Specificity of retention and transmission of viruses by nematodes.

A central problem for anyone concerned with the transmission of viruses by vectors is to understand how it is that a given vector can transmit one virus but not another, and that a given virus can be transmitted by one species of vector though not by another species that may be closely allied to it taxonomically. With a few exceptions, there is still little indication of what features underlie this specificity between virus and vector, although there is enough information to suggest that the mechanism is probably not the same in all instances. As with other vectors, so also with nematode vectors there is evidence of specificity between virus and vector. Thus, two major groups of nematode-borne viruses have been described, and two corresponding groups of vectors. The nepoviruses (8) have small isometric particles (Fig. 1) and are transmitted by nematodes in the genera Xiphinerna Cobb and Longidorus (Micoletzky) Thorne and Swanger, which are both in the superfamily Dorylaimoidea. By contrast, the tobraviruses (8) (previously known as netuviruses) have straight tubular particles (Fig. 2) and are transmitted by species of Trichodorus Cobb, which are in the superfamily Diphtherophoroidea. With the exception of tobacco ringspot virus (17, 31), none of these nematode-transmitted viruses is known to have vectors in other phyla. In each nematode genus there is further evidence of vector specificity. For example, Longidorus elongatus (de Man) Thorne & Swanger transmits two nepoviruses, raspberry ringspot virus (21) and tomato black ring virus (9), but does not transmit a third, arabis mosaic virus (22). Xiphinema diversicaudatum (Micoletzky) Thorne transmits arabis mosaic virus (7, 14), but not