Occurrence of genetic bottlenecks during citrus tristeza virus acquisition by Toxoptera citricida under field conditions

In this study, we address the involvement of T. citricida in strain segregation and genetic bottleneck events by comparing the nucleotide diversity of CTV coat protein (CP) gene variants present in field-grown trees with that of variants retrieved from single apterous aphids. Plant material and aphids were collected in orange orchards in the northern part of Portugal. Shoots from two trees that were found to be positive using ELISA and twenty-four apterous aphids from these same trees were selected for individual molecular assays. CTV was detected in 60% of the aphids by amplification of a 417-bp fragment of the CP gene. Analysis of molecular variance (AMOVA) of this fragment revealed that most of the variation of the virus was found among individual aphids (FSC: 0.766) within each location. Nucleotide diversity comparison between the pool of sequences obtained from a given shoot and sequences obtained from individual aphids present on that shoot showed a reduction of more than one order of magnitude in most cases. Computer simulations of random virus acquisition by single aphids showed that in 54% of the cases only a single CP gene phylogenetic group was acquired. However, a small number of aphids (e.g. 6) was enough to acquire the full complement of phylogenetic groups present.