DNA methylation machinery is involved in development and reproduction in the viviparous pea aphid (Acyrthosiphon pisum).

Epigenetic mechanisms, such as DNA methylation, have been proposed to mediate plastic responses in insects. The pea aphid (Acyrthosiphon pisum), like the majority of extant aphids, displays cyclical parthenogenesis - the ability of mothers to switch the reproductive mode of their offspring from reproducing parthenogenetically to sexually in response to environmental cues. The pea aphid genome encodes two paralogs of the de novo DNA methyltransferase gene, dnmt3a and dnmt3x. Here we show, using phylogenetic analysis, that this gene duplication event occurred at least 150 million years ago, likely after the divergence of the lineage leading to the Aphidomorpha (phylloxerans, adelgids and true aphids) from that leading to the scale insects (Coccomorpha) and that the two paralogs are maintained in the genomes of all aphids examined. We also show that the mRNA of both dnmt3 paralogs is maternally expressed in the viviparous aphid ovary. During development both paralogs are expressed in the germ cells of embryos beginning at stage 5 and persisting throughout development. Treatment with 5-azactyidine, a chemical that generally inhibits the DNA methylation machinery, leads to defects of oocytes and early-stage embryos and causes a proportion of later stage embryos to be born dead or die soon after birth. These phenotypes suggest a role for DNA methyltransferases in reproduction, consistent with that seen in other insects. Taking the vast evolutionary history of the dnmt3 paralogs, and the localisation of their mRNAs in the ovary, we suggest there is a role for dnmt3a and/or dnmt3x in early development, and a role for DNA methylation machinery in reproduction and development of the viviparous pea aphid.

One genome, multiple phenotypes: decoding the evolution and mechanisms of environmentally induced developmental plasticity in insects.

Plasticity in developmental processes gives rise to remarkable environmentally induced phenotypes. Some of the most striking and well-studied examples of developmental plasticity are seen in insects. For example, beetle horn size responds to nutritional state, butterfly eyespots are enlarged in response to temperature and humidity, and environmental cues also give rise to the queen and worker castes of eusocial insects. These phenotypes arise from essentially identical genomes in response to an environmental cue during development. Developmental plasticity is taxonomically widespread, affects individual fitness, and may act as a rapid-response mechanism allowing individuals to adapt to changing environments. Despite the importance and prevalence of developmental plasticity, there remains scant mechanistic understanding of how it works or evolves. In this review, we use key examples to discuss what is known about developmental plasticity in insects and identify fundamental gaps in the current knowledge. We highlight the importance of working towards a fully integrated understanding of developmental plasticity in a diverse range of species. Furthermore, we advocate for the use of comparative studies in an evo-devo framework to address how developmental plasticity works and how it evolves.