With the exception for one farm, the Australian prawn farming industry remains reliant on the use of wild broodstock in hatcheries to generate seed to stock farm ponds,. Due partly to problems with GAV often occurring at high prevalence in North Queensland (NQ) (eg. Etty Bay, Bingil Bay, Bramston Beach, Yorkeys Knob), GAV-free broodstock have increasingly been sought from more remote locations in the Northern Territory (NT) (eg. Joseph Bonaparte Gulf and Melville/Tiwi Island) (Cowley et al. 2016).
Broodstock pairs (male + female) typically cost ~$160 when sourced locally from NQ and ~$600 when sourced from NT. Hatcheries use in the order of 3,000 wild-caught broodstock pairs annually. Thus due to (i) the substantially higher costs of broodstock sourced from remote regions in NT, (ii) the detection of a GAV variant designated Yellow head virus genotype 7 (YHV7) amongst NT stock (Cowley et al. 2016) with commercially relevant pathogenic potential (CSIRO Unpublished data) and (iii) difficulties in supply continuity and transport stress, the use of a hatchery-based RNAi strategy to reduce or clear GAV infection from local NQ broodstock with potential to abrogate or curtail the vertical transmission of GAV to progeny would significantly benefit industry.
Proof-of-concept obtained in this project that RNAi can be up-scaled from experimental to hatchery-scale systems, and that progeny with markedly reduced GAV infection loads can be generated from carrier broodstock injected with dsRNA, will provide industry with the confidence needed to apply the technology commercially once an APVMA permit has been issued for its use.
As these project objectives were revised due to difficulties in sourcing wild broodstock infected with suitably high loads of GAV, also reported are data from agreed alternative project objectives showing that (i) Infectious hypodermal and hematopoietic necrosis virus (IHHNV) is transmitted vertically from infected female broodstock to progeny and that the IHHNV prevalence and infection loads in progeny are influenced by infection loads in their parental female and (ii) the high-load infections that develop in progeny spawned from females with higher-level IHHNV infection result in substantially reduced growth performance and survival of progeny reared in 0.16 ha ponds under simulated commercial conditions.
The screening of batches of wild P. monodon broodstock to identify locations where these might be infected with GAV at moderately-high loads suitable for the original project objectives identified GAV to be present at very low prevalence among prawns captured at various locations in the vicinity of Innisfail between May and June 2016. Similar screening also identified the absence of Yellow head virus genotype 7 (YHV7) in these broodstock.
Further to these objectives and data, it was agreed to include another project variation objective to investigate whether (i) dsRNA(s) injected into tail-muscle of female broodstock at the time eyestalks were ablated to induce ovary maturation/spawning could be detected by TaqMan real-time RT-qPCR in various tissues (i.e. pleopod, ovary and lymphoid organ) several days later when the female spawned and (ii) dsRNA might transfer from injected females to spawned eggs and be maintained or amplified through larval life stages (i.e. nauplii, protozoea, mysis) to an early post-larvae (PL) stage