Unraveling The Molecular Mechanisms Of Insect Diversity

ABSTRACT UNRAVELING THE MOLECULAR MECHANISMS OF INSECT DIVERSITY by STEVEN MICHAEL HRYCAJ April 2010 Advisor: Dr. Aleksandar Popadic´ Major: Biological Sciences Degree: Doctor of Philosophy While it has long been recognized that the arthropods represent the most diverse animal phylum, the molecular bases defining these large-scale differences in body plans and appendages are only now becoming clear. Specifically, the recent merger between the fields of evolutionary and developmental biology ( evo-devo ) have provided several examples illustrating that this extraordinary diversification may be due to evolved variation(s) in the developmental networks that control the formation of these structures. In addition, the delineation of such developmental processes can provide a fresh, unbiased perspective on the robustness of the more traditional relationships within the arthropods that are based on the presence or absence of key group-defining features. For example, evo-devo type studies have determined that all arthropod mandibles are gnathobasic (composed of coxapodite only) and therefore, this feature can no longer be used to more closely group the insects and myriapods to the exclusion of the crustaceans. This result illustrates how evo-devo analyses can effectively establish true homologies of complex morphological traits. All insects possess a tri-partite body plan that consists of a head, thorax and limbless abdomen. Therefore, it would seem likely that the developmental networks controlling the establishment of these three regions would also be conserved. However, we have found lineage-specific variation in the genetic mechanisms that act to maintain one of these key insect features. Specifically, our analyses show that the POU homeodomain gene nubbin (nub) plays a critical role in the establishment of the limbless abdomen via the up-regulation of the hox gene abdominal-A (abd-A) in the milkweed bug (Oncopeltus fasciatus) that is not present in Drosophila. Hence, these data indicate that there are at least two independent mechanisms that act to maintain the establishment of the limbless abdomen in insects. In a similar fashion, we also show that the formation of the dorsal ridge, a highly conserved structure that separates the head and thorax in all insects, may also be maintained by at least two independent molecular mechanisms. In the cockroach Periplaneta americana, the embryonic abolishment of the hox gene Sex combs reduced (Scr) disrupts this ancient boundary, and results in the formation of an ectopic, supernumerary segment between the head and thorax. Interestingly, while Scr mutants Tribiolium yield an identical phenotype, no such ectopic segment develops when Scr is abolished in either Oncopeltus or Drosophila. Hence these data collectively indicate that a fair amount of plasticity may exist in the developmental networks controlling the establishment of class-defining insect features. Previous studies have indicated that the hox genes are required for conveying segmental identity along the antero-posterior axis in insects. However, while the post-embryonic functions of these genes have been well characterized in holometabolous lineages such as Drosophila, virtually no data exist about their roles during post-embryogenesis in more ancestral, hemimetabolous species. Briefly, the hemimetabolous mode development differs from the holometabolous mode in that the first nymph that hatches from the egg at the end of embryogenesis phenotypically resembles the eventual adult. Therefore, the question remains that if the majority of adult phenotypes are already established during embryogenesis, what are the functions of the hox genes during post-embryogenesis in hemimetabolous lineages? Our analyses of Scr in two hemimetabolous lineages (Oncopeltus and Periplaneta) identified novel temporal and spatial differences of function during embryonic and post-embryonic development. Specifically, in both instances the embryonic role of Scr is mainly restricted to the head with no role in the prothoracic (T1) segment. Conversely, during post-embryogenesis, Scr solely functions to provide identity to the T1 segment and has no major role in the head region in either species. In addition, the post-embryonic abolition of Scr in both Oncopeltus and Periplaneta results in the growth and formation of ectopic wings that originate from the paranotal tissue of the dorsal pronotum. This result suggests that the role of Scr in suppressing the normally active wing program on T1 appears to be conserved in both holo- and hemimetabolous insects. Overall, these findings provide important new insights into the current debate on the morphological origin of insect wings

Functional analysis of Scr during embryonic and post-embryonic development in the cockroach, Periplaneta americana

AbstractThe cockroach, Periplaneta americana represents a basal insect lineage that undergoes the ancestral hemimetabolous mode of development. Here, we examine the embryonic and post-embryonic functions of the hox gene Scr in Periplaneta as a way of better understanding the roles of this gene in the evolution of insect body plans. During embryogenesis, Scr function is strictly limited to the head with no role in the prothorax. This indicates that the ancestral embryonic function of Scr was likely restricted to the head, and that the posterior expansion of expression in the T1 legs may have preceded any apparent gain of function during evolution. In addition, Scr plays a pivotal role in the formation of the dorsal ridge, a structure that separates the head and thorax in all insects. This is evidenced by the presence of a supernumerary segment that occurs between the labial and T1 segments of RNAiScr first nymphs and is attributed to an alteration in engrailed (en) expression. The fact that similar Scr phenotypes are observed in Tribolium but not in Drosophila or Oncopeltus reveals the presence of lineage-specific variation in the genetic architecture that controls the formation of the dorsal ridge. In direct contrast to the embryonic roles, Scr has no function in the head region during post-embryogenesis in Periplaneta, and instead, strictly acts to provide identity to the T1 segment. Furthermore, the strongest Periplaneta RNAiScr phenotypes develop ectopic wing-like tissue that originates from the posterior region of the prothoracic segment. This finding provides a novel insight into the current debate on the morphological origin of insect wings

RNA interference in Lepidoptera: an overview of successful and unsuccessful studies and implications for experimental design

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Ubx Regulates Differential Enlargement and Diversification of Insect Hind Legs

Differential enlargement of hind (T3) legs represents one of the hallmarks of insect evolution. However, the actual mechanism(s) responsible are yet to be determined. To address this issue, we have now studied the molecular basis of T3 leg enlargement in Oncopeltus fasciatus (milkweed bug) and Acheta domesticus (house cricket). In Oncopeltus, the T3 tibia displays a moderate increase in size, whereas in Acheta, the T3 femur, tibia, and tarsus are all greatly enlarged. Here, we show that the hox gene Ultrabithorax (Ubx) is expressed in the enlarged segments of hind legs. Furthermore, we demonstrate that depletion of Ubx during embryogenesis has a primary effect in T3 legs and causes shortening of leg segments that are enlarged in a wild type. This result shows that Ubx is regulating the differential growth and enlargement of T3 legs in both Oncopeltus and Acheta. The emerging view suggests that Ubx was co-opted for a novel role in regulating leg growth and that the transcriptional modification of its expression may be a universal mechanism for the evolutionary diversification of insect hind legs

Pogostick: A New Versatile piggyBac Vector for Inducible Gene Over-Expression and Down-Regulation in Emerging Model Systems

Non-traditional model systems need new tools that will enable them to enter the field of functional genetics. These tools should enable the exploration of gene function, via knock-downs of endogenous genes, as well as over-expression and ectopic expression of transgenes.We constructed a new vector called Pogostick that can be used to over-express or down-regulate genes in organisms amenable to germ line transformation by the piggyBac transposable element. Pogostick can be found at www.addgene.org, a non-profit plasmid repository. The vector currently uses the heat-shock promoter Hsp70 from Drosophila to drive transgene expression and, as such, will have immediate applicability to organisms that can correctly interpret this promotor sequence. We detail how to clone candidate genes into this vector and test its functionality in Drosophila by targeting a gene coding for the fluorescent protein DsRed. By cloning a single DsRed copy into the vector, and generating transgenic lines, we show that DsRed mRNA and protein levels are elevated following heat-shock. When cloning a second copy of DsRed in reverse orientation into a flanking site, and transforming flies constitutively expressing DsRed in the eyes, we show that endogenous mRNA and protein levels drop following heat-shock. We then test the over-expression vector, containing the complete cDNA of Ultrabithorax (Ubx) gene, in an emerging model system, Bicyclus anynana. We produce a transgenic line and show that levels of Ubx mRNA expression rise significantly following a heat-shock. Finally, we show how to obtain genomic sequence adjacent to the Pogostick insertion site and to estimate transgene copy number in genomes of transformed individuals.This new vector will allow emerging model systems to enter the field of functional genetics with few hurdles

Hox10 Genes Function in Kidney Development in the Differentiation and Integration of the Cortical Stroma

Organogenesis requires the differentiation and integration of distinct populations of cells to form a functional organ. In the kidney, reciprocal interactions between the ureter and the nephrogenic mesenchyme are required for organ formation. Additionally, the differentiation and integration of stromal cells are also necessary for the proper development of this organ. Much remains to be understood regarding the origin of cortical stromal cells and the pathways involved in their formation and function. By generating triple mutants in the Hox10 paralogous group genes, we demonstrate that Hox10 genes play a critical role in the developing kidney. Careful examination of control kidneys show that Foxd1-expressing stromal precursor cells are first observed in a cap-like pattern anterior to the metanephric mesenchyme and these cells subsequently integrate posteriorly into the kidney periphery as development proceeds. While the initial cap-like pattern of Foxd1-expressing cortical stromal cells is unaffected in Hox10 mutants, these cells fail to become properly integrated into the kidney, and do not differentiate to form the kidney capsule. Consistent with loss of cortical stromal cell function, Hox10 mutant kidneys display reduced and aberrant ureter branching, decreased nephrogenesis. These data therefore provide critical novel insights into the cellular and genetic mechanisms governing cortical cell development during kidney organogenesis. These results, combined with previous evidence demonstrating that Hox11 genes are necessary for patterning the metanephric mesenchyme, support a model whereby distinct populations in the nephrogenic cord are regulated by unique Hox codes, and that differential Hox function along the AP axis of the nephrogenic cord is critical for the differentiation and integration of these cell types during kidney organogenesis

Efficacy and Safety of Ixekizumab in the Treatment of Radiographic Axial Spondyloarthritis:Sixteen-Week Results From a Phase III Randomized, Double-Blind, Placebo-Controlled Trial in Patients With Prior Inadequate Response to or Intolerance of Tumor Necrosis Factor Inhibitors

Objective: To investigate the efficacy and safety of ixekizumab in patients with active radiographic axial spondyloarthritis (SpA) and prior inadequate response to or intolerance of 1 or 2 tumor necrosis factor inhibitors (TNFi). Methods: In this phase III randomized, double-blind, placebo-controlled trial, adult patients with an inadequate response to or intolerance of 1 or 2 TNFi and an established diagnosis of axial SpA (according to the Assessment of SpondyloArthritis international Society [ASAS] criteria for radiographic axial SpA, with radiographic sacroiliitis defined according to the modified New York criteria and ≥1 feature of SpA) were recruited and randomized 1:1:1 to receive placebo or 80-mg subcutaneous ixekizumab every 2 weeks (IXEQ2W) or 4 weeks (IXEQ4W), with an 80-mg or 160-mg starting dose. The primary end point was 40% improvement in disease activity according to the ASAS criteria (ASAS40) at week 16. Secondary outcomes and safety were also assessed. Results: A total of 316 patients were randomized to receive placebo (n = 104), IXEQ2W (n = 98), or IXEQ4W (n = 114). At week 16, significantly higher proportions of IXEQ2W patients (n = 30 [30.6%]; P = 0.003) or IXEQ4W patients (n = 29 [25.4%]; P = 0.017) had achieved an ASAS40 response versus the placebo group (n = 13 [12.5%]), with statistically significant differences reported as early as week 1 with ixekizumab treatment. Statistically significant improvements in disease activity, function, quality of life, and spinal magnetic resonance imaging–evident inflammation were observed after 16 weeks of ixekizumab treatment versus placebo. Treatment-emergent adverse events (AEs) with ixekizumab treatment were more frequent than with placebo. Serious AEs were similar across treatment arms. One death was reported (IXEQ2W group). Conclusion: Ixekizumab treatment for 16 weeks in patients with active radiographic axial SpA and previous inadequate response to or intolerance of 1 or 2 TNFi yields rapid and significant improvements in the signs and symptoms of radiographic axial SpA versus placebo

Hox genes and evolution [version 1; referees: 3 approved]

Hox proteins are a deeply conserved group of transcription factors originally defined for their critical roles in governing segmental identity along the antero-posterior (AP) axis in Drosophila. Over the last 30 years, numerous data generated in evolutionarily diverse taxa have clearly shown that changes in the expression patterns of these genes are closely associated with the regionalization of the AP axis, suggesting that Hox genes have played a critical role in the evolution of novel body plans within Bilateria. Despite this deep functional conservation and the importance of these genes in AP patterning, key questions remain regarding many aspects of Hox biology. In this commentary, we highlight recent reports that have provided novel insight into the origins of the mammalian Hox cluster, the role of Hox genes in the generation of a limbless body plan, and a novel putative mechanism in which Hox genes may encode specificity along the AP axis. Although the data discussed here offer a fresh perspective, it is clear that there is still much to learn about Hox biology and the roles it has played in the evolution of the Bilaterian body plan