Persistent Viral Infections of Aphids
Some Aphids are Infective for Weeks, Others From Birth
Aphids feed on the sap of plants and transmit about fifty percent of all transmittable plant viruses. Both the location in the plant of a viral infection and the method of feeding by the aphid affect the time the virus remains in the aphid. Viral infections of the epithelial cells of plants spend only a short period of time in the aphid body, but those that affect the phloem cells remain in the aphid much longer times – sometimes staying inside aphids and their offspring for an entire season.
Nonpersistent and semipersistent viral infections are discussed on page 1. This page describes the viruses that last inside the aphids for weeks. Most viruses do not reproduce inside the aphid and the lengths of time the aphid remains infective by these viruses are determined by:
- how long the aphid's stylus remains inside the infected plant cell,
- whether the virus resides in the gut or is distributed throughout the aphid's body,
- how much virus was ingested from the original cell,
- whether the aphid becomes reinfected from the plant it infects.
Types of Persistent Infections
Aphids become infected by persistent viruses when they begin feeding on infected phloem cells – about twenty minutes after inserting the stylus into the plant. These viruses move through the foregut of the aphid into the hindgut. Here, they penetrate the wall of the hindgut, entering the body cavity of the aphid. Once in the body cavity, the viruses circulate in the hemolymph (analagous to our blood) throughout the body – a process that usually takes between twenty four and thirty six hours after infection and during which time, the aphid cannot infect new cells. The mode of infection progresses in one of two directions from here: nonpropagative and propagative.
Nonpropagative
From the hemolymph, the nonpropagative virus enters the aphid salivary glands where it is egested when the aphid feeds on uninfected phloem cells. This type of infection may last several days or even weeks depending on the number of viruses the aphid obtained while feeding in infected phloem cells. Thus, the duration of the aphid's infective state depends on how many viruses there are in the hemolymph, and this depends on the length of time the aphid fed from the infected phloem cell.
Some viruses need to be transmitted in conjunction with helper viruses and/or satellite genes that provide one or more proteins needed by the pathogenic virus. An example is the groundnut rosette virus (GRV). This virus, transmitted by Aphis craccivora, requires both a helper virus, groundnut rosette accessory virus (GRAV) and a satellite RNA molecule to become infective – all three of which must be transmitted by the aphid at the same time. The satellite RNA molecule is responsible for producing the characteristics of the disease and different forms of statellite RNA molecules produce different expressions of the infection.
Propagative
Although the propagative viral infection of the aphid begins in the same manner as a nonpropagative infection, once the virus enters the hemolymph, the process diverges. The viruses of the propagative infection actually enter and reproduce inside the cells of the aphid.
In this mode of infection, the aphid can not transmit all of the viruses to plants and remains infective during its entire life. In addition, since the virus is found inside the cells of the aphid, it infects the aphid's offspring before they are born and all offspring of an infected aphid are both infected and capable of transmitting the virus to new plant cells for several generations.
This type of infection is unusual because a virus can reproduce in two hosts with vastly different genetics, and each host is necessary for the viral infection to complete its cycle:
- Virus cannot move on its own from one plant to another.
- Pores in the phloem cells are so small, the virus cannot even migrate from one cell to another within the plant, and each cell must be infected by an aphid.
- Aphids infect their offspring before the offspring are born.
- Aphids cannot transmit the virus to other aphids except to its own offspring.
Although the movements of persistent viral infections through aphids are more complex than nonpersistent or semipersistent infections, the interactions of the virus with the aphid and plants have similar or even identical complexities in producing symptoms in the plants.
Schematic of Virus Movement Through Aphid
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