Genetic typing of M. sydneyi across estuaries of different QX risk-groups
Surveillance undertaken through previous years (2002-2005) has indicated that M. sydneyi is present in the majority of NSW estuaries, although outbreaks of QX disease only occur in some. In order to understand whether variation in the M. sydneyi strain could account for these differences in disease expression, key M. sydneyi genes from archived oyster samples collected from medium and high risk estuaries were sequenced and compared (Table 1).
Sample ID | Year collected | Estuary of origin | Estuary QX risk rating |
|---|---|---|---|
M16-04749-6 | 2016 | Hawkesbury River | High |
M16-06827-85 | 2016 | Georges River | High |
M20-05667-13 | 2020 | Macleay River | High, but infrequent outbreaks |
M20-06326-17 | 2020 | Richmond River | High |
M20-06983-41 | 2020 | Brunswick River | Medium |
M21-11844-24 | 2021 | Richmond River | High |
M22-17260-24 | 2022 | Stradbroke Island, Qld | N/A |
M22-17262-10 | 2022 | Stradbroke Island, Qld | N/A |
M22-17475-7 | 2022 | Stradbroke Island, Qld | N/A |
M23-06441-24 | 2023 | Port Stephens | High |
Preliminary analysis of the sequence data suggests very little sequence diversity amongst M. sydneyi strains in high and medium QX risk estuaries. While some variant nucleotides were observed in the ITS2 region of the sequences, these are likely attributable to allelic variation. Only one strain, M22-17475-7 from Queensland showed a sequence difference (25 bp deletion) in the ITS 2 region; however, this was absent from the 2 other Queensland strains examined.
In addition, SROs from low-risk estuaries, where QX disease does not occur, were collected and screened for the presence of M. sydneyi to obtain material for genetic comparison.
- April 2023 - Wallis Lake (n = 150) and Wagonga Inlet (n = 150)
- March 2024 - Wallis Lake (n = 250), Wagonga Inlet (n = 250) and Tuross Lake (n = 250) [Figure 1]
- March and April 2025 - Wallis Lake (n = 250), Wagonga Inlet (n = 250) and Tuross Lake (n = 250)
Despite prior molecular detection of M. sydneyi in these estuaries at between 7 and 25% prevalence, no PCR positives were detected from the 1,800 oysters collected.
