Comparative transcriptomics in two extreme neopterans reveal general trends in the evolution of modern insects

The success of neopteran insects, with 1 million species described, is associated with developmental innovations like the holometaboly and the evolution from short to long germ-band embryogenesis. To unveil the mechanisms underlining these innovations, we compared gene expression during the ontogeny of two extreme neopterans, the cockroach Blattella germanica (polyneopteran, hemimetabolan and short germ-band species), and the fly Drosophila melanogaster (endopterygote, holometabolan and long germ-band species). Results revealed that genes associated with metamorphosis are predominantly expressed in late nymphal stages in B. germanica and in early-mid embryo in D. melanogaster. In B. germanica the maternal to zygotic transition (MZT) concentrates early in embryogenesis, when juvenile hormone factors are significantly expressed. In D. melanogaster, the MZT extends throughout embryogenesis, during which juvenile hormone factors appear unimportant. These differences possibly reflect broad trends in the evolution of development within neopterans, related to the germ-band type and the metamorphosis mode. For each chosen developmental stage of Blattella germanica (Non-fertilized eggs, Embryos day 0, 1, 2, 6, 13, Nymphs 1, 3, 5, 6 and Adult females), 2 pools of specimens were obtained, the mRNAs fraction was isolated by Poly-A and the cDNA sequenced.