Dose-dependent regulation of pair-rule stripes by gap proteins and the initiation of segment polarity

A key step in Drosophila segmentation is the establishment of periodic patterns of pair-rule gene expression in response to gap gene products. From an examination of the distribution of gap and pair-rule proteins in various mutants, we conclude that the on/off periodicity of pair-rule stripes depends on both the exact concentrations and combinations of gap proteins expressed in different embryonic cells. It has been suggested that the distribution of gap gene products depends on cross-regulatory interactions among these genes. Here we provide evidence that autoregulation also plays an important role in this process since there is a reduction in the levels of Kruppel (Kr) RNA and protein in a Kr null mutant. Once initiated by the gap genes each pair-rule stripe is bell shaped and has ill-defined margins. By the end of the fourteenth nuclear division cycle, the stripes of the pair-rule gene even-skipped (eve) sharpen and polarize, a process that is essential for the precisely localized expression of segment polarity genes. This sharpening process appears to depend on a threshold response of the eve promoter to the combinatorial action of eve and a second pair-rule gene hairy. The eve and hairy expression patterns overlap but are out of register and the cells of maximal overlap form the anterior margin of the polarized eve stripe. We propose that the relative placement of the eve and hairy stripes may be an important factor in the initiation of segment polarity.     

The nuclear receptor E75A has a novel pair-rule-like function in patterning the milkweed bug, Oncopeltus fasciatus

Genetic studies of the fruit fly Drosophila have revealed a hierarchy of segmentation genes (maternal, gap, pair-rule and HOX) that subdivide the syncytial blastoderm into sequentially finer-scale coordinates. Within this hierarchy, the pair-rule genes translate gradients of information into periodic stripes of expression. How pair-rule genes function during the progressive mode of segmentation seen in short and intermediate-germ insects is an ongoing question. Here we report that the nuclear receptor Of'E75A is expressed with double segment periodicity in the head and thorax. In the abdomen, Of'E75A is expressed in a unique pattern during posterior elongation, and briefly resembles a sequence that is typical of pair-rule genes. Depletion of Of'E75A mRNA caused loss of a subset of odd-numbered parasegments, as well as parasegment 6. Because these parasegments straddle segment boundaries, we observe fusions between adjacent segments. Finally, expression of Of'E75A in the blastoderm requires even-skipped, which is a gap gene in Oncopeltus. These data show that the function of Of'E75A during embryogenesis shares many properties with canonical pair-rule genes in other insects. They further suggest that parasegment specification may occur through irregular and episodic pair-rule-like activity.     

Evolutionary flexibility of pair-rule patterning revealed by functional analysis of secondary pair-rule genes, paired and sloppy-paired in the short-germ insect, Tribolium castaneum

In the Drosophila segmentation hierarchy, periodic expression of pair-rule genes translates gradients of regional information from maternal and gap genes into the segmental expression of segment polarity genes. In Tribolium, homologs of almost all the eight canonical Drosophila pair-rule genes are expressed in pair-rule domains, but only five have pair-rule functions. even-skipped, runt and odd-skipped act as primary pair-rule genes, while the functions of paired (prd) and sloppy-paired (slp) are secondary. Since secondary pair-rule genes directly regulate segment polarity genes in Drosophila, we analyzed Tc-prd and Tc-slp to determine the extent to which this paradigm is conserved in Tribolium. We found that the role of prd is conserved between Drosophila and Tribolium; it is required in both insects to activate engrailed in odd-numbered parasegments and wingless (wg) in even-numbered parasegments. Similarly, slp is required to activate wg in alternate parasegments and to maintain the remaining wg stripes in both insects. However, the parasegmental register for Tc-slp is opposite that of Drosophila slp1. Thus, while prd is functionally conserved, the fact that the register of slp function has evolved differently in the lineages leading to Drosophila and Tribolium reveals an unprecedented flexibility in pair-rule patterning.     

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.     

Segmentation gene expression in the housefly Musca domestica.

Drosophila and Musca both belong to the group of higher dipteran flies and show morphologically a very similar early development. However, these two species are evolutionary separated by at least 100 million years. This presents the opportunity for a comparative analysis of segmentation gene expression across a large evolutionary distance in a very similar embryonic background. We have analysed in detail the early expression of the maternal gene bicoid, the gap genes hunchback, Krüppel, knirps and tailless, the pair-rule gene hairy, the segment-polarity gene engrailed and the homoeotic gene Ultrabithorax. We show that the primary expression domains of these genes are conserved, while some secondary expression aspects have diverged. Most notable is the finding of hunchback expression in 11-13 stripes shortly before gastrulation, as well as a delayed expression of terminal domains of various genes. We conclude that the early developmental gene hierarchy, as it has been defined in Drosophila, is evolutionary conserved in Musca domestica.     

Pax group III genes and the evolution of insect pair-rule patterning

Pair-rule genes were identified and named for their role in segmentation in embryos of the long germ insect Drosophila. Among short germ insects these genes exhibit variable expression patterns during segmentation and thus are likely to play divergent roles in this process. Understanding the details of this variation should shed light on the evolution of the genetic hierarchy responsible for segmentation in Drosophila and other insects. We have investigated the expression of homologs of the Drosophila Pax group III genes paired, gooseberry and gooseberry-neuro in short germ flour beetles and grasshoppers. During Drosophila embryogenesis, paired acts as one of several pair-rule genes that define the boundaries of future parasegments and segments, via the regulation of segment polarity genes such as gooseberry, which in turn regulates gooseberry-neuro, a gene expressed later in the developing nervous system. Using a crossreactive antibody, we show that the embryonic expression of Pax group III genes in both the flour beetle Tribolium and the grasshopper Schistocerca is remarkably similar to the pattern in Drosophila. We also show that two Pax group III genes, pairberry1 and pairberry2, are responsible for the observed protein pattern in grasshopper embryos. Both pairberry1 and pairberry2 are expressed in coincident stripes of a one-segment periodicity, in a manner reminiscent of Drosophila gooseberry and gooseberry-neuro. pairberry1, however, is also expressed in stripes of a two-segment periodicity before maturing into its segmental pattern. This early expression of pairberry1 is reminiscent of Drosophila paired and represents the first evidence for pair-rule patterning in short germ grasshoppers or any hemimetabolous insect.     

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.     

Huckebein is part of a combinatorial repression code in the anterior blastoderm

The hierarchy of the segmentation cascade responsible for establishing the Drosophila body plan is composed by gap, pair-rule and segment polarity genes. However, no pair-rule stripes are formed in the anterior regions of the embryo. This lack of stripe formation, as well as other evidence from the literature that is further investigated here, led us to the hypothesis that anterior gap genes might be involved in a combinatorial mechanism responsible for repressing the cis-regulatory modules (CRMs) of hairy (h), even-skipped (eve), runt (run), and fushi-tarazu (ftz) anterior-most stripes. In this study, we investigated huckebein (hkb), which has a gap expression domain at the anterior tip of the embryo. Using genetic methods we were able to detect deviations from the wild-type patterns of the anterior-most pair-rule stripes in different genetic backgrounds, which were consistent with Hkb-mediated repression. Moreover, we developed an image processing tool that, for the most part, confirmed our assumptions. Using an hkb misexpression system, we further detected specific repression on anterior stripes. Furthermore, bioinformatics analysis predicted an increased significance of binding site clusters in the CRMs of h 1, eve 1, run 1 and ftz 1when Hkb was incorporated in the analysis, indicating that Hkb plays a direct role in these CRMs. We further discuss that Hkb and Slp1, which is the other previously identified common repressor of anterior stripes, might participate in a combinatorial repression mechanism controlling stripe CRMs in the anterior parts of the embryo and define the borders of these anterior stripes.     

Mutually repressive interactions between the gap genes giant and Krüppel define middle body regions of the Drosophila embryo

The gap genes play a key role in establishing pair-rule and homeotic stripes of gene expression in the Drosophila embryo. There is mounting evidence that overlapping gradients of gap gene expression are crucial for this process. Here we present evidence that the segmentation gene giant is a bona fide gap gene that is likely to act in concert with hunchback, Krüppel and knirps to initiate stripes of gene expression. We show that Krüppel and giant are expressed in complementary, non-overlapping sets of cells in the early embryo. These complementary patterns depend on mutually repressive interactions between the two genes. Ectopic expression of giant in early embryos results in the selective repression of Krüppel, and advanced-stage embryos show cuticular defects similar to those observed in Krüppel- mutants. This result and others suggest that the strongest regulatory interactions occur among those gap genes expressed in nonadjacent domains. We propose that the precisely balanced overlapping gradients of gap gene expression depend on these strong regulatory interactions, coupled with weak interactions between neighboring genes.     

Establishment of the Deformed expression stripe requires the combinatorial action of coordinate, gap and pair-rule proteins.

In Drosophila embryos, anterior-posterior positional identities are set and maintained by the expression boundaries of homeotic selector genes. The establishment of the initial expression boundaries of the homeotic genes are in turn dependent on earlier acting patterning genes of Drosophila. To define the combinations of early genes that are required to establish a unique blastoderm stripe of expression of the homeotic gene Deformed, we have analysed single and double patterning mutants and heat shock promoter fusion constructs that ectopically express early acting regulators. We find that the activation of Deformed is dependent on combinatorial input from at least three levels of the early hierarchy. The simplest activation code sufficient to establish Deformed expression, given the absence of negative regulators such as fushi-tarazu, consists of a moderate level of expression from the coordinate gene bicoid, in combination with expression from both the gap gene hunchback, and the pair-rule gene even-skipped. In addition, the activation code for Deformed is redundant; other pair-rule genes in addition to even-skipped can apparently act in combination with bicoid and hunchback to activate Deformed.     

Double-segment defining role of even-skipped homologs along the evolution of insect pattern formation

Recent studies on insect patterning suggest that the genetic hierarchy may be roughly conserved in phylogenetically divergent species, but pair-rule genes may not function identically in all insects. In order to understand potential evolutionary changes in the role of the pair-rule genes, a Bombyx even-skipped homolog was cloned and its expression pattern during early embryogenesis studied. Eight stripes of Bombyx even-skipped were progressively expressed in an antero–posterior order. Later, these stripes disappeared anteriorly. Under this detection system, Bombyx even-skipped stripes clearly do not resolve into the corre sponding secondary stripes, an obvious difference from Drosophila and Tribolium . These results suggest that Bombyx even-skipped may serve a double-segment defining role and may determine the odd-numbered engrailed stripes.