Needle in a haystack: involvement of the copepod Paracartia grani in the life-cycle of the oyster pathogen Marteilia refringens

Abstract

Marteilia refringens is a major pathogen of the European flat oyster, Ostrea edulis Linnaeus. Since its description, the life-cycle of this protozoan parasite has eluded discovery. Attempts to infect oysters experimentally have been unsuccessful and led to the hypothesis of a complex life-cycle involving several hosts. Knowledge of this life-cycle is of central importance in order to manage oyster disease. However, the exploration of M. refringens life-cycle has been previously limited by the detection tools available and the tremendous number of species to be screened in enzootic areas. In this study, these two restrictions were circumvented by the use of both molecular detection tools and a mesocosm with low biodiversity. Screening of the entire fauna of the pond for M. refringens DNA was systematically undertaken using PCR. Here, we show that the copepod Paracartia (Acartia) grani is a host of M. refringens. Not only was DNA of M. refringens consistently detected in P. grani but also the presence of the parasite in the ovarian tissues was demonstrated using in situ hybridization. Finally, successful experimental transmissions provided evidence that P. grani can be infected from infected flat oysters.

Marteiliosis in molluscs: A review

“…Basically, paramyxeans have various effects on their host via two major target systems, digestive and reproductive, which we wish to review here. Interestingly, recent investigations on Marteilia refringens show that the digestive gland in the oyster and ovary in the copepod are successively both targeted by the parasite during its life cycle (Audemard et al 2002).…”

“…Marteilia refringens also displays a tropism for ovarian tissue in its copepod host, Paracartia grani, during its life cycle (Audemard et al 2002). Observations were made on female juvenile (copepodid) and adult stages, while M. refringens was never detected in male copepods.…”

“…Infections of the ovaries in copepods by microsporidian parasites has been described (Andreadis 1988;Micieli et al 2000) which apparently do not seriously alter normal metabolic processes, although no experimental data are available in the case of Marteilia refringens. In Paracartia grani, ovarian tissue was frequently overwhelmed by M. refringens in the form of large numbers of small cells within the ovocytes (Audemard et al 2002). This apparent large number of parasitic cells produced in the copepod may increase the probability for the parasite to encounter the next host during its life-cycle.…”

“…In numerous papers published on this paramyxean parasite and on M. sydneyi from Australia, the existence of a complex lifecycle had been postulated, although its transmission routes and life-cycle was unknown and until now remains uncertain (Balouet 1979;Balouet et al 1979;Lauckner 1983;Grizel 1985;Figueras and Montes 1988;Berthe et al 1998;Kleeman and Adlard 2000;Audemard et al 2000Audemard et al , 2002.…”

Validation of in situ hybridisation and histology assays for the detection of the oyster parasite Marteilia refringens

“…It has been a cause of mass mortality in Europe (Grizel 1985), and continues to strongly impede the production of flat oysters. The geographical distribution of the disease ranges from a northern limit in western Brittany on the French Atlantic coast to a southern limit identified as being the Mediterranean Sea (Audemard et al 2002). In Australia, Marteilia sydneyi is responsible for QX disease in the Sydney rock oyster Saccostrea (commercialis) glomearata (Perkins & Wolf 1976).…”