Alice B. Dennis, Gabriel I. Ballesteros, Stéphanie Robin, Lukas Schrader, Jens Bast, Jan Berghöfer, Leo W. Beukeboom, Maya Belghazi, Anthony Bretaudeau, Jan Buellesbach, Elizabeth Cash, Dominique Colinet, Zoé Dumas, Mohammed Errbii, Patrizia Falabella, Jean-Luc Gatti, Elzemiek Geuverink, Joshua D. Gibson, Corinne Hertaeg, Stefanie Hartmann, Emmanuelle Jacquin-Joly, Mark Lammers, Blas I. Lavandero, Ina Lindenbaum, Lauriane Massardier-Galata, Camille Meslin, Nicolas Montagné, Nina Pak, Marylène Poirié, Rosanna Salvia, Chris R. Smith, Denis Tagu, Sophie Tares, Heiko Vogel, Tanja Schwander, Jean-Christophe Simon, Christian C. Figueroa, Christoph Vorburger, Fabrice Legeai, Jürgen Gadau

• Background Parasitoid wasps have fascinating life cycles and play an important role in trophic networks, yet little is known about their genome content and function. Parasitoids that infect aphids are an important group with the potential for biological control. Their success depends on adapting to develop inside aphids and overcoming both host aphid defenses and their protective endosymbionts. Results We present the de novo genome assemblies, detailed annotation, and comparative analysis of two closely related parasitoid wasps that target pest aphids: Aphidius ervi and Lysiphlebus fabarum (Hymenoptera: Braconidae: Aphidiinae). The genomes are small (139 and 141 Mbp) and the most AT-rich reported thus far for any arthropod (GC content: 25.8 and 23.8%). This nucleotide bias is accompanied by skewed codon usage and is stronger in genes with adult-biased expression. AT-richness may be the consequence of reduced genome size, a near absence of DNA methylation, and energy efficiency. We identify missing desaturase genes, whoseBackground Parasitoid wasps have fascinating life cycles and play an important role in trophic networks, yet little is known about their genome content and function. Parasitoids that infect aphids are an important group with the potential for biological control. Their success depends on adapting to develop inside aphids and overcoming both host aphid defenses and their protective endosymbionts. Results We present the de novo genome assemblies, detailed annotation, and comparative analysis of two closely related parasitoid wasps that target pest aphids: Aphidius ervi and Lysiphlebus fabarum (Hymenoptera: Braconidae: Aphidiinae). The genomes are small (139 and 141 Mbp) and the most AT-rich reported thus far for any arthropod (GC content: 25.8 and 23.8%). This nucleotide bias is accompanied by skewed codon usage and is stronger in genes with adult-biased expression. AT-richness may be the consequence of reduced genome size, a near absence of DNA methylation, and energy efficiency. We identify missing desaturase genes, whose absence may underlie mimicry in the cuticular hydrocarbon profile of L. fabarum. We highlight key gene groups including those underlying venom composition, chemosensory perception, and sex determination, as well as potential losses in immune pathway genes. Conclusions These findings are of fundamental interest for insect evolution and biological control applications. They provide a strong foundation for further functional studies into coevolution between parasitoids and their hosts. Both genomes are available at https://bipaa.genouest.org.…