Prediction model for sequence variation in the glycoprotein gene of infectious hematopoietic necrosis virus in California, U.S.A

The influence of spatio-temporal factors on genetic variation of infectious hematopoietic necrosis virus (IHNV) is an active area of research. Using host-isolate pairs collected from 1966 to 2004 for 237 IHNV isolates from California and southern Oregon, we examined genetic variation of the mid-G gene of IHNV that could be quantified across times and geographic locations. Information hypothesized to influence genetic variation was environmental and/or fish host demographic factors, viz. location (inland or coastal), year of isolation, habitat (river, lake, or hatchery), the agent factors of subgroup (LI or LII) and serotype (1, 2, or 3), and the host factors of fish age (juvenile or adult), sex (male or female), and season of spawning run (spring, fall, late fall, winter). Inverse distance weighting (IDW) was performed to create isopleth maps of the genetic distances of each subgroup. IDW maps showed that more genetic divergence was predicted for isolates found inland (for both subgroups: LI and LII) than for coastal watershed isolates. A mixed-effect beta regression with a logit link function was used to seek associations between genetic distances and hypothesized explanatory factors. The model that best described genetic distance contained the factors of location, year of isolation, and the interaction between location and year. Our model suggests that genetic distance was greater for isolates collected from 1966 to 2004 at inland locations than for isolates found in coastal watersheds during the same years. The agreement between the IDW and beta regression analyses quantitatively supports our conclusion that, during this time period, more genetic variation existed within subgroup LII in inland watersheds than within coastal LI isolates.

Comparison of the RNA polymerase genes of marine birnavirus strains and other birnaviruses

The cDNA nucleotide sequence of the genome segment B encoding the VP1 protein, the putative RNA-dependent RNA polymerase (RdRp), was determined for 5 marine birnavirus (MABV) strains from different host or geographic origins and 1 infectious pancreatic necrosis virus (IPNV) strain AM-98. Segment B of the IPNV AM-98 strain and 4 MABV strains, Y-6, YT-01A, H1 and NC1, contained a 2535 bp ORF, which encoded a protein of 845 amino acid residues with a predicted MW of 94.4 kDa. Only the MABV AY-98 RdRp had 1 amino acid shorter RdRp. Pairwise comparisons were made among our data and 4 other known IPNV sequences. The nucleotide sequences of the 5 MABV strains were very similar each other, with identities of 98.3-99.7%. The highest divergence of the nucleotide level was between MABV strains and IPNV SP strain (serotype A2), with 20.4-20.8% divergences in the coding region, which gave 10.1-11.3% divergence in the amino acid level. The aquabirnavirus RdRp was noticeably conserved in amino acid sequences. Though the identities of the nucleotide sequences of encoding region were 85.1-85.9% between MABV strains and IPNV serotype A1 strains, they shared as high as 95.1-95.9% identities in amino acid level. A phylogenetic tree was constructed based on the amino acid sequences of the RdRp gene from different birnaviruses including avibirnavirus and entomobirnavirus. Ten aquabirnavirus strains were clustered into 3 Genogroups. The Genogroup I consisted of four IPNV A1 serotype strains. All MABV strains were clustered into Genogroup II. Only IPNV SP strain was clustered into an independent Genogroup III.