Expression of hunchback during trunk segmentation in the branchiopod crustacean Artemia franciscana

The evolution of the gene regulatory networks patterning the Drosophila Blastoderm

The Drosophila blastoderm gene regulatory network is one of the best studied networks in biology. It is composed of a series of tiered sub-networks that act sequentially to generate a primary segmental pattern. Many of these sub-networks have been studied in other arthropods, allowing us to reconstruct how each of them evolved over the transition from the arthropod ancestor to the situation seen in Drosophila today. I trace the evolution of each of these networks, showing how some of them have been modified significantly in Drosophila relative to the ancestral state while others are largely conserved across evolutionary timescales. I compare the putative ancestral arthropod segmentation network with that found in Drosophila and discuss how and why it has been modified throughout evolution, and to what extent this modification is unusual.

The neuroblast timer gene nubbin exhibits functional redundancy with gap genes to regulate segment identity in Tribolium

The neuroblast timer genes hunchback, Krüppel, nubbin and castor are expressed in temporal sequence in neural stem cells, and in corresponding spatial sequence along the Drosophila blastoderm. As canonical gap genes, hunchback and Krüppel play a crucial role in insect segmentation, but the roles of nubbin and castor in this process remain ambiguous. We have investigated the expression and functions of nubbin and castor during segmentation in the beetle Tribolium. We show that Tc-hunchback, Tc-Krüppel, Tc-nubbin and Tc-castor are expressed sequentially in the segment addition zone, and that Tc-nubbin regulates segment identity redundantly with two previously described gap/gap-like genes, Tc-giant and Tc-knirps. Simultaneous knockdown of Tc-nubbin, Tc-giant and Tc-knirps results in the formation of ectopic legs on abdominal segments. This homeotic transformation is caused by loss of abdominal Hox gene expression, likely due to expanded Tc-Krüppel expression. Our findings support the theory that the neuroblast timer series was co-opted for use in insect segment patterning, and contribute to our growing understanding of the evolution and function of the gap gene network outside of Drosophila.

Summary:nubbin and the gap genes knirps and giant redundantly repress Krüppel expression during segmentation. Simultaneous knockdown of all three genes causes ectopic Krüppel expression and loss of abdominal segment identity.

The Evolution of Gene Regulatory Networks that Define Arthropod Body Plans

Our understanding of the genetics of arthropod body plan development originally stems from work on Drosophila melanogaster from the late 1970s and onward. In Drosophila, there is a relatively detailed model for the network of gene interactions that proceeds in a sequential-hierarchical fashion to define the main features of the body plan. Over the years, we have a growing understanding of the networks involved in defining the body plan in an increasing number of arthropod species. It is now becoming possible to tease out the conserved aspects of these networks and to try to reconstruct their evolution. In this contribution, we focus on several key nodes of these networks, starting from early patterning in which the main axes are determined and the broad morphological domains of the embryo are defined, and on to later stage wherein the growth zone network is active in sequential addition of posterior segments. The pattern of conservation of networks is very patchy, with some key aspects being highly conserved in all arthropods and others being very labile. Many aspects of early axis patterning are highly conserved, as are some aspects of sequential segment generation. In contrast, regional patterning varies among different taxa, and some networks, such as the terminal patterning network, are only found in a limited range of taxa. The growth zone segmentation network is ancient and is probably plesiomorphic to all arthropods. In some insects, it has undergone significant modification to give rise to a more hardwired network that generates individual segments separately. In other insects and in most arthropods, the sequential segmentation network has undergone a significant amount of systems drift, wherein many of the genes have changed. However, it maintains a conserved underlying logic and function.

Expression study of the hunchback ortholog in embryos of the onychophoran Euperipatoides rowelli

Zinc finger transcription factors encoded by hunchback homologs play different roles in arthropods, including maternally mediated control, segmentation, and mesoderm and neural development. Knockdown experiments in spider and insect embryos have also revealed homeotic effects and gap phenotypes, the latter indicating a function of hunchback as a "gap gene". Although the expression pattern of hunchback has been analysed in representatives of all four major arthropod groups (chelicerates, myriapods, crustaceans and insects), nothing is known about its expression in one of the closest arthropod relatives, the Onychophora (velvet worms). We therefore examined the expression pattern of hunchback in embryos of the onychophoran Euperipatoides rowelli. Our transcriptomic and phylogenetic analyses revealed only one hunchback ortholog in this species. The putative Hunchback protein contains all nine zinc finger domains known from other protostomes. We found no indication of maternally contributed transcripts of hunchback in early embryos of E. rowelli. Its initial expression occurs in the ectodermal tissue of the antennal segment, followed by the jaw, slime papilla and trunk segments in an anterior-to-posterior progression. Later, hunchback expression is seen in the mesoderm of the developing limbs. A second "wave" of expression commences later in development in the antennal segment and continues posteriorly along each developing nerve cord. This expression is restricted to the neural tissues and does not show any segmental pattern. These findings are in line with the ancestral roles of hunchback in mesoderm and neural development, whereas we find no evidence for a putative function of hunchback as a "gap gene" in Onychophora.

The gap gene network

Gap genes are involved in segment determination during the early development of the fruit fly Drosophila melanogaster as well as in other insects. This review attempts to synthesize the current knowledge of the gap gene network through a comprehensive survey of the experimental literature. I focus on genetic and molecular evidence, which provides us with an almost-complete picture of the regulatory interactions responsible for trunk gap gene expression. I discuss the regulatory mechanisms involved, and highlight the remaining ambiguities and gaps in the evidence. This is followed by a brief discussion of molecular regulatory mechanisms for transcriptional regulation, as well as precision and size-regulation provided by the system. Finally, I discuss evidence on the evolution of gap gene expression from species other than Drosophila. My survey concludes that studies of the gap gene system continue to reveal interesting and important new insights into the role of gene regulatory networks in development and evolution.

hunchback and Ikaros-like zinc finger genes control reproductive system development in Caenorhabditis elegans

Here we provide evidence for a C2H2 zinc finger gene family with similarity to Ikaros and hunchback. The founding member of this family is Caenorhabditis elegans ehn-3, which has important and poorly understood functions in somatic gonad development. We examined the expression and function of four additional hunchback/Ikaros-like (HIL) genes in C. elegans reproductive system development. Two genes, ehn-3 and R08E3.4, are expressed in somatic gonadal precursors (SGPs) and have overlapping functions in their development. In ehn-3; R08E3.4 double mutants, we find defects in the generation of distal tip cells, anchor cells, and spermatheca; three of the five tissues derived from the SGPs. We provide in vivo evidence that C. elegans HIL proteins have functionally distinct zinc finger domains, with specificity residing in the N-terminal set of four zinc fingers and a likely protein-protein interaction domain provided by the C-terminal pair of zinc fingers. In addition, we find that a chimeric human Ikaros protein containing the N-terminal zinc fingers of EHN-3 functions in C. elegans. Together, these results lend support to the idea that the C. elegans HIL genes and Ikaros have similar functional domains. We propose that hunchback, Ikaros, and the HIL genes arose from a common ancestor that was present prior to the divergence of protostomes and deuterostomes.

Hunchback-like protein is expressed in cleavage blastomeres, gastrula epithelium, and ciliary structures in gastropods

We report the expression of Hunchback (Hb)-like protein during embryonic and larval development in two caenogastropods, Crepidula fornicata and Ilyanassa obsoleta. During the cleavage stages of these species, Hb-like protein is uniformly expressed in micromere and macromere nuclei. At gastrulation, gastropod Hb-like protein is expressed in the surface epithelium that undergoes epiboly. During organogenesis, gastropod Hb-like protein is expressed in the developing ciliated structures associated with feeding and locomotion. We find no detectable gradient or regionalization of Hb-like protein in gastropod embryos or larvae that resembles the graded Hb pattern of expression observed in dipteran insect embryos. Rather we found that the spatiotemporal expression profile of gastropod Hb-like protein is nearly identical to the Hb-like patterns obtained from the polychaete Capitella sp. I and is highly similar to those reported for clitellate annelids. Based upon the comparative data collected from both ecdysozoans and lophotrochozoan lineages, our results support the hypothesis that the role of Hb in anteroposterior patterning is a derived trait specific to arthropods, and that the ancestral function of lophotrochozoan Hb-like protein played a role in the formation of the cleavage-stage blastomeres and the gastrula epithelium and in structures associated with larval feeding and locomotion.