Abstract
In insect species that undergo long germ segmentation, such as Drosophila, all segments are specified simultaneously at the early blastoderm stage. As embryogenesis progresses, the expression boundaries of Hox genes are established by repression of gap genes, which is subsequently replaced by Polycomb group (PcG) silencing. At present, however, it is not known whether patterning occurs this way in a more ancestral (short germ) mode of embryogenesis, where segments are added gradually during posterior elongation. In this study, two members of the PcG family, Enhancer of zeste (E(z)) and Suppressor of zeste 12 (Su(z)12), were analyzed in the short germ cricket, Gryllus bimaculatus. Results suggest that although stepwise negative regulation by gap and PcG genes is present in anterior members of the Hox cluster, it does not account for regulation of two posterior Hox genes, abdominal-A (abd-A) and Abdominal-B (Abd-B). Instead, abd-A and Abd-B are predominantly regulated by PcG genes, which is the mode present in vertebrates. These findings suggest that an intriguing transition of the PcG-mediated silencing of Hox genes may have occurred during animal evolution. The ancestral bilaterian state may have resembled the current vertebrate mode of regulation, where PcG-mediated silencing of Hox genes occurs before their expression is initiated and is responsible for the establishment of individual expression domains. Then, during insect evolution, the repression by transcription factors may have been acquired in anterior Hox genes of short germ insects, while PcG silencing was maintained in posterior Hox genes.
Highlights
Though all insects possess highly conserved adult body plans, there are two different ways developmental patterning can be accomplished
Su(z)12RNAi embryos exhibited a similar but less severe phenotype. This observation was consistent with real-time quantitative PCR (RT-qPCR) results showing the lesser reduction of Su(z)12 mRNA levels in Su(z)12RNAi embryos compared to E(z) mRNA levels in E(z)RNA interference (RNAi) embryos
Functional conservation of the PRC2 complex in Gryllus In the present study, Gryllus embryos treated with RNAi directed against E(z) displayed reduced H3K27me3 signals during embryogenesis (Fig. 2), suggesting that the E(z) protein plays an important role in histone methylation activity of the PRC2 complex
Summary
Though all insects possess highly conserved adult body plans, there are two different ways developmental patterning can be accomplished. On the other hand, the middle Hox genes, Sex combs reduced (Scr), Antennapedia (Antp), and Ultrabithorax (Ubx), are expressed in the anterior regions preceding initiation of posterior elongation. This is followed by expression of their posterior counterparts, abdominal-A (abd-A) and Abdominal-B (Abd-B), which are associated with the formation of posterior segments (Tear et al, 1990; Kelsh et al, 1993; Shippy et al, 1998; Peterson et al, 1998; Zhang et al, 2005). These observed differences in the temporal activation of posterior Hox genes might reflect different regulatory mechanisms of Hox genes
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