In immunosorbent electron microscopy (ISEM) tests, strong relationships were detected between five whitefly-transmitted geminiviruses: African cassava mosaic (ACMV), bean golden mosaic, euphorbia mosaic, squash leaf curl and tomato golden mosaic. Among five leafhopper-transmitted geminiviruses, beet curly top and tobacco yellow dwarf viruses were distantly related but no relationship was detected between either chloris striate mosaic, maize streak or wheat dwarf viruses and any of the other four. No relationship was detected between any whitefly-transmitted and any leafhopper-transmitted virus. A similar pattern of relationships was found by spot hybridization experiments in which extracts from infected leaves were tested with probes for ACMV DNA-1 or DNA-2. Imperfect nucleotide sequence homologies were found between ACMV DNA-1, which contains the particle protein gene, and the DNA of five other whitefly-transmitted viruses: bean golden mosaic, tomato golden mosaic, tobacco leaf curl, tomato leaf curl and tomato yellow leaf curl, the last three of which are not sap-transmissible. Thus, relationships were established between sap-transmissible and sap non-transmissible geminiviruses. No homologies were detected with a full-length probe for ACMV DNA-2. Extracts from plants infected with three leafhopper- transmitted viruses (beet curly top, maize streak and wheat dwarf) did not react with probes for ACMV DNA-1 or DNA-2. Because each of the leafhopper-transmitted geminiviruses has a different vector species whereas the whitefly-transmitted geminiviruses all have the same vector, , the genome homologies and antigenic relationships detected among members of the group could be explained if their coat proteins have a key role in transmission by vectors.


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