It is suggested that the isolates of Oryctes Nudivirus (OrNV), cultured for decades in cells of Heteronychus arator (F.) (HA), be checked to verify genomic changes have not occurred which adapt them to culture but reduce or cancel their ability to infect the target pest, the coconut rhinoceros beetle (CRB), Oryctes rhinoceros (L.). Full genomes of field-caught OrNV isolates, and their infectivity against larvae and adults, could be compared with those of HA-cultured isolates. Further data to correlate OrNV dosage indices with doses in number of virions/ml could be advantageous so as to explore if CRB larvae or adults may resist infection by a sub-threshold dose. Also the possibility of changes in the HA culture cells which alter the outer coat of the resulting virion, hence perhaps its infectivity towards CRB cells, could be checked. Might it be possible to move beyond HA-culture and develop tissue culture of Oryctes rhinoceros cells for mass production of OrNV as this beetle species is the target? Nuclear genomes of OrNV-resistant and OrNV-susceptible strains of the CRB could be examined for changes perhaps correlated with resistance. The possibility of endosymbiotic bacteria affecting CRB susceptibility to OrNV might be checked.
Oryctes rhinoceros (L.) (Coleoptera: Scarabaeidae: Dynastinae) is an important pest of coconut palms. The adults bore into the heart of the palm to feed on sap, damaging immature fronds which emerge subsequently showing typical V-shaped cuts with reduced photosynthetic area. Repeated attacks may destroy the growing point and kill the palm. Young oil palms are also attacked and may be killed. While it is endemic in Asia, the pest had also been accidentally introduced into the South Pacific in the early 20th century, and had then spread and become established in a number of South Pacific countries (summarised in [
An isolate of Oryctes Nudivirus (OrNV) was discovered in Malaysia and described in 1966 [
Infection is peroral, and the virus multiplies in the fat body and midgut of larvae, and in the midgut of adults which defecate the virus and thus are “flying virus factories” which disseminate it [
Since its discovery, OrNV has been released in CRB-affected South Pacific countries as a biopesticide/biocontrol pathogen against CRB, and from the 1970’s particularly by releasing infected adults. As a biocontrol agent OrNV has been able to reduce and hold down beetle populations hence beetle damage to palms (summarised in [
A recent informative contribution [
The current article aims to build on this contribution by suggesting lines of investigation which may explore questions and advance the topic so as to enable continuing benefit to come from the use of OrNV. It is thus a commentary rather than giving new data.
Oryctes Nudivirus (OrNV) has been cultured in Heteronychus arator (F.) (HA) cells since 1981 [
In HA tissue culture the pathogen has no contact with cells of the target species (CRB) Oryctes rhinoceros. HA-cultured OrNV is the form in which the virus is produced and provided for testing against CRB, and for widespread release in the South Pacific. Might it be conceivable that with a replication cycle needing only hours, yet being in HA culture for 36 years, genomic changes may evolve in cultured OrNV strains or isolates adapting them so as to facilitate their ability to infect HA cells, but concomitantly reducing even disabling their ability to infect O. rhinoceros & its midgut cells, as in HA cells this ability is thus redundant? While various isolates of OrNV can be readily produced in HA tissue culture [
The OrNV genome has plasticity, and is well able to undergo genome changes, which arise, establish, & persist e.g. 12 isolates have been reported, with some even co-existing within the same geographical area e.g. Luzon, Philippines [
The CRB-G OrNV-resistant haplotype has been identified by bioassay peroral test feeding of OrNV to adult O. rhinoceros from the Guam population [
Also, the CRB-S populations of O. rhinoceros (established for many decades in South Pacific countries such as Samoa, American Samoa, Tonga, Fiji, Tokelau Isl, Wallis Isl, PNG (New Britain, New Ireland, Manus Isl) could be checked, both larvae, & adults, as to their current susceptibility, to whatever OrNV strains may exist & be isolated from larvae, & adults, from current O. rhinoceros populations in those countries into which OrNV isolates have been released in decades past. Has this geographic and reproductive isolation begun to cause local differentiation in the genome of these CRB populations, perhaps affecting their susceptibility to OrNV [
The CRB-G and CRB-S strains have so far been separated based on differences in the mitochondrial COI gene [
OrNV Malaysian geographical isolates A, B and C infecting larvae & adults of Malaysian CRB-S have been propagated in HA [
Apart from possible genomic changes, another possibility may affect the coat of the virions. Fresh virions are shown in
A further consideration is that while it has been advocated [
or can its immune system fight off a potential infection from lower doses of virions?
One could check whether CRB-G may harbour an endosymbiont bacteria (perhaps paralleling Wolbachia?) which may give resistance to OrNV [
It would be worthwhile to compare the full genome of OrNV isolates freshly extracted from field populations of O. rhinoceros, with full genomes of OrNV long cultured in HA, to look for changes. Also the infectivity of these isolates against CRB larvae and adults could be compared with that of HA-cultured isolates. One could look into the possibility of establishing tissue culture of O. rhinoceros cells (thus moving beyond HA culture) for the propagation of OrNV isolates since O. rhinoceros is the ultimate target. The nuclear genome of CRB strains and geographical races could be checked for mutations which may affect resistance or susceptibility to OrNV.
I thank Prof. A. Hoffmann, University of Melbourne, Prof. E. Holmes, University of Sydney, and Prof. R. Frankham, Macquarie University, for helpful comments.
There is no financial interest affecting this study.
Bedford, G.O. (2018) Possibility of Evolution in Culture of the Oryctes Nudivirus of the Coconut Rhinoceros Beetle Oryctes rhinoceros (Coleoptera: Scarabaeidae: Dynastinae). Advances in Entomology, 6, 27-33. https://doi.org/10.4236/ae.2018.61004