The ring-legged earwig Euborellia annulipes as a new model for oogenesis and development studies in insects

Insects are the dominant group of animals on Earth. Despite this abundance, most of our knowledge about many aspects of their biology and development come from a unique model, the vinegar fly, Drosophila melanogaster. Nevertheless, in the last years, the advances in molecular tools and imaging techniques have allowed the emergence of new insect models, adding valuable information to decipher the morphogenetic bases behind the formation and evolution of the vast diversity of shapes, sizes, and patterns that characterize them. Earwigs belong to Dermaptera which is a small order clustered in the Polyneopteran group. They are hemimetabolous insects with a flattened body, characteristic abdominal pincers, and maternal care behavior. This last feature and their role in agroecosystems have been studied in cosmopolitan species such as Forficula auricularia and Euborellia annulipes; however, their reproduction and embryonic development have been poorly addressed in laboratory conditions. In response, here we describe the ring-legged earwig Euborellia annulipes embryogenesis and life cycle from nymphal to adult stages, its reproduction, and essential morphological and behavioral characters. Additionally, using confocal and transmission electron microscopy we analyzed in detail the morphogenesis of its peculiar meroistic polytrophic ovary. Our aim is to provide an emerging model system to perform comparative studies on insect oogenesis, development, and morphological evolution.

The organizing role of Wnt signaling pathway during arthropod posterior growth

Wnt signaling pathways are recognized for having major roles in tissue patterning and cell proliferation. In the last years, remarkable progress has been made in elucidating the molecular and cellular mechanisms that underlie sequential segmentation and axial elongation in various arthropods, and the canonical Wnt pathway has emerged as an essential factor in these processes. Here we review, with a comparative perspective, the current evidence concerning the participation of this pathway during posterior growth, its degree of conservation among the different subphyla within Arthropoda and its relationship with the rest of the gene regulatory network involved. Furthermore, we discuss how this signaling pathway could regulate segmentation to establish this repetitive pattern and, at the same time, probably modulate different cellular processes precisely coupled to axial elongation. Based on the information collected, we suggest that this pathway plays an organizing role in the formation of the body segments through the regulation of the dynamic expression of segmentation genes, via controlling the caudal gene, at the posterior region of the embryo/larva, that is necessary for the correct sequential formation of body segments in most arthropods and possibly in their common segmented ancestor. On the other hand, there is insufficient evidence to link this pathway to axial elongation by controlling its main cellular processes, such as convergent extension and cell proliferation. However, conclusions are premature until more studies incorporating diverse arthropods are carried out.

Contribution of cell proliferation to axial elongation in the red flour beetle Tribolium castaneum

Most arthropods generate their posterior bodies by adding segments periodically, as the embryo grows, from a posteriorly located region called the segment addition zone. This mode of segmentation is shared with vertebrates and relies on oscillatory mechanisms, where the temporal periodicity of a clock is translated into repetitive spatial patterns. This ordered anterior-to-posterior pattern is achieved at the same time as the tissue elongates, opening the question of the functional coordination between the mechanisms of segmental patterning and posterior growth. The study of these processes in different arthropods has played an important role in unravelling some of the molecular mechanisms of segment formation. However, the behavior of cells during elongation and how cellular processes affect this segmental patterning has been poorly studied. Cell proliferation together with cell rearrangements are presumed to be the major forces driving axis elongation in the red flour beetle Tribolium castaneum. However, there still no strong evidence about the role and distribution of cell proliferation within the embryo. In this study, we propose to address these questions by using whole embryo cultures and pharmacological manipulation. We show that considerable cell proliferation occurs during germband elongation, measured by incorporation of the nucleoside analog of thymidine 5-Ethynyl-2’-deoxyuridine, EdU. Moreover, proliferating cells appeared to be spread along the elongating embryo with a posterior bias at early segmentation. In addition, when we blocked cell division, treated germbands were always shorter than controls and in some cases not able to fully elongate, even when control embryos already started to retract and leg buds are evident. Finally, we found that the absence of cell proliferation has no apparent effect on segmental patterning, as evidenced by Tc-engrailed (Tc-en) gene expression.

Evolutionary Developmental Biology (Evo-Devo) Research in Latin America

Famous for its blind cavefish and Darwin's finches, Latin America is home to some of the richest biodiversity hotspots of our planet. The Latin American fauna and flora inspired and captivated naturalists from the nineteenth and twentieth centuries, including such notable pioneers such as Fritz Müller, Florentino Ameghino, and Léon Croizat who made a significant contribution to the study of embryology and evolutionary thinking. But, what are the historical and present contributions of the Latin American scientific community to Evo-Devo? Here, we provide the first comprehensive overview of the Evo-Devo laboratories based in Latin America and describe current lines of research based on endemic species, focusing on body plans and patterning, systematics, physiology, computational modeling approaches, ecology, and domestication. Literature searches reveal that Evo-Devo in Latin America is still in its early days; while showing encouraging indicators of productivity, it has not stabilized yet, because it relies on few and sparsely distributed laboratories. Coping with the rapid changes in national scientific policies and contributing to solve social and health issues specific to each region are among the main challenges faced by Latin American researchers. The 2015 inaugural meeting of the Pan-American Society for Evolutionary Developmental Biology played a pivotal role in bringing together Latin American researchers eager to initiate and consolidate regional and worldwide collaborative networks. Such networks will undoubtedly advance research on the extremely high genetic and phenotypic biodiversity of Latin America, bound to be an almost infinite source of amazement and fascinating findings for the Evo-Devo community.

A Tribolium castaneum whole-embryo culture protocol for studying the molecular mechanisms and morphogenetic movements involved in insect development

The development of the red flour beetle Tribolium castaneum is more representative of arthropods than the evolutionarily derived fly, Drosophila melanogaster. Thus, Tribolium is becoming an emerging organism model for studying the evolution of the mechanisms that control embryonic development in arthropods. In this regard, diverse genetic and molecular tools are currently available for Tribolium, as well as imaging and embryonic techniques. Recently, we developed a method for culturing embryos in order to study specific stages during Tribolium development. In this report, we present a detailed and "easy-to-follow" protocol for embryo handling and dissection, extending the use of whole-embryo culture to functional analysis by performing in vivo pharmacological manipulations. This experimental accessibility allowed us to study the relevance of microtubules in axis elongation, using nocodazole and taxol drugs to interfere with microtubule networks, followed by length measurement analysis. Additionally, we demonstrated that embryo handling had no effect on the development of Tribolium embryos, and we checked viability after dissection and bisection and during incubation using propidium iodide. The embryo culture protocol we describe here can be applied to study diverse developmental processes in Tribolium. We expect that this protocol can be adapted and applied to other arthropods.

Cystein-serine-rich nuclear protein 1, Axud1/Csrnp1, is essential for cephalic neural progenitor proliferation and survival in zebrafish

The CSRNP (cystein-serine-rich nuclear protein) family has been conserved from Drosophila to human. Although knockout mice for each of the mammalian proteins have been generated, their function during vertebrate development has remained elusive. As an alternative to obtain insights on CSRNP's role in development, we have analysed the expression pattern and function of one member of this family, axud1, during zebrafish development. Our expression analysis indicates that axud1 is expressed from cleavage to larval stages in a dynamic pattern, becoming restricted after gastrulation to anterior regions of the developing neuraxis and later on concentrated predominantly in proliferating domains of the brain. Knockdown analysis using antisense morpholinos shows that reducing Axud1 levels impairs neural progenitor cell proliferation and survival, revealing an essential function of this gene for the growth of cephalic derivatives. The brain growth phenotypes elicited by decreasing Axud1 expression are specific and independent of anterior-posterior patterning events, initial establishment of neural progenitors, or neural differentiation occurring in this tissue. However, Axud1 is necessary for six3.1 expression and is positively regulated by sonic hedgehog. Phylogenetic examination shows that axud1 is likely to be the ortholog of the only member of this family present in Drosophila, as well as to the previously described mouse CSRNP1 and to human AXUD1 (Axin upregulated-1). Thus, we provide evidence as to the role of axud1 in brain growth in vertebrates.

Proneural gene requirement for hair cell differentiation in the zebrafish lateral line

The lateral line system comprises an array of mechanosensory organs, the neuromasts, distributed over the body surface. Each neuromast consists of a patch of mechanosensory hair cells surrounded by support cells. We show that, in the zebrafish, two proneural genes are essential for differentiation of the hair cells, neuroD (nrd) and atonal homolog 1 (ath1). Gene knockdown experiments demonstrate that loss of function of either gene, but not of the related proneural gene neurogenin1 (ngn1), abrogate the appearance of hair cell markers. This is in contrast to other sensory systems, such as the neurons of the lateral line ganglion, where nrd is regulated by ngn1 and not by ath1. Overexpression of ath1 can induce nrd, and the phenotype produced by loss of ath1 function can be partially rescued by injection of nrd mRNA. This supports the conclusion that the activation of nrd probably requires ath1 in the hair cell lineage, whereas in sensory neurons nrd activation requires ngn1. We propose that the emergence of two atonal homologs, ath1 and ngn1, allowed the cellular segregation of mechanoreception and signal transmission that were originally performed by a single cell type as found in insects.