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Abstract
BackgroundMicrocell parasites are small intracellular protozoans mostly detected in molluscs and can be associated with mortalities. In 2010 and 2011, strong increases in mortality events were reported in different wild beds of the wedge clam Donax trunculus Linnaeus, along the Atlantic coast of France and the presence of potential pathogens, including microcells, was investigated.MethodsClams collected in different beds showing mortality were examined by histology. Based on histological observations, confirmatory analyses were carried out, including transmission electron microscopy (TEM) and molecular characterization.ResultsHistological analyses revealed the presence of small protozoans similar to microcell parasites in different tissues of Donax trunculus, particularly in muscular and connective tissues. TEM examination confirmed the intracellular localization of the protozoans. Moreover, the lack of haplosporosomes and mitochondria suggested that the observed parasites belong to the genus Mikrocytos Farley, Wolf & Elston, 1988. Mikrocytos genus-specific PCR and in situ hybridization results supported the microscopic observations. Sequence fragments of the 18S rRNA gene shared 75–83% identity with the different Mikrocytos spp. described previously, including Mikrocytos mackini Farley, Wolf & Elston, 1988 and M. boweri Abbott, Meyer, Lowe, Kim & Johnson, 2014. Phylogenetic analyses confirmed that the microcell parasites observed in Donax trunculus in France belong to the genus Mikrocytos and suggest the existence of two distinct species.ConclusionsBased on morphological, ultrastructural, molecular data and host information, the two microcell parasites detected in Donax trunculus belong to the genus Mikrocytos and are distinct from previously described members of this genus. This is the first report of Mikrocytos spp. found in France and infecting the clam Donax trunculus. Mikrocytos veneroïdes n. sp. was detected in different wild beds and Mikrocytos donaxi n. sp. was detected only in Audierne Bay.
Highlights
Microcell parasites are small intracellular protozoans mostly detected in molluscs and can be associated with mortalities
Based on morphological, ultrastructural, molecular data and host information, the two microcell parasites detected in Donax trunculus belong to the genus Mikrocytos and are distinct from previously described members of this genus
This is the first report of Mikrocytos spp. found in France and infecting the clam Donax trunculus
Summary
Microcell parasites are small intracellular protozoans mostly detected in molluscs and can be associated with mortalities. The protozoan Mikrocytos mackini was first reported in 1961 [8] and was validly described and named in 1988 [9] This parasite was responsible for Denman Island disease in the wild and cultured Pacific oyster Crassostrea gigas Thunberg, 1793 in British Columbia, Canada, where it induced severe oyster mortalities [8]. It could naturally affect other oysters, such as the Olympia oyster Ostrea lurida Carpenter, 1864 [10] or the Kumamoto oyster Crassostrea sikamea Amemiya, 1928 [11]. Because of its potential impact on oyster farming, this parasite is regulated in Europe and surveillance is implemented in different European countries in order to prevent its introduction
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