we suggest that they may be required in multiple developmental contexts to induce epithelial cellular changes.     

even-skipped has gap-like, pair-rule-like, and segmental functions in the cricket Gryllus bimaculatus, a basal, intermediate germ insect (Orthoptera)

Developmental mechanisms of segmentation appear to be varied among insects in spite of their conserved body plan. Although the expression patterns of the segment polarity genes in all insects examined imply well conserved function of this class of genes, expression patterns and function of the pair-rule genes tend to exhibit diversity. To gain further insights into the evolution of the segmentation process and the role of pair-rule genes, we have examined expression and function of an ortholog of the Drosophila pair-rule gene even-skipped (eve) in a phylogenetically basal insect, Gryllus bimaculatus (Orthoptera, intermediate germ cricket). We find that Gryllus eve (Gb'eve) is expressed as stripes in each of the prospective gnathal, thoracic, and abdominal segments and as a broad domain in the posterior growth zone. Dynamics of stripe formation vary among Gb'eve stripes, representing one of the three modes, the segmental, incomplete pair-rule, and complete pair-rule mode. Furthermore, we find that RNAi suppression of Gb'eve results in segmentation defects in both anterior and posterior regions of the embryo. Mild depletion of Gb'eve shows a pair-rule-like defect in anterior segments, while stronger depletion causes a gap-like defect showing deletion of anterior and posterior segments. These results suggest that Gb'eve acts as a pair-rule gene at least during anterior segmentation and also has segmental and gap-like functions. Additionally, Gb'eve may be involved in the regulation of hunchback and Krüppel expression. Comparisons with eve functions in other species suggest that the Gb'eve function may represent an intermediate state of the evolution of pair-rule patterning by eve in insects.     

Zygotically active genes that affect the spatial expression of the fushi tarazu segmentation gene during early Drosophila embryogenesis

The establishment of the segmental body pattern of Drosophila requires the coordinated functions of three classes of zygotically active genes early in development. We have examined the effects of mutations in these genes on the spatial expression of the fushi tarazu (ftz) pair-rule segmentation gene. Mutations in four gap loci and in three pair-rule loci dramatically affect the initial pattern of transverse stripes of ftz-containing nuclei. Five other pair-rule genes and several other loci that affect the larval cuticular pattern do not detectably affect ftz expression. No simple regulatory relationships can be deduced. Rather, expression of the ftz gene depends upon the interactions among the different segmentation genes active at each position along the anterior-posterior axis of the early embryo.     

Expression of pair-rule gene homologues in a chelicerate: early patterning of the two-spotted spider mite Tetranychus urticae

Embryo segmentation has been studied extensively in the fruit fly, DROSOPHILA: These studies have demonstrated that a mechanism acting with dual segment periodicity is required for correct patterning of the body plan in this insect, but the evolutionary origin of the mechanism, the pair-rule system, is unclear. We have examined the expression of the homologues of two Drosophila pair-rule genes, runt and paired (Pax Group III), in segmenting embryos of the two-spotted spider mite (Tetranychus urticae Koch). Spider mites are chelicerates, a group of arthropods that diverged from the lineage leading to Drosophila at least 520 million years ago. In T. urticae, the Pax Group III gene Tu-pax3/7 was expressed during patterning of the prosoma, but not the opisthosoma, in a series of stripes which appear first in even numbered segments, and then in odd numbered segments. The mite runt homologue (Tu-run) in contrast was expressed early in a circular domains that resolved into a segmental pattern. The expression patterns of both of these genes also indicated they are regulated very differently from their Drosophila homologues. The expression pattern of Tu-pax3/7 lends support to the possibility that a pair-rule patterning mechanism is active in the segmentation pathways of chelicerates.     

Analysis of maternal effect mutant combinations elucidates regulation and function of the overlap of hunchback and Krüppel gene expression in the Drosophila blastoderm embryo

The metameric organisation of the Drosophila embryo is generated early during development, due to the action of maternal effect and zygotic segmentation and homeotic genes. The gap genes participate in the complex process of pattern formation by providing a link between the maternal and the zygotic gene activities. Under the influence of maternal gene products they become expressed in distinct domains along the anteroposterior axis of the embryo; negative interactions between neighboring gap genes are thought to be involved in establishing the expression domains. The gap gene activities in turn are required for the correct patterning of the pair-rule genes; little is known, however, about the underlying mechanisms. We have monitored the distribution of gap and pair-rule genes in wild-type embryos and in embryos in which the anteroposterior body pattern is greatly simplified due to combinations of maternal effect mutations (staufen exuperantia, vasa exuperantia, vasa exuperantia, bicoid oskar, bicoid oskar torsolike, vasa torso exuperantia). We show that the domains of protein distribution of the gap genes hunchback and Krüppel overlap in wild-type embryos. Based on the analysis of the maternal mutant combinations, we suggest an explanation of how this overlap is generated. Furthermore, our data show that different constellations of gap gene activities provide different input for the pair-rule genes, and thus strongly suggest that the overlap of hunchback and Krüppel in wild-type is functional in the formation of the patterns of pair-rule genes.     

even-skipped is not a pair-rule gene but has segmental and gap-like functions in Oncopeltus fasciatus, an intermediate germband insect

The pair-rule gene even-skipped is required for the initiation of metameric pattern in Drosophila. But Drosophila segmentation is evolutionarily derived and is not representative of most insects. Therefore, in order to shed light on the evolution of insect segmentation, homologs of the pair-rule gene even-skipped have been studied in several insect taxa. However, most of these studies have reported the expression eve but not its function. We report the isolation, expression and function of the homolog of Drosophila even-skipped from the intermediate germband insect Oncopeltus fasciatus. We find that in Oncopeltus, even-skipped striped expression initiates in a segmental and not pair-rule pattern. Weak RNAi suppression of Oncopeltus even-skipped shows no apparent pair-rule like phenotype, while stronger RNAi suppression shows deletion of nearly the entire body. These results suggest that in Oncopeltus, even-skipped is not acting as a pair-rule gene. In almost all insects, prior to its striped expression, even-skipped is expressed in a conserved broad gap-like domain but its function has been largely ignored. We find that this early broad domain is required for activation of the gap genes hunchback and Krüppel. Given the large RNAi deletion phenotype and its regulation of hunchback and Krüppel, even-skipped seems to act as an über-gap gene in Oncopeltus, indicating that it may have both upstream and downstream roles in segmentation.     

Gradients of Krüppel and knirps gene products direct pair-rule gene stripe patterning in the posterior region of the Drosophila embryo

Abdominal segmentation of the Drosophila embryo requires the activities of the gap genes Krüppel (Kr), knirps (kni), and tailless (tll). They control the expression of the pair-rule gene hairy (h) by activating or repressing independent cis-acting units that generate individual stripes. Kr activates stripe 5 and represses stripe 6, kni activates stripe 6 and represses stripe 7, and tll activates stripe 7. Kr and kni proteins bind strongly to h control units that generate stripes in areas of low concentration of the respective gap gene products and weakly to those that generate stripes in areas of high gap gene expression. These results indicate that Kr and kni proteins form overlapping concentration gradients that generate the periodic pair-rule expression pattern.     

Insect segmentation: Genes, stripes and segments in "Hoppers".

Recent work has revealed that orthologues of several segmentation genes are expressed in the grasshopper embryo, in patterns resembling those shown in Drosophila. This suggests that, despite great differences between the embryos, a hierarchy of gap/pair-rule/segment polarity gene function may be a shared and ancestral feature of insect segmentation.