Understanding the mechanism and evolution of vector transmission, vector specificity and host specificity for soybean dwarf virus

Soybean Dwarf Virus (SbDV), a persistently aphid-transmitted luteovirus, is an important plant pathogen causing economic losses on soybean crops in Asia. In North America, indigenous strains of SbDV are commonly infecting clover, but have not emerged as a threat to soybean production. Failure of SbDV to spread from clover to soybean was believed due to the fact that SbDV was poorly vectored by Aphis glycines, the only aphid that colonizes soybeans. However, there are multiple isolates of SbDV in the United States that are A. glycines-transmissible. In order to evaluate the risk of SbDV as an emerging plant pathogen capable of threatening soybean production, the clover isolate SbDV-MD6 was serially transmitted from clover to pea or soybean by different aphid vectors, Acrythosipon pisum, Nearctaphis bakeri, and A. glycines. Virus titer, symptom severity, and transmission efficiency were evaluated for each passage. Sequence analyses of SbDV-MD6 from both pea and soybean passages identified 6 non-synonymous consistent mutations in pea, compared to 11 and 16 mutations in soybean when transmitted by N. bakeri and A. glycines , respectively. The dN/dS analysis indicated that SbDV was under strong selective pressures in soybean, but not in pea. Significantly increasing virus titers with each sequential transmission supports this analysis. However, aphid transmission efficiency on soybean decreased with each passage from 53% to 0%, until the virus was no longer aphid transmissible. Although virus titers increased, serial transmission by A. glycines ceased after only one or two passages in soybean. Results indicated that the clover strain of SbDV-MD6 adapted readily to soybean by improved replication and/or movement, but selection for host adaptation created tradeoff factors decreasing host-to-host transmissibility by aphid vectors. This may explain the reason SbDV-MD6 could not be sequentially transmitted by certain aphid vectors. In addition, to explain the cellular regulation mechanism of the aphid vector transmission, two isolates, SbDV-MD16 and SbDV-MD6, were used in the transmission assays. Virus titers in different aphids were examined by RT-qPCR during each passage. All aphids are able to acquire the virus even after 2 days of non-host feeding. Virus titers in aphids that fed on infected soybeans decreased with each sequential transmission suggesting that the interaction between the virus and the aphid is a major bottleneck for the spread of SbDV-MD6. It is suggested that the most probable site is at the accessory saliva gland that is known for specific interaction between aphid and luteovirus in many systems. The selection on SbDV in soybean is probably leading to loss of specificity in this interaction. This may explain why clover strains of SbDV have not yet caused a severe epidemic in soybeans in N. America. Finally, SbDV-MD6 population diversity was examined in different host species. The diversity of SbDV populations increased slightly when at the beginning of shifting to a new host, and was relatively constant over serial passages. It also provided evidences that SbDV has the potential of expanding into a new niche and thus poses a threat of emerging as a new crop pathogen. However, the bottleneck imposed by persistent aphid transmission is the major limitation of SbDV spread in nature.