A sub-set of the Hox genes in the pea aphid, Acyrthosiphon pisum

Abstract

How developmental pathways evolve to produce different morphologies is an exciting area of study. Most knowledge of invertebrate development is from D. melanogaster, or other holometabolous insects. However, the more ancestral hemimetabolous insects are a useful model for learning more about the evolution of development in insects. By comparing hemimetabolous insects’ developmental mechanisms to that of the holometabolous insects, this data can be used to resolve which insect developmental pathways are conserved, and which are derived. The pea aphid (a hemimetabolous insect), can reproduce either sexually (which is oviparous) or asexually (which is viviparous). This is a polyphenism; reproduction is usually asexual, but sexual reproduction is induced by cooler temperatures and shorter daylengths. These two developmental trajectories differ in many ways, including the length of time it takes, the structure of the ovary, and molecular mechanisms underpinning early development. And yet, the pea aphid nymphs produced by these two pathways are morphologically identical. The pea aphid therefore provides a unique opportunity to examine the expression and function of developmental genes in different modes of development, in the same organism.

Hox genes are highly conserved homeobox-containing transcription factors. Their usual role is in specifying the morphology along the anterior to posterior axis, however several Hox genes are ‘fast-evolving’ and have gained different roles in insect embryogenesis. The pea aphid also has an aphid-specific Hox gene, HoxR. We hypothesised the Hox genes were likely to play a role in the buffering or convergence of two developmental pathways, to allow the production of morphologically identical offspring. The aim of this study was to examine the expression and function of the pea aphid, and in particular, to see if there were any differences in the expression of these genes between sexual and asexual embryogenesis.

HoxR is shown to be a true Hox protein, and to likely be a duplication of the posterior Hox gene, Abd-B. Attempts at expression analyses were not successful, and further work is required to gather this data.

The expression of two Hox genes, Ultrabithorax and zerknüllt were investigated, and compared between sexual and asexual ovaries and embryos. These genes showed some conserved features of expression, but also several novel and unusual expression domains, which suggests a novel role for these genes in pea aphid embryogenesis.

Functional studies are needed to elucidate the roles of these genes and to test whether they have a role in buffering or convergence of the developmental pathways in the pea aphid. Steps were taken towards performing RNAi in the pea aphid sexual egg. There are no technical hurdles in terms of injection, as this study shows the eggs remain syncytial for an extended period of time. However, the rate of egg laying is very low, and to produce reproducible phenotypes by RNAi, hundreds of eggs are needed. Future work on stimulating egg laying is needed to be able to carry out RNAi in the pea aphid egg.

Together, the results from the project support the hypothesis that the fast-evolving Hox genes may have a role in buffering early differences between sexual and asexual development. This study provides a framework for further expression and functional studies in the pea aphid to investigate this hypothesis.