Mud crab susceptibility to disease from white spot syndrome virus is species-dependent

<p>Abstract</p> <p>Background</p> <p>Based on a report for one species (<it>Scylla serrata</it>), it is widely believed that mud crabs are relatively resistant to disease caused by white spot syndrome virus (WSSV). We tested this hypothesis by determining the degree of susceptibility in two species of mud crabs, <it>Scylla olivacea </it>and <it>Scylla paramamosain</it>, both of which were identified by mitochondrial 16 S ribosomal gene analysis. We compared single-dose and serial-dose WSSV challenges on <it>S. olivacea </it>and <it>S. paramamosain</it>.</p> <p>Findings</p> <p>In a preliminary test using <it>S. olivacea </it>alone, a dose of 1 × 10⁶WSSV copies/g gave 100% mortality within 7 days. In a subsequent test, 17 <it>S. olivacea </it>and 13 <it>S. paramamosain </it>were divided into test and control groups for challenge with WSSV at 5 incremental, biweekly doses starting from 1 × 10⁴and ending at 5 × 10⁶copies/g. For 11 <it>S. olivacea </it>challenged, 3 specimens died at doses between 1 × 10⁵and 5 × 10⁵copies/g and none died for 2 weeks after the subsequent dose (1 × 10⁶copies/g) that was lethal within 7 days in the preliminary test. However, after the final challenge on day 56 (5 × 10⁶copies/g), the remaining 7 of 11 <it>S. olivacea </it>(63.64%) died within 2 weeks. There was no mortality in the buffer-injected control crabs. For 9 <it>S. paramamosain </it>challenged in the same way, 5 (55.56%) died after challenge doses between 1 × 10⁴and 5 × 10⁵copies/g, and none died for 2 weeks after the challenge dose of 1 × 10⁶copies/g. After the final challenge (5 × 10⁶copies/g) on day 56, no <it>S. paramamosain </it>died during 2 weeks after the challenge, and 2 of 9 WSSV-infected <it>S. paramamosain </it>(22.22%) remained alive together with the control crabs until the end of the test on day 106. Viral loads in these survivors were low when compared to those in the moribund crabs.</p> <p>Conclusions</p> <p><it>S. olivacea </it>and <it>S. paramamosain </it>show wide variation in response to challenge with WSSV. <it>S. olivacea </it>and <it>S. paramamosain </it>are susceptible to white spot disease, and <it>S. olivacea </it>is more susceptible than <it>S. paramamosain</it>. Based on our single-challenge and serial challenge results, and on previous published work showing that <it>S. serrata </it>is relatively unaffected by WSSV infection, we propose that susceptibility to white spot disease in the genus <it>Scylla </it>is species-dependent and may also be dose-history dependent. In practical terms for shrimp farmers, it means that <it>S. olivacea </it>and <it>S. paramamosain </it>may pose less threat as WSSV carriers than <it>S. serrata</it>. For crab farmers, our results suggest that rearing of <it>S. serrata </it>would be a better choice than <it>S. paramamosain </it>or <it>S. olivacea </it>in terms of avoiding losses from seasonal outbreaks of white spot disease.</p

Assessment of population genetic structure in the arbovirus vector midge, Culicoides brevitarsis (Diptera Ceratopogonidae), using multi-locus DNA microsatellites

Bluetongue virus (BTV) is a major pathogen of ruminants that is transmitted by biting midges (Culicoides spp.). Australian BTV serotypes have origins in Asia and are distributed across the continent into two distinct episystems, one in the north and another in the east. Culicoides brevitarsis is the major vector of BTV in Australia and is distributed across the entire geographic range of the virus. Here, we describe the isolation and use of DNA microsatellites and gauge their ability to determine population genetic connectivity of C. brevitarsis within Australia and with countries to the north. Eleven DNA microsatellite markers were isolated using a novel genomic enrichment method and identified as useful for genetic analyses of sampled populations in Australia, northern Papua New Guinea (PNG) and Timor-Leste. Significant (P < 0.05) population genetic subdivision was observed between all paired regions, though the highest levels of genetic sub-division involved pair-wise tests with PNG (PNG vs. Australia (F-ST = 0.120) and PNG vs. Timor-Leste (F-ST = 0.095)). Analysis of multi-locus allelic distributions using STRUCTURE identified a most probable two-cluster population model, which separated PNG specimens from a cluster containing specimens from Timor-Leste and Australia. The source of incursions of this species in Australia is more likely to be Timor-Leste than PNG. Future incursions of BTV positive C. brevitarsis into Australia may be genetically identified to their source populations using these microsatellite loci. The vector's panmictic genetic structure within Australia cannot explain the differential geographic distribution of BTV serotypes

Towards a global barcode library for Lymantria (Lepidoptera: Lymantriinae) tussock moths of biosecurity concern

Background: Detecting and controlling the movements of invasive species, such as insect pests, relies upon rapid and accurate species identification in order to initiate containment procedures by the appropriate authorities. Many species in the tussock moth genus Lymantria are significant forestry pests, including the gypsy moth Lymantria dispar L., and consequently have been a focus for the development of molecular diagnostic tools to assist in identifying species and source populations. In this study we expand the taxonomic and geographic coverage of the DNA barcode reference library, and further test the utility of this diagnostic method, both for species/subspecies assignment and for determination of geographic provenance of populations. Methodology/Principal Findings: Cytochrome oxidase I (COI) barcodes were obtained from 518 individuals and 36 species of Lymantria, including sequences assembled and generated from previous studies, vouchered material in public collections, and intercepted specimens obtained from surveillance programs in Canada. A maximum likelihood tree was constructed, revealing high bootstrap support for 90% of species clusters. Bayesian species assignment was also tested, and resulted in correct assignment to species and subspecies in all instances. The performance of barcoding was also compared against the commonly employed NB restriction digest system (also based on COI); while the latter is informative for discriminating gypsy moth subspecies, COI barcode sequences provide greater resolution and generality by encompassing a greater number of haplotypes across all Lymantria species, none shared between species. Conclusions/Significance: This study demonstrates the efficacy of DNA barcodes for diagnosing species of Lymantria and reinforces the view that the approach is an under-utilized resource with substantial potential for biosecurity and surveillance. Biomonitoring agencies currently employing the NB restriction digest system would gather more information by transitioning to the use of DNA barcoding, a change which could be made relatively seamlessly as the same gene region underlies both protocols

Phytoplasmas and their insect vectors: implications for date palm.

Phytoplasmas are transmitted chie\ufb02 y by insects, most commonly planthoppers and leafhoppers. Molecular genetic analyses have improved the understanding of phytoplasma taxonomy, and also enhanced the ability to identify phytoplasmas detected in hosts and insect vectors. Date palm is affected by Al-Wijam disease in Saudi Arabia and molecularly indistinguishable phytoplasmas (16SrI group) were recovered from affected palms and from Cicadulina bipunctata , an insect commonly found on the palms. The phytoplasma that is associated with the lethal yellowing disease in coconut palm (16SrIV-A) can also infect date palm. In the Americas lethal yellowing is likely to be transmitted by Haplaxius crudus (formerly Myndus crudus ). Texas phoenix decline is reported from warm regions of South-East USA and may be transmitted by two species of Derbidae. Phytoplasmas belonging to the 16SrIV-F and 16SrXIV groups have also been identi\ufb01 ed from date palm growing in the USA and Africa respectively, though vectors have not been identi\ufb01 ed. Preventing spread in infected vegetative planting material and of vectors is key to limiting the impact of phytoplasma diseases. Management in affected areas can use antibiotics on high value trees, but this is not economical for extensive crops. In these situations, vector control by insecticide use or habitat management can be useful, but the long lifespan of individual palms means that even low vector pressure can lead to infection over successive years. The development of resistant varieties and replanting is the most effective long-term approach developed so far for phytoplasma disease management in this plant species

First report of the presence of Verticillium dahliae VCG6 in Australia

Verticillium dahliae was isolated from Noogoora burr (Xanthium occidentale) seeds collected from mature asymptomatic plants in 2017 growing along the riverbank at Condobolin, NSW. V. dahliae was isolated from seeds by transferring surfaced-disinfested seed components onto 25% Potato Dextrose Agar (PDA) plus Novobiocin. Morphological and molecular analysis, as well as fulfilment of Koch’s postulates, confirmed the causal pathogen was V. dahliae. Additional analysis was carried out to assign the isolates into a vegetative compatibility group (VCG) and pathotype. This is the first report of V. dahliae VCG6 in Australia infecting X. occidentale.This research was funded through the Agricultural Competitiveness White Paper, Control Tools and Technologies for Established Pest Animals and Weeds Program.Peer reviewe

Statistical phylogeographic tests of competing 'Lake Carpentaria hypotheses' in the mouth-brooding freshwater fish, Glossamia aprion (Apogonidae)

Glacial cycles during the Pleistocene reduced sea levels and created new land connections in northern Australia, where many currently isolated rivers also became connected via an extensive paleo-lake system, 'Lake Carpentaria'. However, the most recent period during which populations of freshwater species were connected by gene flow across Lake Carpentaria is debated: various 'Lake Carpentaria hypotheses' have been proposed. Here, we used a statistical phylogeographic approach to assess the timing of past population connectivity across the Carpentaria region in the obligate freshwater fish, Glossamia aprion. Results for this species indicate that the most recent period of genetic exchange across the Carpentaria region coincided with the mid- to late Pleistocene, a result shown previously for other freshwater and diadromous species. Based on these findings and published studies for various freshwater, diadromous and marine species, we propose a set of 'Lake Carpentaria' hypotheses to explain past population connectivity in aquatic species: (1) strictly freshwater species had widespread gene flow in the mid- to late Pleistocene before the last glacial maximum; (2) marine species were subdivided into eastern and western populations by land during Pleistocene glacial phases; and (3) past connectivity in diadromous species reflects the relative strength of their marine affinity