In this article we will discuss about the relationship between virus and vector.
1. Stylet-Borne Viruses:
Stylet is a long, slender, hollow feeding organ of insects and nematodes and the stylet-borne viruses arc the viruses borne on stylet of their vectors. Majority of the stylet-borne viruses are transmitted by aphids. The acquisition period of these viruses by aphids varies from a few minutes to many hours and during this period they remain ‘viruliferous’.
After transmitting the viruses, aphids become virus- free or ‘aviruliferous’. Because of the limited company of these viruses with their vectors they are also called ‘non-persistent’ viruses. Stylet-borne virus transmission is purely a mechanical process.
Few stylet-borne viruses can only be transmitted by a particular vector. Myzus ornatus is a specific vector for cauliflower mosaic virus and M. ascolonicus transmits only cucumber mosaic virus and the henbane mosaic virus.
Many viruses are stylet-borne and great majority of them induce mosaic symptoms and are saptransmissible. Cucumoviruses, carlaviruses and potyviruses are aphid-borne stylet-borne viruses.
2. Circulative Viruses:
When the viruses are taken in by vectors, accumulate within them, do not undergo multiplication, enter the blood by passing through gut wall and reach back to salivary glands (i.e., circulate through the tissues of the vectors) and then are transmitted via their mouth parts, they are called ‘circulative viruses’.
These viruses are not transmitted immediately after the acquisition but the vectors have to wait several hours (latent or incubation period; the time taken by the virus to complete the above mentioned route in vector-body) to transmit such viruses.
Since these viruses are retained in the body during latent or incubation period; they are also called ‘semi-persistent’ viruses. Circulative viruses are mainly transmitted by aphids. Once the transmission starts it may be continued by vectors many days.
Maize chlorotic dwarf virus, rice tungro virus, pea enation mosaic virus, potato leaf-roll virus, lettuce mosaic yellow virus etc. are circulative viruses.
3. Propagative Viruses:
When the acquired viruses start multiplying within their respective vectors and establish a biological relationship, they are called ‘propagative viruses’. These viruses possess an incubation period which is presumably the time needed by them to multiply and to reach a definite concentration to become transmissible. They thus have a definite biological relationship with their vectors.
Four aphid- transmitted viruses and almost all leafhopper transmitted viruses are propagative. Most insect vectors become viruliferous after a feeding period of one to several days and remain viruliferous throughout their life and when such insect vector starts transmitting viruses once, it may continue the same for the rest of its life.
Since the propagative viruses establish a definite biological relationship with the vector and remain persistently viable inside vector’s body, they are also called ‘persistent’ viruses.
Propagative viruses follow a prefixed route and sequence to spread systemically within the body of their respective vectors and, finally, spread throughout the entire parts of the body of the vectors. Wheat striate mosaic virus, wound tumor virus, potato yellow dwarf virus, rice dwarf virus etc. are some propagative viruses.