Analysis of ribosomal RNA intergenic spacer (IGS) sequences in species and populations of Gyrodactylus (Platyhelminthes: Monogenea) from salmonid fish in northern Europe

Larsen²,

et al. 2007

Dis. Aquat. Org.

Gyrodactylus salaris was isolated from rainbow trout in a Danish freshwater trout farm, and a laboratory population of this particular parasite form was established on rainbow trout. Challenge infections were performed using different salmonid strains and species, including East Atlantic salmon Salmo salar (from the Danish River Skjernå), Baltic salmon S. salar (from the Swedish River Ume älv) and rainbow trout Oncorhynchus mykiss (from the Danish rainbow trout farm Fousing). These were compared to infection studies on the Norwegian Laerdalselva parasite form kept under exactly the same conditions in the laboratory. The Danish G. salaris form had low virulence towards both Atlantic and Baltic salmon, whereas rainbow trout proved susceptible to the parasite. The Danish G. salaris form was able to maintain a very low infection on East Atlantic salmon, but not on the Baltic salmon, which eliminated the infection within 2 wk. Rainbow trout developed infection intensities ranging up to several hundred parasites per host. The host colonization patterns of the parasite differed clearly from those of previous studies on microhabitats of the Norwegian form of G. salaris. A comparative study on morphological characters (opisthaptoral hard parts) from the Danish parasite form and Norwegian G. salaris showed no significant differences. Selected genes comprising internal transcribed spacers 1 and 2 (ITS), ribosomal RNA intergenic spacer (IGS) and cytochrome c oxidase subunit I (COI) regions were cloned and sequenced. Five sequenced ITS clones from 5 individuals of the Danish strain consistently revealed a single base substitution compared to ITS sequences from all other known species and strains of Gyrodactylus. Mitochondrial COI gene sequences demonstrated that the Danish G. salaris form is closely similar to the Laerdalselva parasite form found in Norway. The IGS sequences were highly variable, but very similar to those obtained from German isolates of G. salaris.

Section: Methodsmentioning

confidence: 99%

Characterisation of a low pathogenic form of Gyrodactylus salaris from rainbow trout

Larsen²,

et al. 2007

Dis. Aquat. Org.

“…Its hook morphology closely resembles those of other related species. Limited success has been achieved with the sequencing of the intergenic spacer (IGS) of rDNA (Sterud et al 2002 ;Cunningham et al 2003 ;Hansen et al 2006), but a suite of alternative markers is still needed for the detection of population variation, to further understand the taxonomy and biology of the parasite, and to study its transmission and dispersion in space and time. However, the discrimination between G. salaris and G. thymalli Zitnan, 1960 has proved difficult, since at least 1 Norwegian G. thymalli population fell within the morphological variation of G. salaris unpublished observations).…”

Section: Introductionmentioning

confidence: 99%

The mitochondrial genome ofGyrodactylus salaris(Platyhelminthes: Monogenea), a pathogen of Atlantic salmon (Salmo salar)

et al. 2006

In the present study, we describe the complete mitochondrial (mt) genome of the Atlantic salmon parasite Gyrodactylus salaris, the first for any monogenean species. The circular genome is 14,790 bp in size. All of the 35 genes recognized from other flatworm mitochondrial genomes were identified, and they are transcribed from the same strand. The protein-coding and ribosomal RNA (rRNA) genes share the same gene arrangement as those published previously for neodermatan mt genomes (representing cestodes and digeneans only), and the genome has an overall A+T content of 65%. Three transfer RNA (tRNA) genes overlap with other genes, whereas the secondary structure of 3 tRNA genes lack the DHU arm and 1 tRNA gene lacks the TphiC arm. Eighteen regions of non-coding DNA ranging from 4 to 112 bp in length, totalling 278 bp, were identified as well as 2 large non-coding regions (799 bp and 768 bp) that were almost identical to each other. The completion of the mt genome offers the opportunity of defining new molecular markers for studying evolutionary relationships within and among gyrodactylid species.

“…The Gyrodactylus specialists M almberg and M almberg (1986) and Mo (1991) (reviewed recently in C unningham et al 2003) have always maintained that the rainbow trout farm type of G. salaris can be separated from other strains by morphology or morphometrics. As long as the type was recognized only in rainbow trout farms, this suggestion remained both true and unfalsifiable.…”

Section: Discussionmentioning

confidence: 99%

Escape from an evolutionary dead end: a triploid clone of Gyrodactylus salaris is able to revert to sex and switch host (Platyhelminthes, Monogenea, Gyrodactylidae)

2006

Diploid parthenogenesis, with rare sex, is considered as the basic mode of reproduction among the hermaphroditic and viviparous Gyrodactylus. A particular strain of the monogenean parasite Gyrodactylus salaris (RBT clone) was recognized by an invariable, unique mitochondrial DNA haplotype in rainbow trout (Oncorhynchus mykiss) farms. The RBT clone was shown to be triploid and asexual by analyzing a 493 bp sequence of a nuclear DNA marker. Three alleles were present as heterozygous in all 237 individuals sampled in years 2001-2005 from five isolated Finnish farms. The triploid clone probably originated from a diploid oocyte fertilized by a non-self hermaphrodite, most probably in a fish farm. Identical mitochondrial COI gene (1606 bp) was also found in G. salaris parasites on landlocked salmon (Salmo salar) in two rivers draining to the lake Kuitozero, Russian Karelia. In the river Pisto, the clone was triploid, but the diagnostic "short" nuclear allele of the RBT clone was replaced by an allele typical for salmon specific parasites in the Lake Onega. The clone in the river Kurzhma was diploid, having lost the "short" allele, but still heterozygous for the other two alleles of the RBT clone. Evidently, the triploid parthenogenetic RBT clone had produced diploid oocytes, when (as a female) stimulated by a non-self mate in the new environment. The genetic reorganization coincided with a switch to the salmon host. Participation of triploids into the gene pool of the species is rarely reported in animals, and the triploidy is generally considered as an irreversible dead-end of the evolution. Liberalism in ploidy level may significantly add to the evolutionary options available for a parasite in ever-changing environments.