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.

An elongated and segmented body plan is a common morphological characteristic of all arthropods and is probably responsible for their high adaptation ability to diverse environments. Most arthropods form their bodies by progressively adding segments, resembling vertebrate somitogenesis. This sequential segmentation relies on a molecular clock that operates in the posterior region of the elongating embryo that combines dynamically with cellular behaviors and tissue rearrangements, allowing the extension of the developing body along its main embryonic axis. Even though the molecular mechanisms involved in elongation and segment formation have been found to be conserved in a considerable degree, cellular processes such as cell division are quite variable between different arthropods. In this study, we show that cell proliferation in the beetle Tribolium castaneum has a nonuniform spatiotemporal patterning during axial elongation. We found that dividing cells are preferentially oriented along the anterior–posterior axis, more abundant and posteriorly localized during thoracic segments formation and that this cell proliferation peak was triggered at the onset of axis elongation. This raise in cell divisions, in turn, was correlated with an increase in the elongation rate, but not with changes in cell density. When DNA synthesis was inhibited over this period, both the area and length of thoracic segments were significantly reduced but not of the first abdominal segment. We discuss the variable participation that different cell division patterns and cell movements may have on arthropod posterior growth and their evolutionary contribution.

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.