Alfalfa leaf curl virus is efficiently acquired by its aphid vector Aphis craccivora but inefficiently transmitted

Alfalfa leaf curl virus (ALCV) is the first geminivirus for which an aphid transmission was reported. Transmission by Aphis craccivora was determined previously to be highly specific and circulative. Using various complementary techniques, the transmission journey of ALCV was monitored from its uptake in an infected plant tissue up to the head of its vector. ALCV was shown to be restricted in the phloem using fluorescent in situ hybridization (FISH) and electropenetrography (EPG) monitoring of virus acquisition. Furthermore, the virus is heterogeneously distributed in the phloem as revealed by FISH and qPCR quantification of the viral DNA acquired by aphids monitored by EPG. In spite of the efficient ingestion of viral DNA, about 106 in a 15-hour feeding on ALCV infected plants, the individual transmission rate was at a maximum of 12%. Transmission success was related to a critical viral accumulation, around 1.6x107 viral DNA copies per insect, a threshold that needs generally more than 48 hours to be reached. Moreover, whereas the amount of acquired virus does not decrease over time in the whole aphid body, it decreased in hemolymph and heads. ALCV was not detected in progenies of viruliferous aphids and had no effect on aphid fitness. Compared to geminiviruses transmitted by whiteflies or leafhoppers or to luteovirus transmitted by aphids, the transmission efficiency of ALCV by A. craccivora is low. This result is discussed in relation to the aphid vector of this geminivirus and the agroecological features of alfalfa, a hardy perennial host plant.