Biology subjects

Tagu, D.

Publications and source records attributed to Tagu, D..

4 recordsLinked to original sources

bioRxiv2022

A quarter-million-year-old polymorphism drives reproductive mode variation in the pea aphid

Although asexual linages evolved from sexual lineages in many different taxa, the genetics of sex loss remains poorly understood. We addressed this issue in the pea aphid Acyrthosiphon pisum, whose natural populations encompass lineages performing cyclical parthenogenesis (CP) and producing one sexual generation per year, as well as obligate parthenogenetic (OP) lineages that can no longer produce sexual females but can still produce males. A SNP-based, whole-genome scan of CP and OP populations sequenced in pools (103 individuals from six populations) showed that a single X-linked region controls the variation in reproductive mode. This 840-kb region is highly divergent between CP and OP populations (FST = 34.9%), with >2000 SNPs or short Indels showing a high degree of association with the phenotypic trait. Comparison of de novo genome assemblies built from long reads did not reveal large structural rearrangements between CP and OP lineages within the candidate region. This reproductive polymorphism still appears relatively ancient, as we estimated its age at ~0.25 million years from the divergence between cp and op alleles. The low genetic differentiation between CP and OP populations at the rest of the genome (FST = 2.4%) suggests gene flow between them. Males from OP lineages thus likely transmit their op allele to new genomic backgrounds. This "contagious asexuality", combined with environment-induced selection (each reproductive mode being favored under different climates) probably contributes to the long-term persistence of the cp and op alleles. SignificanceAsexual taxa occur in all major clades of Eukaryotes and derive from related sexual species. Yet, the genetic basis for these transitions is poorly known because crosses cannot generally be performed to genetically map the ability to propagate asexually. As a result, only one gene responsible for sex loss has been identified in one animal species. Here, using pooled genome sequencing, we identified an 840kb region (carrying 32 genes) that controls the transition to permanent asexuality in the pea aphid. We also revealed that sexual and asexual alleles diverged 0.25 million years ago and that asexual lineages likely persist through contagious asexuality, providing new insights into the mechanisms of coexistence of sexual and asexual lineages.

evolutionary biology↗

bioRxiv2019

Conserved and specific genomic features of endogenous polydnaviruses revealed by whole genome sequencing of two ichneumonid wasps

Polydnaviruses (PDVs) are mutualistic endogenous viruses associated with some lineages of parasitoid wasps that allow successful development of the wasps within their hosts. PDVs include two taxa resulting from independent virus acquisitions in braconid (bracoviruses) and ichneumonid wasps (ichnoviruses). PDV genomes are fully incorporated into the wasp genomes and comprise (1) virulence genes located on proviral segments that are packaged into the viral particle, and (2) genes involved in the production of the viral particles, which are not encapsidated. Whereas the genomic organization of bracoviruses within the wasp genome is relatively well known, the architecture of endogenous ichnoviruses remains poorly understood. We sequenced the genome of two ichnovirus-carrying wasp species, Hyposoter didymator and Campoletis sonorensis. Complete assemblies with long scaffold sizes allowed identification of the integrated ichnovirus, highlighting an extreme dispersion within the wasp genomes of the viral loci, i.e. isolated proviral segments and clusters of replication genes. Comparing the two wasp species, proviral segments harbor distinct gene content and variable genomic environment, whereas viral machinery clusters show conserved gene content and order, and can be inserted in collinear wasp genomic regions. This distinct architecture is consistent with the biological properties of the two viral elements: proviral segments producing virulence proteins allowing parasitism success are fine-tuned to the host physiology, while an ancestral viral architecture was likely maintained for the genes involved in virus particle production. Finding a distinct genomic architecture of ichnoviruses and bracoviruses highlights different evolutionary trajectories leading to virus domestication in the two wasp lineages.

genomics↗

bioRxiv2019

Functional insights from the GC-poor genomes of two aphid parasitoids, Aphidius ervi and Lysiphlebus fabarum

BackgroundParasitoid 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 biocontrol, and infecting aphids requires overcoming both aphid defenses and their defensive endosymbionts. ResultsWe 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), highly syntenic, 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 age-specific energy demands. We identify expansions of F-box/Leucine-rich-repeat proteins, suggesting that diversification in this gene family may be associated with their broad host range or with countering defenses from aphids endosymbionts. The absence of some immune genes (Toll and Imd pathways) resembles similar losses in their aphid hosts, highlighting the potential impact of symbiosis on both aphids and their parasitoids. ConclusionsThese findings are of fundamental interest for insect evolution and beyond. This will provide a strong foundation for further functional studies including coevolution with respect to their hosts, the basis of successful infection, and biocontrol. Both genomes are available at https://bipaa.genouest.org.

genomics↗

bioRxiv2019

Selection following gene duplication shapes recent genome evolution in the pea aphid Acyrthosiphon pisum

Ecology of insects is as wide as their diversity, which reflects their high capacity of adaptation in most of the environments of our planet. Aphids, with over 4,000 species, have developed a series of adaptations including a high phenotypic plasticity and the ability to feed on the phloem-sap of plants, which is enriched in sugars derived from photosynthesis. Recent analyses of aphid genomes have indicated a high level of shared ancestral gene duplications that might represent a basis for genetic innovation and broad adaptations. In addition, there is a large number of recent, species-specific gene duplications whose role in adaptation remains poorly understood. Here, we tested whether duplicates specific to the pea aphid Acyrthosiphon pisum are related to genomic innovation by combining comparative genomics, transcriptomics, and chromatin accessibility analyses. Consistent with large levels of neofunctionalization, we found that most of the recent pairs of gene duplicates evolved asymmetrically, showing divergent patterns of positive selection and gene expression. Genes under selection involved a plethora of biological functions, suggesting that neofunctionalization and tissue specificity, among other evolutionary mechanisms, have orchestrated the evolution of recent paralogs in the pea aphid and may have facilitated host-symbiont cooperation. Our comprehensive phylogenomics analysis allowed to tackle the history of duplicated genes to pave the road towards understanding the role of gene duplication in ecological adaptation.

genomics↗