Nai et al. BMC Genomics 2010, 11:116 http://www.biomedcentral.com/1471-2164/11/116 The result indicated that LyxyMNPV and LdMNPV are grouped together. These results correspond to our pre- vious studies [12,16] and indicate that LyxyMNPV is a baculovirus distinct from LdMNPV but the two are clo- sely related based on the pairwise distances of the nucleotide sequences of polh, lef-8 and lef-9 [12]. Comparison of LyxyMNPV to LdMNPV The most significant difference between LyxyMNPV and LdMNPV is a large genomic fragment (29.3 kb in length) of LyxyMNPV that includes 32 ORFs and one hr, which range from Lyxy110 (Ld145) to Lyxy141 (Ld116) and are inverted compared to those of LdMNPV (Fig. 2a; Fig. 5). The LyxyMNPV genome is 4702 bp smaller than the LdMNPV genome and con- tains six fewer ORFs. LyxyMNPV contains six ORFs that are absent in LdMNPV (Table 1), whereas LdMNPV contains 22 ORFs that are absent in Lyx- yMNPV, namely rr1, rr2a, and ctl-1, as mentioned pre- viously (see Additional file 3). LyxyMNPV and LdMNPV share 14 ORFs (Lyxy7, Lyxy8, Lyxy9, Lyxy22, Lyxy25, Lyxy30, Lyxy45, Lyxy46, Lyxy65, Lyxy73, Lyxy124, Lyxy136, Lyxy144 and Lyxy157) with unknown functions, and the homologues of these genes are not present in the other baculoviruses. These shared genes range from 50 to 370 aa in LyxyMNPV, and 10 of them contain recognisable promoters. It is possible that these genes are host range genes that are involved specifically in LyxyMNPV or other NPV infection in LD cells or other Lymantria species. There are 151 ORFs in com- mon between LyxyMNPV and LdMNPV, and their order is mostly identical. Several of these ORFs are of different lengths, as shown in Additional file 4. These genes include p74, dbp1, bro-c to -f, bro-m, vp91/p95, vp80, egt, pif-2, arif-1 and odv-e27 as well as other genes of unassigned functions. The hrs of LyxyMNPV are not all in the same position and contain different numbers and orientations of repeat units compared to LdMNPV. The differences in gene content, ORF length and hr are possible candidates for regulators of the different degree of virulence exhibited by the two similar species [68], and it might be the same case between LyxyMNPV and LdMNPV, and even LdMNPV-like virus. There are three Lymantriidae-derived NPV species, including LyxyMNPV, LdMNPV and OpMNPV, which have a common in vitro host cell, the LD cell line [16,17,19,23]. Comparing further the LyxyMNPV gene content to those of LdMNPV and OpMNPV, we found that these three NPVs shared 99 ORFs (Additional file 2); three of these genes were absent in the AcMNPV genome, namely Lyxy116/Op35/Ld139 (iap-3), Lyxy137/Op34/ Ld120 (rr2b), and Lyxy141/Op31/Ld116 (dutpase). Of these three genes, iap-3 is one of the baculovirus genes that affect the viral host range and prevent apoptosis in Page 11 of 14 baculovirus-infected cells [54]. In OpMNPV, op-iap3 could rescue AcMNPV mutants lacking p35 from induced LD cell apoptosis [56]. Silencing of op-iap3 during the OpMNPV infection of LD cells induces apoptosis [69]. These data support a possibly important role for iap-3 during the early stage of baculovirus infection in LD cells. However, LdMNPV and OpMNPV have 22 and 55 genes, respectively, with no homologues in the Lyx- yMNPV genome. Among them, an important gene, hrf- 1 (Ld67 and Op143), that is involved in baculoviral host range and infectivity was not found in the LyxyMNPV genome (Table 1). Studies on hrf-1 revealed that it could promote NPV infectivity (including SeMNPV, HycuNPV, BmNPV, and AcMNPV) of LD cells [69], and recombinant AcMNPV bearing hrf-1 also exhibited increased infectivity towards Helicoverpa zea and L. dis- par larvae [70,71]. A new NPV species, MaviMNPV, which also lacks the hrf-1 gene, could infect LD cells with a low infection rate (<1%) by its egfp-recombinant virus, but it could not infect LY cells [24]. Baculovirus host range likely involves a complicated array of viral and cellular factors. However, through data from genomic analyses, we speculate that hrf-1 maybe more important for group-I NPVs during infec- tion in LD cells than group-II NPVs. In addition, iap-3 from LyxyMNPV, LdMNPV and OpMNPV may be an important factor during NPV infection in LD cells. Conclusion In conclusion, LyxyMNPV showed a high degree of coli- nearity and sequence identity with LdMNPV. However, these two viruses came from different geographical loca- tions. The results of our previous studies and previous in vitro infection assay revealed that LyxyMNPV could be propagated in both LY and LD cell lines. Further- more, the genome sequence analysis revealed that Lyx- yMNPV lacks hrf-1. Thus, the genes that are involved in the host range expansion of LyxyMNPV and LdMNPV are very interesting and worth further study. LyxyMNPV was highly virulent to L. xylina larvae, which suggests that it could be a promising agent for inclusion in I.P. M. programs for the biological control of L. xylina in Taiwan. Methods Insect cell lines, virus and viral DNA Wild-type LyxyMNPV was isolated from infected larvae in Taiwan [16,17], and a LyxyMNPV clone (Ly-5) was isolated using the L. xylina cell line [NTU-LY-1 cells (LY) [18]] and used in this study. Viral occlusion bodies (OBs) and viral DNA were determined and prepared fol- lowing the protocol described by Summers & Smith [72]. The quantity and quality of extracted DNA were determined spectrophotometrically and by