The crayfish plague pathogen, Aphanomyces astaci Schikora, has become one of the most well-studied pathogens of invertebrates. Since its introduction to Europe in the mid-19th century, it has caused mass crayfish mortalities, resulting in drastic declines of local populations. In contrast, North American crayfish usually serve as latent carriers, although they may also be negatively affected by A. astaci infections under some circumstances. Recent research benefiting from molecular tools has improved our knowledge about various aspects of A. astaci biology. In this review, we summarize these advances, particularly with respect to the host range and transmission. We highlight several aspects that have recently received particular attention, in particular newly confirmed or suspected A. astaci hosts, latent A. astaci infections in populations of European crayfish, and the relationship between A. astaci genotype groups and host taxa.

International audienceThe oomycete Aphanomyces astaci is generally considered a parasite specific to freshwater crayfish, and it has become known as the crayfish plague pathogen. Old experimental work that reported transmission of crayfish plague to the Chinese mitten crab Eriocheir sinensis, and the ability of A. astaci to grow in non-decapod crustaceans, has never been tested properly. * We re-evaluated the host range of A. astaci by screening for the presence of A. astaci in two crab species cohabiting with infected crayfish in fresh waters, as well as in other higher crustaceans from such localities. The animals were tested with species-specific quantitative PCR, and the pathogen determination was confirmed by sequencing of an amplified fragment of the nuclear internal transcribed spacer. Furthermore, we examined microscopically cuticle samples from presumably infected crab individuals for the presence of A. astaci-like hyphae and checked for the presence of pathogen DNA in such samples. * Screenings of benthopelagic mysids, amphipods and benthic isopods did not suggest infection by A. astaci in non-decapod crustaceans. In contrast, both studied lake populations of crabs (a native semiterrestrial species Potamon potamios in Turkey, and an invasive catadromous E. sinensis in Sweden) were infected with this parasite according to both molecular and microscopic evidence. * Analyses of polymorphic microsatellite loci demonstrated that A. astaci strains in the crabs and in cohabiting crayfish belonged to the same genotype group, suggesting crayfish as the source for crab infection. * The potential for A. astaci transmission in the opposite direction, from crabs to crayfish, and potential impact of this pathogen on populations of freshwater crabs require further investigations, because of possible consequences for crayfish and freshwater crab conservation and aquaculture

Many populations of the narrow-clawed crayfish Astacus leptodactylus in Turkey, including those inhabiting Lake Eg irdir, declined drastically in the mid-1980s due to introduction of crayfish plague Aphanomyces astaci. However, unlike many other localities, there has been some recovery in the A. leptodactylus population inhabiting this lake even though crayfish plague has been suspected to have persisted since then. In support of this, DNA from 5 of 34 healthy-looking crayfish sampled recently from the lake tested positive by both conventional and real-time PCR using species-specific primers targeting the rDNA internal transcribed spacer region, and product sequence analysis confirmed the identification of A. astaci. This complies with other recent reports of coexistence of native European crayfish with this pathogen, and further research is now needed to identify the key mechanisms allowing it.KEY WORDS: Aphanomyces astaci · Astacus leptodactylus · Host−pathogen coexistence · Melanisation · rDNA-internal transcribed spacer sequence · Lake Eg irdir Resale or republication not permitted without written consent of the publisherDis Aquat Org 98: [255][256][257][258][259] 2012 dramatically reduced. However, there were exceptional cases where crayfish numbers either remained continuously high or recovered enough to sustain commercial harvesting despite the presumed presence of the crayfish plague pathogen (Rahe & Soylu 1989, Harliog lu 2004, 2008.Crayfish resistance and/or unique water chemistries have been assumed as reasons facilitating the long-term coexistence of Aphanomyces astaci in crayfish inhabiting such lakes (Rahe & Soylu 1989, Harliog lu 2008. A. astaci was unambiguously proven as the cause of crayfish mortalities in the 1980s (Baran & Soylu 1989, Rahe & Soylu 1989, Huang et al. 1994. However, its later presence in Turkish lakes (e.g. Aydin & Dilek 2004, Harliog lu 2004, 2008 has only been assumed from symptoms that are not specific for the crayfish plague pathogen (Cerenius et al. 1988, Oidtmann et al. 1999. As A. astaci should not survive at a locality when its crayfish hosts disappear (Söderhäll & Cerenius 1999) and its presence in Turkey after 1989 has not been proven definitively, its distribution in the country might now be more restricted than assumed.The narrow-clawed crayfish population inhabiting Lake Eg irdir (Isparta Province; 38°N, 31°E, altitude 917 m above sea level [a.s.l.], area 482 km 2 ) collapsed in 1986, presumably due to crayfish plague (Fürst & Söderhäll 1987). By 1999, numbers recovered sufficiently to allow commercial harvesting, although catch sizes have been lower and prone to fluctuations (Fig. 1). The aim of this study was to examine Lake Eg irdir crayfish using PCR and rDNA sequence analysis to test for the contemporary presence of A. astaci. MATERIALS AND METHODSOver 2 occasions in 2009, a total of 32 crayfish (mean body length ± SD: 111 ± 22 mm) were caught in fyke-net traps in the southern part of Lake Eg irdir (37°53' N, 30°53' E). In March, 2 egg-bearing fem...

Using a high-resolution mapping approach, we identified a candidate gene for ZYMV resistance in cucumber. Our findings should assist the development of high-versatility molecular markers for MAS for ZYMV resistance. Zucchini yellow mosaic virus (ZYMV) causes significant disease, which leads to fruit yield loss in cucurbit crops. Since ZYMV resistance is often inherited recessively in cucumber, marker-assisted selection (MAS) is a useful tool for the development of resistant cucumber cultivars. Using 128 families of an F2:3 population derived from a cross between susceptible 'CS-PMR1' and resistant 'A192-18' cucumber inbred lines, we confirmed that ZYMV resistance is conferred by a single recessive locus: zym (A192-18) . We constructed a cucumber genetic linkage map that included 125 simple sequence repeat (SSR) markers segregating into 7 linkage groups (chromosomes). The zym (A192-18) locus was mapped to chromosome 6, at genetic distances of 0.9 and 1.3 cM from two closely linked SSR markers. For high-resolution genetic mapping, we identified new molecular markers cosegregating with the zym (A192-18) locus; using cucumber genomic and molecular marker resources and screening an F2 population of 2,429 plants, we narrowed down the zym (A192-18) locus to a <50-kb genomic region flanked by two SSR markers, which included six candidate genes. Sequence analysis of the candidate genes' coding regions revealed that the vacuolar protein sorting-associated protein 4-like (VPS4-like) gene had two SNPs between the parental lines. Based on SNPs of the VPS-4-like gene, we developed zym (A192-18) -linked DNA markers and found that genotypes associated with these markers were correlated with the ZYMV resistance phenotype in 48 cucumber inbred lines. According to our data, the gene encoding VPS4-like protein is a candidate for the zym (A192-18) locus. These results may be valuable for MAS for ZYMV resistance in cucumber.

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The crayfish plague pathogen, Aphanomyces astaci, is one of the most serious threats to indigenous European crayfish species. The North American invasive spiny-cheek crayfish, Orconectes limosus, is an important source of this pathogen in central and western Europe. We evaluated potential changes in A. astaci spore release rate from infected individuals of this species by experiments investigating the pathogen transmission to susceptible noble crayfish, Astacus astacus. We filtered defined volumes of water regularly to quantify spore concentration, and sampled crayfish tissues at the end of the experiment. The filters and tissues were then tested for the presence of A. astaci DNA by species-specific quantitative PCR. Additionally, we tested the efficiency of horizontal transmission to apparently uninfected O. limosus. The experiments confirmed that A. astaci can be transmitted to susceptible crayfish during intermoult periods, and that the pathogen was more frequently detected in noble crayfish recipients than in American ones. The pathogen spore concentrations substantially varied in time, and significantly increased during moulting of infected hosts. Our study strengthens the evidence that although the likelihood of crayfish plague transmission by water transfer from localities with infected American crayfish might increase when these are moulting or dying, no time-periods can be proclaimed safe.

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