Chapter IV 



Biological Cycles of Plant Viruses in Insect Vectors 



L. M. Black 

 Department of Botany, University of Illinois, Urbana, Illinois 



I. Introduction 157 



II. The Nature of the Lisects and Viruses Involved in Biological Cycles 159 



III. History of Research on the Problem 161 



rV. Evaluation of Kinds of Evidence for Multiplication 173 



V. Evaluation of Kinds of Evidence for Absence of Multiplication 175 



VI. Significance of Biological Cycles 179 



References 184 



I, Introduction 



In 1931, Huff proposed that the transmission of parasites by arthropods be 

 differentiated into four categories, three biological and one mechanical. The 

 first biological type he termed cyclo-propagative and characterized it as 

 including organisms that undergo both cycKcal change, that is, morphological 

 transformation to a different form, and multiphcation in the arthropod 

 vector (carrier). The second type, cy do-developmental, encompassed parasites 

 which change to a different form in the arthropod disseminator but do not 

 multiply in it. In his third type, propagative, he included organisms which 

 undergo no change in form in the arthropod vector but which multiply in 

 it. Transmission of parasites which undergo neither change in form nor 

 multiphcation in the arthropod carrier he classed as mechanical. 



Probably most, if not all plant virologists, would consider those plant 

 viruses which multiply in their insect vectors as the only ones having a true 

 biological cycle in their vectors. It is with these that we shall be principally 

 concerned. From what is known about virus multiphcation in general, it is 

 to be expected that multiphcation in the arthropod vector would involve a 

 change in the form of the virus during the process of multiplication itself. We 

 know something about the form of the end products of this multiphcation in 

 both plant and insect in only one case, that of wound tumor virus (Brakke et 

 al., 1954; Black and Brakke, 1954). Here the evidence indicates that both the 

 virus and viral soluble antigen are the same in both hosts. We, therefore, 

 have reason for supposing that the process of multiplication in the insect is 

 essentially the same as that in the plant. On the basis of present knowledge, 

 the plant viruses with biological cycles would therefore belong in Huff's 



157 



