Abstracti mb_944 47..62Aphids exhibit unique attributes, such as polyphenisms and specialized cells to house endosymbionts, that make them an interesting system for studies at the interface of ecology, evolution and development. Here we present a comprehensive characterization of the developmental genes in the pea aphid, Acyrthosiphon pisum, and compare our results to other sequenced insects. We investigated genes involved in fundamental developmental processes such as establishment of the body plan and organogenesis, focusing on transcription factors and components of signalling pathways. We found that most developmental genes were well conserved in the pea aphid, although many lineage-specific gene duplications and gene losses have occurred in several gene families. In particular, genetic components of transforming growth factor beta (TGFb) Wnt, JAK/STAT (Janus kinase/signal transducer and activator of transcription) and EGF (Epidermal Growth Factor) pathways appear to have been significantly modified in the pea aphid.

Background The Hemiptera (aphids, cicadas, and true bugs) are a key insect order, with high diversity for feeding ecology and excellent experimental tractability for molecular genetics. Building upon recent sequencing of hemipteran pests such as phloem-feeding aphids and blood-feeding bed bugs, we present the genome sequence and comparative analyses centered on the milkweed bug Oncopeltus fasciatus , a seed feeder of the family Lygaeidae. Results The 926-Mb Oncopeltus genome is well represented by the current assembly and official gene set. We use our genomic and RNA-seq data not only to characterize the protein-coding gene repertoire and perform isoform-specific RNAi, but also to elucidate patterns of molecular evolution and physiology. We find ongoing, lineage-specific expansion and diversification of repressive C2H2 zinc finger proteins. The discovery of intron gain and turnover specific to the Hemiptera also prompted the evaluation of lineage and genome size as predictors of gene structure evolution. Furthermore, we identify enzymatic gains and losses that correlate with feeding biology, particularly for reductions associated with derived, fluid nutrition feeding. Conclusions With the milkweed bug, we now have a critical mass of sequenced species for a hemimetabolous insect order and close outgroup to the Holometabola, substantially improving the diversity of insect genomics. We thereby define commonalities among the Hemiptera and delve into how hemipteran genomes reflect distinct feeding ecologies. Given Oncopeltus ’s strength as an experimental model, these new sequence resources bolster the foundation for molecular research and highlight technical considerations for the analysis of medium-sized invertebrate genomes. Electronic supplementary material The online version of this article (10.1186/s13059-019-1660-0) contains supplementary material, which is available to authorized users.

The germarium, oocytes and embryos of the parthenogenetic viviparous pea aphid Acyrthosiphon pisum are contained within a single ovariole. This species provides an excellent model for studying how maternally-inherited germ plasm is specified and how it is transferred to primordial germ cells. Previous studies have shown that germ cells are first segregated at the embryonic posterior after formation of the blastoderm. We used two cross-reacting antibodies against the conserved germline markers Vasa and Nanos, which specifically identified these presumptive germ cells, to investigate whether germ cells were determined during early development. We observed randomly-distributed weak expression of Vasa signals in the developing oocyte but no localization in the oocyte segregated from the germarium. Localized Vasa was not apparent until it was detected at the posterior in the embryo undergoing the second nuclear division. Nanos, on the other hand, was localized to a nuage-like structure surrounding the nucleus in the developing and segregated oocytes. At the beginning of the oocyte maturation division, Nanos localization shifted to the posterior and could be identified in successive stages until it was incorporated into the germ cells. Taken together, our results suggest that germ plasm is specified in the developing oocyte and that Nanos is an earlier germline marker than Vasa. Presumptive germ cells stained for Vasa remained at a dorsal location in the egg during middevelopment and then were guided into abdominal segments A1 to A6 during germ-band retraction. We infer that germ cells coalesce with segmented gonadal mesoderm during this period.

Vasa is a widely conserved germline marker, both in vertebrates and invertebrates. We identify a vasa orthologue, Sgvasa, and use it to study germline development in the grasshopper Schistocerca gregaria, a species in which no germ plasm has been identified. In adults, Sgvasa is specifically expressed in the ovary and testis. It is expressed at high levels during early oogenesis, but no detectable vasa RNA and little Vasa protein are present in mature unlaid eggs. None appears to be localized to any defined region of the egg cortex, suggesting that germline specification may not depend on maternal germ plasm expressing vasa. Vasa protein is expressed in most cleavage energids as they reach the egg surface and persists at high levels in most cells aggregating to form the embryonic primordium. However, after gastrulation, Vasa protein persists only in extraembryonic membranes and in cells at the outer margin of the late heart-stage embryo. In the embryo, it then become restricted to cells at the dorsal margin of the forming abdomen. In older embryos, these Vasa-positive cells move toward the midline; Vasa protein accumulates asymmetrically in their cytoplasm, a pattern closely resembling that of germ cells in late embryonic gonads. Thus, we suggest that the Vasa-stained cells in the abdominal margin are germ cells, as proposed by Nelson (1934), and not cardioblasts, as has been proposed by others.

In the parthenogenetic and viviparous pea aphid Acyrthosiphon pisum, germline specification depends on the germ plasm localized to the posterior region of the egg chamber before the formation of the blastoderm. During blastulation, germline segregation occurs at the egg posterior, and in early gastrulation germ cells are pushed inward by the invaginating germ band. Previous studies suggest that germ cells remain dorsal in the embryo in subsequent developmental stages. In fact, though, it is not known whether germ cells remain in place or migrate dynamically during katatrepsis and germ-band retraction. We cloned Apvasa, a pea aphid homologue of Drosophila vasa, and used it as a germline marker to monitor the migration of germ cells. Apvasa messenger RNA (mRNA) was first restricted to morphologically identifiable germ cells after blastoderm formation but that expression soon faded. Apvasa transcripts were again identified in germ cells from the stage when the endosymbiotic bacteria invaded the embryo, and after that, Apvasa mRNA was present in germ cells throughout all developmental stages. At the beginning of katatrepsis, germ cells were detected at the anteriormost region of the egg chamber as they were migrating into the body cavity. During the early period of germ-band retraction, germ cells were separated into several groups surrounded by a layer of somatic cells devoid of Apvasa staining, suggesting that the coalescence between migrating germ cells and the somatic gonadal mesoderm occurs between late katatrepsis and early germ-band retraction.