Submitted to: Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2010
Publication Date: 6/11/2010
Citation: Spear, A., Sisterson, M.S., Yokomi, R.K., Stenger, D.C. 2010. Plant-feeding insects harbor double-stranded RNA viruses encoding a novel proline-alanine rich protein and a polymerase distantly related to that of fungal viruses. Virology. 404:304-311. Interpretive Summary: In an effort to discover new biological control agents for plant feeding hemipteran pests, three cornered alfalfa hopper (TCAH, Spissistilus festinus), and beet leafhopper (BLH, Circulifer tenellus) were evaluated for presence of viral double-stranded (ds) RNA. TCAH and BLH are of economic importance and common in the San Joaquin Valley of California. TCAH has a preference for legumes and in California is mainly a pest of alfalfa. BLH has a broad host range but is primarily of concern as a vector of curtoviruses causing curly top disease and Spiroplasma citri causing citrus stubborn disease. Both insect species were found to harbor novel dsRNA viruses, designated Spissistilus festinus virus 1 (SpFV1) and Circulifer tenellus virus 1 (CiTV1). The genomes of both viruses were sequenced and shown to be distantly related to certain unclassified fungal viruses and the plant virus Curcubit yellows associated virus. As neither virus produced overt disease in their respective insect hosts, SpFV1 and CiTV1 are not considered viable biological control agents. Nonetheless, high incidence of SpFV1 and CiTV1 in populations of TCAH and BLH, coupled with substantially intraspecies sequence diversity of viruses, indicates that the two new viruses may have utility as novel genetic markers to assess population structure, gene flow, and migration patterns of TCAH and BLH.
Technical Abstract: Novel double-stranded RNAs (~8 kbp) were isolated from three cornered alfalfa hopper (Spissistilus festinus) and beet leafhopper (Circulifer tenellus), two plant-feeding hemipteran insect pests. Genomes of the two new viruses, designated as Spissistilus festinus virus 1 (SpFV1) and Circulifer tenellus virus 1 (CiTV1), were organized similarly and shared 54% nucleotide sequence identity. The presumptive positive-sense strand contained a leader sequence of 572 nts (SpFV1) or 642 nts (CiTV1) followed by an open reading frame (ORF) encoding a proline alanine rich protein (PArp). Expression of the 3’ proximal RNA directed RNA polymerase (RdRp) ORF appears to result from -1 translational frameshifting of the PArp ORF. BLASTX searches using SpFV1 and CiTV1 RdRp as queries returned as subjects the RdRp core domain of totiviruses, chrysoviruses, several unclassified fungal viruses, and Cucurbit yellows associated virus (CuYAV). Phylogenetic analysis of the RdRp indicated that SpFV1 and CiTV1 were most closely related to each other and, along with CuYAV, represent a unique lineage that may constitute a new genus not affiliated with any established family. Although BLASTX searches using PArp sequences as queries failed to return subjects with significant e values, genome organization of SpFV1 and CiTV1 resembled that of the unclassified fungal dsRNA viruses Phlebiopsis gigantea virus 1 (PgV1), Phlebiopsis gigantea virus 2 (PgV2), and Fusarium graminearum virus 3 (FgV3). Attempts to purify SpFV1 and CiTV1 virions were unsuccessful, suggesting lack of encapsidation in conventional particles, a trait shared with PgV1, PgV2, and FgV3. As SpFV1 and CiTV1 do not appear to cause disease but are common in their respective hosts, these two new viruses may be considered benign molecular symbionts.