Low genetic variation in the salmon and trout parasite Loma salmonae (Microsporidia) supports marine transmission and clarifies species boundaries

Abstract

Loma salmonae is a microsporidian parasite prevalent in wild and farmed salmon species of the genus Oncorhynchus. This study compared ribosomal RNA (rDNA) and elongation factor-1 alpha (EF-1α) gene sequences to look for variation that may provide a basis for distinguishing populations. Specimens were collected from laboratory, captive (sea netpen farm and freshwater hatchery) and wild populations of fish. The host range included rainbow trout O. mykiss, Pacific salmon Oncorhynchus spp. and brook trout Salvelinus fontinalis from British Columbia, Prince Edward Island, Canada, from California, Colorado, Idaho, USA and from Chile. Both loci suggested that a variant in S. fontinalis (named 'SV') was a separate species. This was supported by the absence of similar variants in the source material (isolated from laboratory-held O. tshawytscha) and high divergence (1.4 to 2.3% in the rDNA and EF-1α) from L. salmonae in the type host and locality (O. mykiss in California). L. salmonae from freshwater and anadromous Oncorhynchus spp. were distinguished, providing a basis on which to evaluate possible sources of infection and suggesting geographic boundaries are important. Higher genetic variation occurred among samples of freshwater origin and from a sea netpen farm in Chile, suggesting these environments may present greater population diversity. Invariance in rDNA sequence across 17 samples from anadromous salmon in rivers, lakes, ocean, farms and hatcheries supports the hypothesis that marine transmission occurs and effectively prevents population substructuring caused by freshwater transmission.KEY WORDS: Loma salmonae · Microsporidia · Oncorhynchus · Pacific salmon · Salmon farm · Salvelinus fontinalis · rDNA · EF-1 α Resale or republication not permitted without written consent of the publisherDis Aquat Org 91: [35][36][37][38][39][40][41][42][43][44][45][46] 2010 since the recent development of a vaccine against L. salmonae . Non-genetic studies have suggested there may be strains that differ in host preference (e.g. preferring coho and Chinook salmon to brook or rainbow trout) or showing different infection patterns in salmon versus trout (Bader et al. 1998, Shaw et al. 2000a,c, Sánchez et al. 2001b, Ramsay et al. 2002, Lovy et al. 2004, Kent & Speare 2005.The present study examined genetic variation across Loma salmonae from the wild, hatcheries, sea netpen farms and laboratories in British Columbia (BC) and Prince Edward Island (PEI), Canada, in California, Colorado, and Idaho, USA, and in Chile to establish the baseline level of variation that we might expect within this species and to look for genetic variants that may correspond to different populations. Genetic data from the type host, rainbow trout Oncorhynchus mykiss in California, is essential as this parasite species is morphologically indistinguishable across its geographic and host range (Morrison & Sprague 1983, Shaw et al. 1997, Brown & Kent 2002 and distinct strains or cryptic species may exist. For example, some data suggest L. salmonae...

Verification of Intraovum Transmission of a Microsporidium of Vertebrates: Pseudoloma neurophilia Infecting the Zebrafish, Danio rerio

“…Loma salmonae is a microsporidian pathogen of salmon and trout, and has been observed in the ovigerous stroma, but not eggs [20]. The translocation of Loma salmonae to fish farms in Chile which had been populated using only surface decontaminated, fertilized eggs is also suggestive of vertical transmission [20], [21].…”

Expansion of the Known Host Range of the Microsporidium, Pseudoloma neurophilia

“…This can especially occur in microparasites that have few morphological characters upon which to base speciation. For example, Loma is a microsporidian genus with relatively narrow host specificity, 12 with Loma salmonae only infecting Pacific salmon Oncorhynchus spp. and not other salmonids, such as Salmo spp.…”