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A Pair-Rule Gene Circuit Defines Segments Sequentially in the Short-Germ Insect Tribolium Castaneum

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A Pair-Rule Gene Circuit Defines Segments Sequentially in the Short-Germ Insect Tribolium Castaneum A pair-rule gene circuit defines segments sequentially in the short-germ insect Tribolium castaneum Chong Pyo Choe, Sherry C. Miller, and Susan J. Brown* Division of Biology, Kansas State University, Manhattan, KS 66506 Edited by Kathryn V. Anderson, Sloan–Kettering Institute, New York, NY, and approved March 7, 2006 (received for review December 4, 2005) In Drosophila, a hierarchy of maternal, gap, pair-rule, and segment (11). However, pair-rule expression of prd homologs is con- polarity gene interactions regulates virtually simultaneous blasto- served in Drosophila, Tribolium, and Schistocerca (12). These derm segmentation. For the last decade, studies have focused on results suggest that if insect segments are prepatterned in units revealing the extent to which Drosophila segmentation mecha- of double segment periodicity, then the genetic regulatory nisms are conserved in other arthropods where segments are interactions of pair-rule mechanisms differ in each species. added sequentially from anterior to posterior in a cellular envi- Interactions among pair-rule genes in insects other than ronment. Despite our increased knowledge of individual segmen- Drosophila have not been investigated to date. We have used tation genes, details of their interactions in non-Drosophilid insects parental RNA interference (RNAi) (13) to functionally analyze are not well understood. We analyzed the Tribolium orthologs of pair-rule gene orthologs and their interactions in the short-germ Drosophila pair-rule genes, which display pair-rule expression beetle T. castaneum. Here, we describe the genetic interactions patterns. Tribolium castaneum paired (Tc-prd) and sloppy-paired of pair-rule patterning in the short-germ insect T. castaneum and (Tc-slp) genes produced pair-rule phenotypes when their tran- discuss implications for insect segmentation. scripts were severely reduced by RNA interference. In contrast, similar analysis of T. castaneum even-skipped (Tc-eve), runt (Tc- Results run), or odd-skipped (Tc-odd) genes produced severely truncated, Two Classes of Pair-Rule Genes in Tribolium. Classic pair-rule mu- almost completely asegmental phenotypes. Analysis of interac- tant phenotypes in Drosophila include loss of alternating seg- tions between pair-rule components revealed that Tc-eve, Tc-run, ments or defects displaying double segment periodicity, which and Tc-odd form a three-gene circuit to regulate one another as are consistent with the normal expression pattern of the corre- well as their downstream targets, Tc-prd and Tc-slp. The comple- sponding gene. Because Tribolium orthologs of these genes are ment of primary pair-rule genes in Tribolium differs from Drosoph- expressed in pair-rule patterns (see Fig. 5 and Supporting Results, ila in that it includes Tc-odd but not Tc-hairy. This gene circuit which are published as supporting information on the PNAS web defines segments sequentially in double segment periodicity. Fur- site, for expression of Tc-odd, Tc-prd, and Tc-slp), we expected thermore, this single mechanism functions in the early blastoderm RNAi to produce similar phenotypes. Surprisingly, however, stage and subsequently during germ-band elongation. The peri- strong knock-down of Tc-eve, Tc-run,orTc-odd transcripts odicity of the Tribolium pair-rule gene interactions reveals com- produced truncated, almost completely asegmental embryos ponents of the genetic hierarchy that are regulated in a repetitive instead of pair-rule phenotypes. Tc-eveRNAi embryonic cuticles circuit or clock-like mechanism. This pair-rule gene circuit provides contain labrum, antennae, and telson (Fig. 1b) but no gnathal or insight into short-germ segmentation in Tribolium that may be trunk segments. In addition to labrum and antennae, Tc-runRNAi more generally applicable to segmentation in other arthropods. cuticles contain mandibles (Fig. 1c), whereas Tc-oddRNAi cuticles contain mandibles and maxilla (Fig. 1d). Consistent with these insect segmentation ͉ even-skipped ͉ runt ͉ odd-skipped phenotypes, there are no gnathal or trunk Tc-Engrailed (Tc-En) stripes in Tc-eveRNAi germ-band embryos, and only one (man- n Drosophila, a hierarchy of maternal, gap, pair-rule, and dibular) or two gnathal (mandibular and maxillary) stripes in Isegment polarity genes regulates segmentation (1). Pair-rule Tc-run and Tc-odd RNAi embryos (Fig. 2 g, m, and s), respec- genes transform regional gradients of maternal and gap gene tively. The homeotic gene Tc-Dfd, which serves as a molecular information into cellular domains that define parasegmental marker for mandibular and maxillary segments, is expressed boundaries (2), ultimately producing segments by means of normally in Tc-odd RNAi embryos (Fig. 2s). Knock-down of any regulation of segment polarity genes. Genetic and molecular of these three genes blocked segmentation and elongation. In analyses reveal a complex pair-rule gene network, which Drosophila, eve null mutants produce asegmental cuticles, operates in units of double segment periodicity. even-skipped whereas null mutants of run or odd cause typical pair-rule (eve), hairy (h), and runt (run) are essential in setting para- phenotypes (14). The similar truncated, asegmental phenotypes segmental boundaries. These primary pair-rule genes are of Tc-eve RNAi, Tc-runRNAi, and Tc-oddRNAi embryos suggest that regulated by the maternal and gap genes; in turn, they regulate these genes function at the same level in the segmentation other, secondary pair-rule genes such as fushi-tarazu ( ftz), hierarchy. paired (prd), sloppy-paired (slp), and odd-skipped (odd) (3, 4). In contrast, Tc-prdRNAi and Tc-slpRNAi generated typical pair- In general, loss of primary pair-rule gene function affects the rule phenotypes (Fig. 1 e and f) that phenocopy previously expression of secondary pair-rule genes, whereas the expres- described mutants (15). Similar to Drosophila prd mutants (14), sion of primary pair-rule genes is not altered in secondary Tc-prdRNAi embryonic cuticles lacked odd-numbered segments, pair-rule gene mutants. Comparative studies of pair-rule gene homologs in other insects reveal a wide variety of expression patterns. In the Conflict of interest statement: No conflicts declared. grasshopper Schistocerca, homologs of eve and ftz are not This paper was submitted directly (Track II) to the PNAS office. expressed in pair-rule stripes (5, 6). In the milkweed bug Abbreviations: En, Engrailed; RNAi, RNA interference. Oncopeltus fasciatus, the eve homolog is expressed in segmental, Data deposition: The sequences reported in this paper have been deposited in the GenBank not pair-rule, stripes (7). In the beetle Tribolium castaneum, database [accession nos. DQ414246 (Tc-odd), DQ414247 (Tc-prd), and DQ414248 (Tc-slp)]. where eve, ftz, h, and run orthologs are expressed in pair-rule *To whom correspondence should be addressed. E-mail: [email protected]. stripes (8–10), loss of ftz does not produce a pair-rule phenotype © 2006 by The National Academy of Sciences of the USA 6560–6564 ͉ PNAS ͉ April 25, 2006 ͉ vol. 103 ͉ no. 17 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0510440103 Downloaded by guest on October 2, 2021 Fig. 1. Cuticle preparations of severely affected T. castaneum pair-rule gene RNAi embryos. (a) This WT first-instar larval cuticle contains a head, three thoracic segments (T1–T3), eight abdominal segments (A1–A8), and terminal structures. Lr, labrum; Ant, antennae; Md, mandibles; Mx, maxillae; Lb, la- bium. (b) This spherical, asegmental Tc-eveRNAi cuticle contains labrum and antennae but no trunk segments. (c) In this severely affected Tc-runRNAi cuticle, the preoral and mandibular segments developed normally, but all other segments are missing, resulting in a spherical body similar to that of the Tc-eveRNAi embryo in b.(d) Preoral, mandibular, and maxillary segments developed normally in this severely affected Tc-oddRNAi cuticle, but the ab- sence of posterior segments produced a spherical body shape similar to the Tc-eveRNAi and Tc-runRNAi embryos. (e) This severely affected Tc-prdRNAi cuticle contains maxillary, T1, T3, and four abdominal segments. (f) In this severely affected Tc-slpRNAi cuticle, T2 and four abdominal segments formed, whereas all gnathal and even-numbered trunk segments are missing. including mandibular, labial, thoracic (T2), and four abdominal Fig. 2. Expression of Tribolium pair-rule genes in primary pair-rule gene RNAi embryos. Expression of Tc-En and pair-rule genes in WT (a–f), Tc-eveRNAi segments (Fig. 1e). Corresponding germ-band embryos lacked (g–l), Tc-runRNAi (m–r), and Tc-oddRNAi (s-x) embryos is shown. (g) Antennal and odd-numbered Tc-En stripes (Fig. 3 c and d), suggesting that intercalary Tc-En stripes formed in this severely affected Tc-eveRNAi embryo. Tc-prd is essential for the expression of Tc-En in odd-numbered (h–l) In severely affected Tc-eveRNAi embryos, expression of Tc-eve (h), Tc-run parasegments. Complementary to Tc-prdRNAi, Tc-slpRNAi cuti- (i), and Tc-odd (j) were severely reduced or abolished, and Tc-prd (k) and Tc-slp cles lacked even-numbered segments (Fig. 1f). Corresponding (l) failed to resolve into stripes. (m) In this severely affected Tc-runRNAi embryo, germ-band embryos lacked even-numbered Tc-En stripes (Fig. only antenna and mandibular Tc-En stripes formed. (n–r) In severely affected Tc-runRNAi embryos, Tc-eve (n), Tc-prd (q), and Tc-slp (r) were ectopically 3 e and f), indicating that Tc-slp is required for the expression of expressed, but Tc-run (o) and Tc-odd (p) expression was strongly reduced. (s) Tc-En in even-numbered parasegments. Interestingly, hypomor- Tc-Deformed (purple) and Tc-En are expressed normally in the mandibular and phic slp mutants in Drosophila affect odd-numbered segments maxillary segments of this severely affected Tc-oddRNAi embryo. (t–x)Inse- BIOLOGY RNAi RNAi (16), whereas Tc-slp affects even-numbered segments, im- verely affected Tc-odd embryos, Tc-eve (t) and Tc-run (u) were expressed DEVELOPMENTAL plying that the requirement for slp function is different in flies in broad continuous domains, but Tc-odd (v), Tc-prd (w), and Tc-slp (x) expres- and beetles.
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