Imaging of Transgenic Cricket Embryos Reveals Cell Movements Consistent with a Syncytial Patterning Mechanism

Abstract

The mode of insect embryogenesis varies among species, reflecting adaptations to different life history strategies [1, 2]. In holometabolous insects, which include the model systems, such as the fruit fly and the red flour beetle, a large proportion of the blastoderm produces an embryo, whereas hemimetabolous embryos generally arise from a small region of the blastoderm [3]. Despite their importance in evolutionary studies, information of early developmental dynamics of hemimetabolous insects remains limited. Here, to clarify how maternal and gap gene products act in patterning the embryo of basal hemimetabolous insects, we analyzed the dynamic segmentation process in transgenic embryos of an intermediate-germ insect species, the cricket, Gryllus bimaculatus. Our data based on live imaging of fluorescently labeled embryonic cells and nuclei suggest that the positional specification of the cellular blastoderm may be established in the syncytium, where maternally derived gradients could act fundamentally in a way that is similar to that of Drosophila, namely throughout the egg. Then, the blastoderm cells move dynamically, retaining their positional information to form the posteriorly localized germ anlage. Furthermore, we find that the anterior head region of the cricket embryo is specified by orthodenticle in a cellular environment earlier than the gnathal and thoracic regions. Our findings imply that the syncytial mode of the early segmentation in long-germ insects evolved from a dynamic syncytial-to-cellular mode found in the present study, accompanied by a heterochronic shift of gap gene action.

Targeted gene delivery in the cricket brain, using in vivo electroporation

“…The Gb'-act promoter is an 897 base pair (bp) genomic DNA fragment that is found upstream of the putative start codon of the Gb′-act gene (accession number: AB087882.1). Its strong activity in G. bimaculatus embryonic cells has been reported previously (Zhang et al, 2002;Shinmyo et al, 2004;Nakamura et al, 2010). A Gb′-act promoter-eGFP fragment was subcloned into a vector derived from a C. elegans plasmid, pttx-3::GFP (pPD95.75 vector; Tsalik and Hober, 2003), using the XbaI and EcoRI sites.…”

“…There have been several successful attempts to introduce exogenous genes and other constructs into crickets (Zhang et al, 2002;Shinmyo et al, 2004;Nakamura et al, 2010;Mito et al, 2010;Watanabe et al, 2012). In these studies, which were performed to clarify development and regeneration mechanisms, eggs or nymphs were the main targets of gene transfer.…”

TGF-β signaling in insects regulates metamorphosis via juvenile hormone biosynthesis

“…However, the molecular mechanisms underlying regulation of the temporal expression profile of jhamt, a long-standing area of research in entomology (10), remain unknown. To elucidate the mechanisms underlying the regulation of JH titer, the cricket Gryllus bimaculatus (15,16) was used as a model system of hemimetabolous ancestors that evolved into holometabolous insects (2,17). In the present study,…”

Changing cell behaviours during beetle embryogenesis correlates with slowing of segmentation

“…As the name implies, elongation in the growth zone has been traditionally assumed to be due to high rates of cell division in the posterior [25][26][27][28] . However, other cellular behaviours-cell migration, cell rearrangements, cell shape change-could be used singly or in combination [29][30][31][32][33][34] . Actual mechanisms of elongation remain poorly described for most arthropods (with the exception of malacostracan crustaceans, which elongate using posterior stem cells 35,36 and the centipede Strigamia maritima, which recruits existing tissue to the elongating germ band 37 ).…”