Jekyll or Hyde? The genome (and more) of Nesidiocoris tenuis, a zoophytophagous predatory bug that is both a biological control agent and a pest.

Nesidiocoris tenuis (Reuter) is an efficient predatory biological control agent used throughout the Mediterranean Basin in tomato crops but regarded as a pest in northern European countries. From the family Miridae, it is an economically important insect yet very little is known in terms of genetic information and no genomic or transcriptomic studies have been published. Here, we use a linked-read sequencing strategy on a single female N. tenuis. From this, we assembled the 355 Mbp genome and delivered an ab initio, homology-based and evidence-based annotation. Along the way, the bacterial "contamination" was removed from the assembly. In addition, bacterial lateral gene transfer (LGT) candidates were detected in the N. tenuis genome. The complete gene set is composed of 24 688 genes; the associated proteins were compared to other hemipterans (Cimex lectularis, Halyomorpha halys and Acyrthosiphon pisum). We visualized the genome using various cytogenetic techniques, such as karyotyping, CGH and GISH, indicating a karyotype of 2n = 32. Additional analyses include the localization of 18S rDNA and unique satellite probes as well as pooled sequencing to assess nucleotide diversity and neutrality of the commercial population. This is one of the first mirid genomes to be released and the first of a mirid biological control agent.

Maternal provision of non-sex-specific transformer messenger RNA in sex determination of the wasp Asobara tabida.

In many insect species maternal provision of sex-specifically spliced messenger RNA (mRNA) of sex determination genes is an essential component of the sex determination mechanism. In haplodiploid Hymenoptera, maternal provision in combination with genomic imprinting has been shown for the parasitoid Nasonia vitripennis, known as maternal effect genomic imprinting sex determination (MEGISD). Here, we characterize the sex determination cascade of Asobara tabida, another hymenopteran parasitoid. We show the presence of the conserved sex determination genes doublesex (dsx), transformer (tra) and transformer-2 (tra2) orthologues in As. tabida. Of these, At-dsx and At-tra are sex-specifically spliced, indicating a conserved function in sex determination. At-tra and At-tra2 mRNA is maternally provided to embryos but, in contrast to most studied insects, As. tabida females transmit a non-sex-specific splice form of At-tra mRNA to the eggs. In this respect, As. tabida sex determination differs from the MEGISD mechanism. How the paternal genome can induce female development in the absence of maternal provision of sex-specifically spliced mRNA remains an open question. Our study reports a hitherto unknown variant of maternal effect sex determination and accentuates the diversity of insect sex determination mechanisms.

Genomic imprinting and maternal effect genes in haplodiploid sex determination.

The research into the Drosophila melanogaster sex-determining system has been at the basis of all further research on insect sex determination. This further research has made it clear that, for most insect species, the presence of sufficient functional Transformer (TRA) protein in the early embryonic stage is essential for female sexual development. In Hymenoptera, functional analysis of sex determination by knockdown studies of sex-determining genes has only been performed for 2 species. The first is the social insect species Apis mellifera, the honeybee, which has single-locus complementary sex determination (CSD). The other species is the parasitoid Nasonia vitripennis, the jewel wasp. Nasonia has a non-CSD sex-determining system, described as the maternal effect genomic imprinting sex determination system (MEGISD). Here, we describe the arguments that eventually led to the formulation of MEGISD and the experimental data that supported and refined this model. We evaluate the possibility that DNA methylation lies at the basis of MEGISD and briefly address the role of genomic imprinting in non-CSD sex determination in other Hymenoptera.

DNA methylation plays a crucial role during early Nasonia development.

Although the role of DNA methylation in insect development is still poorly understood, the number and role of DNA methyltransferases in insects vary strongly between species. DNA methylation appears to be widely present among the social hymenoptera and functional studies in Apis have suggested a crucial role for de novo methylation in a wide variety of developmental processes. The sequencing of three parasitoid Nasonia genomes revealed the presence of three Dnmt1 (Dnmt1a, Dnmt1b and Dnmt1c) genes and one Dnmt2 and Dnmt3 gene, suggesting a role of DNA methylation in Nasonia development. In the present study we show that in Nasonia vitripennis all Dnmt1 messenger RNAs (mRNAs) and Dnmt3 mRNA are maternally provided to the embryo and, of these, Dnmt1a is essential during early embryogenesis. Lowering of maternal Dnmt1a mRNA results in embryonic lethality during the onset of gastrulation. This dependence on maternal Dnmt1a during embryogenesis in an organismal group outside the vertebrates, suggests evolutionary conservation of the function of Dnmt1 during embryogenesis.

Identification and characterization of the doublesex gene of Nasonia.

The doublesex (dsx) gene of the parasitic wasp Nasonia vitripennis is described and characterized. Differential splicing of dsx transcripts has been shown to induce somatic sexual differentiation in Diptera and Lepidoptera, but not yet in other insect orders. Two spliceforms of Nasonia dsx mRNA are differentially expressed in males and females. In addition, in a gynandromorphic line that produces haploids (normally males) with full female phenotypes, these individuals show the female spliceform, providing the first demonstration of a direct association of dsx with somatic sex differentiation in Hymenoptera. Finally, the DNA binding (DM) domain of Nasonia dsx clusters phylogenetically with dsx from other insects, and Nasonia dsx shows microsynteny with dsx of Apis, further supporting identification of the dsx orthologue in Nasonia.