Reproduction and plasticity within the reproductive polyphenism of an aphid, Acyrthosiphon pisum

Cyclical parthenogenesis is a key evolutionary novelty of the aphids and a striking example of phenotypic plasticity – a phenomena which is exemplified in the insects. Within cyclical parthenogenesis, aphids use both asexual and sexual reproduction, two fundamentally different approaches to producing offspring, with the asexual, viviparous mode being unusual among insects and animals. The primary aims of this thesis were to first, investigate how epigenetic factors may be involved in the asexual reproductive mode and potentially have contributed to its uptake as a strategy or underpin the ability to switch from it to sexual reproduction, and secondly, to improve our understanding more generally of the molecular underpinnings of cyclical parthenogenesis, primarily at the level of the ovaries of aphids that will produce asexual or sexual offspring, thus determining some of the potentially more proximate causes of effecting the switch and linking known and putative modulators of the switch to these putative proximate causes.
To this end, I explored the use of RNAi as a tool to investigate the functions of genes and discern their potential involvement in the reproductive switch, in order to causatively link suspected factors to it – however, in doing so I contributed to a growing body of evidence to suggest that RNAi susceptibility is highly variable, which I discussed, in chapter 3. The ovary-specific expression pattern and evolutionary history of two A. pisum dnmt3 paralogs were explored in chapter 4, revealing an Aphidomorpha specific duplication and expression in germ cells and early embryos, and additionally, a role of DNA methyltransferases in asexual reproduction. Genes relating to known and putative modulators of the reproductive switch were then assayed at the level of the ovary, revealing differential expression of several insulin and juvenile hormone related genes, and the two dnmt3 paralogs between ovaries containing embryonic aphids fated to produce asexual and sexual offspring, associating them with the reproductive switch, though few of them appeared directly responsive to artificial elevation of juvenile hormone signalling, which was able to redirect reproductive mode. Finally, a technique to target specific cell types was optimised and directed at the germ cells in order to characterise the expression profiles of these cells derived from aphids producing asexual and sexual offspring, the germ cells being a possible key place for the switch to be effected.