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

Marek S Ziętara,Jussi Kuusela,Jaakko Lumme

doi:10.1111/j.2006.0018-0661.01956.x

Marek S Ziętara, Jussi Kuusela + Show 1 more

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https://doi.org/10.1111/j.2006.0018-0661.01956.x

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Journal: Hereditas	Publication Date: May 22, 2006
Citations: 29	License type: cc-by

Affiliation: University of Oulu, University of Gdańsk

Abstract

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.

Highlights

Diploid parthenogenesis, with rare sex, is considered as the basic mode of reproduction among the hermaphroditic and viviparous Gyrodactylus
Many forms of parthenogenesis are correlated with elevated ploidy, and the clones are usually considered evolutionary dead-ends: the meiosis of triploids never succeeds, and the clone is excluded from the gene pool and further evolution (BELL 1982; SUOMALAINEN et al 1987)
The internal transcribed spacer (ITS) regions of the nuclear ribosomal genes are nearly uniform in parasites on different hosts, grayling (Thymallus thymallus ), salmon (Salmo salar ), Arctic char (Salvelinus alpinus ) and rainbow trout (Oncorhynchus mykiss ) (CUNNINGHAM 1997; CABLE et al 1999; ZIE TARA and LUMME 2002; ZIE TARA et al 2002; LINDENSTRØM et al 2003)

Summary

Introduction

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. 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. Many forms of parthenogenesis are correlated with elevated ploidy, and the clones are usually considered evolutionary dead-ends: the meiosis of triploids never succeeds, and the clone is excluded from the gene pool and further evolution (BELL 1982; SUOMALAINEN et al 1987). Escape from an evolutionary dead-end 85 were causing the Atlantic Ocean and White Sea gyrodactylosis epidemics (MEINILAet al. 2004)

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