It's a bug's life

Light Sheet-based Fluorescence Microscopy of Living or Fixed and Stained Tribolium castaneum Embryos

The red flour beetle Tribolium castaneum has become an important insect model organism in developmental genetics and evolutionary developmental biology. The observation of Tribolium embryos with light sheet-based fluorescence microscopy has multiple advantages over conventional widefield and confocal fluorescence microscopy. Due to the unique properties of a light sheet-based microscope, three dimensional images of living specimens can be recorded with high signal-to-noise ratios and significantly reduced photo-bleaching as well as photo-toxicity along multiple directions over periods that last several days. With more than four years of methodological development and a continuous increase of data, the time seems appropriate to establish standard operating procedures for the usage of light sheet technology in the Tribolium community as well as in the insect community at large. This protocol describes three mounting techniques suitable for different purposes, presents two novel custom-made transgenic Tribolium lines appropriate for long-term live imaging, suggests five fluorescent dyes to label intracellular structures of fixed embryos and provides information on data post-processing for the timely evaluation of the recorded data. Representative results concentrate on long-term live imaging, optical sectioning and the observation of the same embryo along multiple directions. The respective datasets are provided as a downloadable resource. Finally, the protocol discusses quality controls for live imaging assays, current limitations and the applicability of the outlined procedures to other insect species. This protocol is primarily intended for developmental biologists who seek imaging solutions that outperform standard laboratory equipment. It promotes the continuous attempt to close the gap between the technically orientated laboratories/communities, which develop and refine microscopy methodologically, and the life science laboratories/communities, which require 'plug-and-play' solutions to technical challenges. Furthermore, it supports an axiomatic approach that moves the biological questions into the center of attention.

Long-term fluorescence live imaging of Tribolium castaneum embryos: principles, resources, scientific challenges and the comparative approach

Light sheet-based fluorescence microscopy became an important tool in insect developmental biology due to its high acquisition speed, low photo-bleaching rate and the high survival probability of the specimens. Initially applied to document the embryogenesis of Drosophila melanogaster, it is now used to investigate the embryonic morphogenesis of emerging model organisms such as the red flour beetle Tribolium castaneum. Here, we discuss the principles of light sheet-based fluorescence microscopy and outline Tribolium as a model organism for developmental biology. We summarize labeling options and present two custom-made transgenic lines suitable for live imaging. Finally, we highlight studies on Tribolium that address scientific questions with fluorescence live imaging and discuss the comparative approach to investigate insect morphogenesis in an evolutionary context.

Non-invasive long-term fluorescence live imaging of Tribolium castaneum embryos

Insect development has contributed significantly to our understanding of metazoan development. However, most information has been obtained by analyzing a single species, the fruit fly Drosophila melanogaster. Embryonic development of the red flour beetle Tribolium castaneum differs fundamentally from that of Drosophila in aspects such as short-germ development, embryonic leg development, extensive extra-embryonic membrane formation and non-involuted head development. Although Tribolium has become the second most important insect model organism, previous live imaging attempts have addressed only specific questions and no long-term live imaging data of Tribolium embryogenesis have been available. By combining light sheet-based fluorescence microscopy with a novel mounting method, we achieved complete, continuous and non-invasive fluorescence live imaging of Tribolium embryogenesis at high spatiotemporal resolution. The embryos survived the 2-day or longer imaging process, developed into adults and produced fertile progeny. Our data document all morphogenetic processes from the rearrangement of the uniform blastoderm to the onset of regular muscular movement in the same embryo and in four orientations, contributing significantly to the understanding of Tribolium development. Furthermore, we created a comprehensive chronological table of Tribolium embryogenesis, integrating most previous work and providing a reference for future studies. Based on our observations, we provide evidence that serosa window closure and serosa opening, although deferred by more than 1 day, are linked. All our long-term imaging datasets are available as a resource for the community. Tribolium is only the second insect species, after Drosophila, for which non-invasive long-term fluorescence live imaging has been achieved.

Evolution of male sexual characters in the oriental Drosophila melanogaster species group

Understanding the genetic and molecular mechanisms of morphological evolution is one of the greatest challenges in evolutionary biology. Sexually dimorphic traits, which often evolve at a high rate due to their involvement in mate choice and sexual selection, present unique opportunities for investigating changes in development over short evolutionary distances. Phylogenetic analysis is essential to provide a historical framework for comparative studies of development by establishing the order and polarity of morphological changes. In this report, we apply a new molecular phylogeny to reconstruct the evolution of male sexual characters in a group of species closely related to the model species Drosophila melanogaster. These highly variable traits include wing melanin patterns, the sex comb, and the structure of external genitalia and analia. We show that sexually dimorphic characters can diverge very rapidly among closely related species. More surprisingly, we also find a pervasive pattern of independent origin and secondary loss of male sexual traits in different evolutionary lineages.

DNA sequence variation at a duplicated gene: excess of replacement polymorphism and extensive haplotype structure in the Drosophila melanogaster bicoid region

The bicoid (bcd) gene of Drosophila has played an important role in understanding the system of developmental regulatory genes that controls segmentation in the fruit fly. Several studies in Drosophila and closely related insects suggest that bcd may be the result of a gene duplication in the Dipteran lineage. In addition, the presence of a large, conserved secondary structure in the 3' untranslated region (UTR) makes the bcd gene a good candidate for studying compensatory evolution and the relationship between RNA secondary structure and patterns of standing variation in natural populations. Despite these interesting aspects, a population-level analysis has until now not been performed on bcd. In this study, DNA sequence variation was examined for a 4-kb region of the bcd gene, including a portion of the 5' UTR, the entire coding region, and the 3' UTR, for 25 Drosophila melanogaster isofemale lines from Zimbabwe and one allele from D. simulans. Statistical tests revealed a significant excess of replacement polymorphisms in the D. melanogaster lineage that are clustered in two putative linker regions of the Bicoid protein. This result is consistent with a relaxation of selective constraints in these regions. In addition, we found a distinct haplotype structure and a significantly smaller number of haplotypes than predicted by the standard neutral model. It is unlikely that the haplotype structure is maintained by epistatic selection acting on the secondary structure in the 3' UTR or by the association of the bcd gene with polymorphic inversions. Instead, our two main observations, namely the occurrence of a haplotype structure and the excess of replacement polymorphisms, may indicate that the selective history of this gene is rather complex, involving both the relaxation of purifying selection in some parts of the protein and the action of positive selection in other parts of the gene region.

Grasshopper hunchback expression reveals conserved and novel aspects of axis formation and segmentation

While the expression patterns of segment polarity genes such as engrailed have been shown to be similar in Drosophila melanogaster and Schistocerca americana (grasshopper), the expression patterns of pair-rule genes such as even-skipped are not conserved between these species. This might suggest that the factors upstream of pair-rule gene expression are not conserved across insect species. We find that, despite this, many aspects of the expression of the Drosophila gap gene hunchback are shared with its orthologs in the grasshoppers S. americana and L. migratoria.

We have analyzed both mRNA and protein expression during development, and find that the grasshopper hunchback orthologs appear to have a conserved role in early axial patterning of the germ anlagen and in the specification of gnathal and thoracic primordia. In addition, distinct stepped expression levels of hunchback in the gnathal/thoracic domains suggest that grasshopper hunchback may act in a concentration-dependent fashion (as in Drosophila), although morphogenetic activity is not set up by diffusion to form a smooth gradient.

Axial patterning functions appear to be performed entirely by zygotic hunchback, a fundamental difference from Drosophila in which maternal and zygotic hunchback play redundant roles. In grasshoppers, maternal hunchback activity is provided uniformly to the embryo as protein and, we suggest, serves a distinct role in distinguishing embryonic from extra-embryonic cells along the anteroposterior axis from the outset of development – a distinction made in Drosophila along the dorsoventral axis later in development.

Later hunchback expression in the abdominal segments is conserved, as are patterns in the nervous system, and in both Drosophila and grasshopper, hunchback is expressed in a subset of extra-embryonic cells. Thus, while the expected domains of hunchback expression are conserved in Schistocerca, we have found surprising and fundamental differences in axial patterning, and have identified a previously unreported domain of expression in Drosophila that suggests conservation of a function in extra-embryonic patterning.

A polychaete hunchback ortholog

We report the first characterization of a segmentation gene homologue in the basal polychaete Capitella capitata using a pan-annelid cross-species antibody to the hunchback-like gene product. In flies, the gap segmentation gene hunchback (hb) encodes a C(2)H(2) zinc-finger transcription factor that plays a pivotal role in patterning the anterior region of the fly body plan. The hb orthologue in Capitella (Cc-hb) is expressed maternally and in all micromere and macromere cells throughout cleavage. At gastrulation, nuclear Cc-hb protein is expressed in the micromere-derived surface epithelium that undergoes epiboly and in the large vegetal blastomeres that gradually become internalized. During organogenesis, Cc-hb is expressed in the developing gut epithelium, the prostomial and pygidial epithelium, and in a subset of differentiated neurons in the adult central nervous system. Cc-hb is not expressed in the segmental precursor cells in the trunk. The Cc-hb expression domains in Capitella are similar to those reported for the leech hb orthologue (LZF2), and many of the observed differences between the annelid classes correlate with changes in life history. The lack of detectable annelid hb protein in the trunk at the time of AP pattern formation in leech and in polychaete suggests that the anterior organizing function of hb in flies originated in the arthropod or insect lineage.

Polarity and form regulation in development and reconstitution

In the literature, it is often assumed, for example with respect to Hydra, that several Turing systems coexist and it is also assumed that maintaining the polar profile, even when the system increases in size, is important for the polarity of the final phenotype. It is shown here that in reality there is only one Turing system, Child's system. To obtain a complete polar individual or organ, whether in reconstitution or development, it is essential that the complete succession of metabolic patterns occurs. Child's concepts of physiological dominance, subordination and isolation are interpreted in the light of Turing theory and in particular the Turing wavelength. It is emphasised, particularly by pointing to Child's metabolic patterns in coelenterates, both in development and in reconstitution, that it is the elongation that drives the succession polar metabolic pattern-->bipolar metabolic pattern, and this corresponds to the prediction of Turing theory supporting the thesis that Child's metabolic pattern is a Turing pattern. It is shown that if we assume that ATP is the Turing inhibitor then the many results of Child about the reduction of the scale of organisation with the decrease in the intensity of the energy metabolism correspond to the reduction of the Turing wavelength. The interplay between the Turing wavelength and the length of the form explains the conditions of reconstitution under which partial forms, wholes and form regulation are obtained. It is suggested that higher metabolism is responsible for both larger size and larger Turing wavelength thus securing form regulation. The results could be of importance in modern 'regenerative biology'. Heteromorphosis, i.e. animals with two heads (or two tails), one at each end, is explained by a bipolar Turing-Child metabolic pattern replacing a polar metabolic pattern. This can be brought about by chemical or by genetic means and indeed the prediction for Drosophila that the transition, wild type-->Bicaudal D, occurs because a polar Turing pattern is replaced by bipolar Turing pattern is confirmed, again if we accept that Child's metabolic pattern is the underlying Turing pattern. Child's experiments on Drosophila, including the requirement of critical length for metabolic polarity, are explained by Turing theory. Phenocopies and phenotypes are explained by the Turing-Child theory. It is shown that both Child's results about metabolic patterns and modern results for Hydra about gap junctions, 'endogeneous inhibitor' and gene expression, are correlated and explained by (cAMP, ATP) Turing theory. It is argued that the double-gradient hypothesis is incorrect in its original formulation and that it is Child's conception of succeeding metabolic patterns that is the correct one and that it corresponds to the prediction of the Turing theory.

"Alien" wasps and evolution of development

A comparative analysis of early developmental programs in a group of parasitic wasps reveals that closely related species can undergo dramatic evolutionary shifts in their patterns of embryogenesis. Developmental changes detected include alterations in early cleavage divisions, the establishment of embryonic anteroposterior polarity and modifications of the segmentation gene hierarchy described from Drosophila. These changes appear to be adaptations to parasitic development, taking place within the body of the host. Wasps illustrate a surprising plasticity in their early development and embryogenesis. The alterations associated with different parasitic strategies suggest that ecological adaptations may have profound influences on developmental processes in animals.