title: Adenine methylation may contribute to endosymbiont selection in a clonal aphid population.
abstract: BACKGROUND
  The pea aphid Acyrthosiphon pisum has two modes of reproduction: parthenogenetic during the spring and summer and sexual in autumn. This ability to alternate between reproductive modes and the emergence of clonal populations under favorable conditions make this organism an interesting model for genetic and epigenetic studies. The pea aphid hosts different types of endosymbiotic bacteria within bacteriocytes which help the aphids survive and adapt to new environmental conditions and habitats. The obligate endosymbiont Buchnera aphidicola has a drastically reduced and stable genome, whereas facultative endosymbionts such as Regiella insecticola have large and dynamic genomes due to phages, mobile elements and high levels of genetic recombination. In previous work, selection toward cold adaptation resulted in the appearance of parthenogenetic A. pisum individuals characterized by heavier weights and remarkable green pigmentation.
  
  
  RESULTS
  Six adenine-methylated DNA fragments were isolated from genomic DNA (gDNA) extracted from the cold-induced green variant of A. pisum using deoxyadenosine methylase (Dam) by digesting the gDNA with the restriction enzymes DpnI and DpnII, which recognize the methylated and unmethylated GATC sites, respectively. The six resultant fragments did not match any sequence in the A. pisum or Buchnera genomes, implying that they came from facultative endosymbionts. The A1 fragment encoding a putative transposase and the A6 fragment encoding a putative helicase were selected for further comparison between the two A. pisum variants (green and orange) based on Dam analysis followed by PCR amplification. An association between adenine methylation and the two A. pisum variants was demonstrated by higher adenine methylation levels on both genes in the green variant as compared to the orange one.
  
  
  CONCLUSIONS
  Temperature selection may affect the secondary endosymbiont and the sensitive Dam involved in the survival and adaptation of aphids to cold temperatures. There is a high degree of adenine methylation at the GATC sites of the endosymbiont genes at 8°C, an effect that disappears at 22°C. We suggest that endosymbionts can be modified or selected to increase host fitness under unfavorable climatic conditions, and that the phenotype of the newly adapted aphids can be inherited.
authors.forename: Victoria
authors.initials: V
authors.lastname: Reingold
authors.forename: Neta
authors.initials: N
authors.lastname: Luria
authors.forename: Alain
authors.initials: A
authors.lastname: Robichon
authors.affiliation: INRA/CNRS/UNSA University Nice Sophia Antipolis, 400 routes de Chappes, BP 167, Sophia Antipolis 06903, France. aviv@agri.gov.il.
authors.forename: Aviv
authors.initials: A
authors.lastname: Dombrovsky
mesh_terms: D000225:Adenine / Q000378:metabolism*; D000818:Animals; D001042:Aphids / Q000235:genetics*; D001483:Base Sequence; D002999:Clone Cells; D004265:DNA Helicases / Q000235:genetics; D019175:DNA Methylation / Q000235:genetics*; D015321:Gene Rearrangement; D049750:Genome, Insect; D010858:Pigmentation / Q000235:genetics; D016133:Polymerase Chain Reaction; D015203:Reproducibility of Results; D012641:Selection, Genetic; D013559:Symbiosis / Q000235:genetics*; D013696:Temperature; D019895:Transposases / Q000235:genetics
chemical_list: D019895:Transposases; D004265:DNA Helicases; D000225:Adenine