Molecular characterization of pea aphid facultative parthenogenesis

Meiosis is a highly conserved process in which a diploid genome is recombined and assorted into haploid gametes. Remarkably, the pea aphid Acyrthosiphon pisum exhibits a reproductive polyphenism whereby environmental signals trigger a switch between apomixis (parthenogenetic reproduction) and meiosis (sexual reproduction). Aphid apomixis results in daughter embryo clones with 2n genome content without male contribution or recombination. This important adaptation allows aphid populations to not only rapidly expand upon abundant resources during summer but also survive winter. How aphids have evolved this ability to switch between parthenogenesis and sexual meiosis is unknown. To arrive at a mechanistic explanation for this developmental plasticity, I determined meiosis gene activity in sexuals and asexuals. I first identified homologs of a core set of meiosis genes from the pea aphid genome. Next, I tested the expression of these core meiosis genes by PCR spanning across at least one intron from cDNA isolated from asexual and sexual ovaries. Surprisingly, meiosis specific genes (e.g., Spo11, Msh4, Msh5, Hop2 and Mnd1) are expressed in asexual ovaries. Additionally, the Spo11 PCR product contained intronic sequence, thus representing unspliced mRNA. Future experiments looking at the quantities and localizations of mRNA and protein will help to distinguish among several possible explanations for these results. Further molecular characterization of this phenotypic plasticity will be helpful in understanding how multiple interacting pathways can evolve to create alternate developmental phenotypes.