A strategy for mapping bicoid on the phylogenetic tree.
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Amino Acid Sequence↗
Biomedical subjects
R Denell
Publications and source records attributed to R Denell.
Comparative insect developmental genetics: phenotypes without mutants.
The last decade has seen a dramatic increase in interest in the extent to which morphological evolution depends on changes in regulatory pathways. Insects provide a fertile ground for study because of their diversity and our high level of understanding of the genetic regulation of development in Drosophila melanogaster. However, comparable genetic approaches are presently possible in only a small number of non-Drosophilid insects. In a recent paper, Hughes and Kaufman have used a new methodology, RNA interference, in the milkweed bug, Oncopeltus fasciatus, to phenocopy the effects of mutations in Hox genes. RNA interference involves the injection of double-stranded RNA of the same sequence as the relevant mRNA resulting in a depletion of that transcript. Hughes and Kaufman focused on the gnathal segments, which elaborate specialized appendages important to feeding. Their results indicate that gnathal adaptations in this bug are correlated with changes in Hox gene functions and interactions.
Animals↗
Molecular characterization of Tclabial and the 3' end of the Tribolium homeotic complex.
The homeotic selector gene labial is located at the 3' end of the Antennapedia complex (ANTC) and is required for proper head development in Drosophila. We have cloned and characterized the Tribolium ortholog of labial, Tclabial (Tclab). Similar to Drosophila labial, Tclab contains a single large intron upstream of the homeobox. In contrast, Tclab lacks an intron within the homeobox. The Tribolium ortholog of chaoptic, Tcchaoptic, transcribed from the opposite strand, is located immediately downstream of the homeotic complex, and its 3'UTR overlaps that of Tclab by 50 nucleotides. We have also sequenced the 13.5 kb interval between Tclab and maxillopedia (the Tribolium ortholog of Drosophila proboscipedia). In contrast to Drosophila, there is not a cluster of cuticle genes in this region. Finally, we have examined the expression of Tclab transcripts in Tribolium embryos. As previously described for Drosophila and other insects, the expression of Tclab is specific to the intercalary segment.
Amino Acid Sequence↗
Implications of the Tribolium Deformed mutant phenotype for the evolution of Hox gene function.
Among insects, the genetic regulation of regional identities in the postoral head or gnathal segments (mandibular, maxillary, and labial) is best understood in the fly Drosophila melanogaster. In part, normal gnathal development depends on Deformed (Dfd) and Sex combs reduced (Scr), genes in the split Drosophila homeotic complex. The gnathal segments of Dfd and Scr mutant larvae are abnormal but not homeotically transformed. In the red flour beetle, Tribolium castaneum, we have isolated loss-of-function mutations of the Deformed ortholog. Mutant larvae display a strong transformation of mandibular appendages to antennae. The maxillary appendages, normally composed of an endite and a telopodite, develop only the telopodite in mutant larvae. We previously reported that mutations in the beetle Scr and Antennapedia orthologs cause the labial and thoracic appendages, respectively, to be transformed to antennae. Moreover, a deficiency of most of the beetle homeotic complex causes all gnathal (as well as thoracic and abdominal) segments to develop antennae. These and other observations are consistent with the hypothesis that ancestral insect homeotic gene functions have been modified considerably during the evolution of the highly specialized maggot head. One of the ancestral homeobox genes that arose close to the root of the Eumetazoa appears to have given rise to Dfd, Scr, and the Antennapedia homeobox-class homeotic genes. Evidence from both Tribolium and Drosophila suggests that this ancestral gene served to repress anterior development as well as confer a trunk-specific identity.
Amino Acid Sequence↗
Characterization of the Tribolium Deformed ortholog and its ability to directly regulate Deformed target genes in the rescue of a Drosophila Deformed null mutant.
We have analyzed the Tribolium castaneum ortholog of the Drosophila homeotic gene Deformed (Dfd) and determined its expression pattern during embryogenesis in this beetle. Tc Deformed (Tc Dfd) is expressed in the blastoderm and the condensing germ rudiment in a region that gives rise to gnathal segments. During germ band extension Tc Dfd is expressed in the mandibular and maxillary segments, their appendages, and the dorsal ridge. Comparison of insect Dfd protein sequences reveals several highly conserved regions. To determine whether common molecular features reflect conserved regulatory functions we used the Gal4 system to express the Tribolium protein in Drosophila embryos. When Tc Dfd is expressed throughout embryonic ectoderm under the control of P69B, the beetle protein autoregulates the endogenous Dfd gene. In addition, the Drosophila proboscipedia gene (a normal target of Dfd) is ectopically activated in the antennal and thoracic segments. We also compared the ability of the beetle and fly proteins to rescue defects in Dfd- mutants by expressing each throughout the embryonic during embryogenesis. Both proteins rescued Dfd- defects to the same extent in that they each restore the development of mouth hooks and cirri, as well as cause gain-of-function abnormalities of posterior mouth parts. As before, pb was ectopically activated in the antennal segment. This is the first demonstration of the ability of a heterologous homeotic selector protein to directly regulate a target gene independent of an endogenous Drosophila autoregulatory loop.
Amino Acid Sequence↗
Class 3 Hox genes in insects and the origin of zen.
We have cloned, from a beetle and a locust, genes that are homologous to the class 3 Hox genes of vertebrates. Outside the homeobox they share sequence motifs with the Drosophila zerknüllt (zen) and z2 genes, and like zen, are expressed only in extraembryonic membranes. We conclude that the zen genes of Drosophila derive from a Hox class 3 sequence that formed part of the common ancestral Hox cluster, but that in insects this (Hox) gene has lost its role in patterning the anterio-posterior axis of the embryo, and acquired a new function. In the lineage leading to Drosophila, the zen genes have diverged particularly rapidly.
Amino Acid Sequence↗
Discovery and genetic definition of the Drosophila Antennapedia complex.
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Animals↗
The Drosophila ribosomal protein S6 gene includes a 3' triplication that arose by unequal crossing-over.
Ribosomal protein S6 (rpS6) is the major phosphoprotein of the small ribosomal subunit of eukaryotes and is phosphorylated in response to treatment with mitogens and other stimuli. We have examined the organization of the rpS6 gene of Drosophila melanogaster. Comparisons of a cDNA with genomic DNA identify a transcription unit including three exons. Two tandem repeats downstream of this transcription unit reiterate divergent copies of the third exon and flanking regions. Comparisons of these three repeats with respect to nucleotide base substitutions and deletions or insertions show clearly that they arose via a duplication and subsequent crossing-over between misaligned copies. Although no direct evidence exists that the downstream exons are transcribed, the maintenance of open reading frames in spite of extensive genetic changes is consistent with a protein-coding function.
Amino Acid Sequence↗
Mutations in the Drosophila gene encoding ribosomal protein S6 cause tissue overgrowth.
We have characterized two P-element-induced, lethal mutations in Drosophila melanogaster which affect the larval hemocytes, mediators of the insect immune response. Each mutant displays larval melanotic tumors characteristic of mutations affecting the insect cellular immune system, and the moribund animals develop grossly hypertrophied hematopoietic organs because of increased cell proliferation and extra rounds of endoreduplication in some hematopoietic cells. Surprisingly, these mutations are due to P element insertions in the 5' regulatory region of the Drosophila gene encoding ribosomal protein S6 and cause a reduction of S6 transcript abundance in mutant larvae.
Animals↗