Virus transmission as an evolutionary adaptation of aphids

Myzus persicae is an efficient vector of Potato leafroll virus (PLRV); however, PLRV transmission efficiency is significantly reduced when aphids are reared on turnip as compared to Physalis floridana. Turnip-reared aphids are also larger and have a fitness advantage. To glean insight into the molecular differences between aphids reared on turnip or P. floridana, we identified differentially expressed proteins by  2-D Difference Gel Electrophoresis (DIGE) coupled with mass spectrometry  as well as a 1-D, reverse phase, nanoscale liquid chromatograph separation coupled to high resolution mass spectrometry and spectral counting.  Both approaches revealed that cathepsin B cysteine protease was upregulated in aphids reared on turnip.  DIGE analysis also revealed multiple size and charge cathepsin B isoforms were differentially expressed.  Three distinct cathepsin B proteins derived from three cathepsin B-encoding transcripts were identified. The expression and up-regulation of each protein isoform in M. persciae reared on turnips was validated using selected reaction monitoring mass spectrometry.  Fluorescence in situ hybridization revealed that the cathepsin B genes are expressed in both the salivary and gut tissues of M. persciae and experiments are in progress to compare the levels of these transcripts among aphids reared on both plants. Functional assays using RNA interference and protease inhibitors are in progress to study the role of cathepsin in virus transmission. We hypothesize that the high levels of cathepsin in turnip-fed aphids degrade PLRV during its passage through the aphid gut or salivary tissues, impairing its transmission to a healthy host.