Iridoviridae genomes from Spodoptera frugiperda
Invertebrate iridescent viruses (IIVs, family Iridoviridae) are icosahedral double-stranded DNA viruses that infect a wide range of invertebrates, particularly in humid and aquatic environments. During field trials in Chiapas, southern Mexico, larvae of the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae) displayed an unexpected lavender iridescence, leading to the discovery of novel IIV isolates from this major agricultural pest. Restriction endonuclease analysis revealed evident diversity among isolates from individual infected larvae. Field data revealed a negative correlation with the prevalence of parasitoids and a positive correlation with entomopathogenic nematodes (probably Hexamermis sp.), suggesting distinct ecological routes through which the virus may transmit across host species. IIV infection of S. frugiperda was also identified in Veracruz State, Mexico, and northern Argentina, revealing a previously unrecognized geographic and host range for these viruses. The genomic and evolutionary features of the three isolates from S. frugiperda were compared with those of two other lepidopteran isolates from Helicoverpa zea (IIV30C obtained from CSIRO) and Anticarsia gemmatalis (AgIIV). Genome sizes ranged between 196.1 and 205.4 kbp (~28% GC content), with several large inversions and were rich in tandem repeats. Average amino acid identity and phylogenetic analyses of 26 core gene sequences placed all five isolates within the genus Chloriridovirus, closely related to IIV22 and IV22a isolated from blackflies (Diptera) in Wales and a previously sequenced isolate of IIV30 from the USA. We conclude that these lepidopterans are all infected by strains of the virus species Chloriridovirus simulium1 across their native geographical range. These findings highlight the unexpected ecological breadth and evolutionary adaptability of IIVs.