Parasites Chromidina et Dicyémides des sacs rénaux de Céphalopodes : évaluation de leur biodiversité morphologique et moléculaire

Abstract : The renal sacs of cephalopods are an uncommon habitat for two phylogenetically distant parasites, dicyemid mesozoa and ciliate chromidinids. These are wormlike organisms, which can reach a few millimetres in length and are attached to the renal epithelia by their anterior part. Information was mainly based on morphological criteria and little is known about the biodiversity and taxonomy of these enigmatic parasites. Concerning the chromidinids, only 4 morpho-species have been described and there are no molecular data available. Dicyemids have been a little more studied, but morphological criteria are still incomplete for some species and not all life stages have been studied. The molecular data are sparse and the phylogenetic position of these organisms is uncertain.During this thesis, we have undertaken to study these two groups with regard to their biodiversity in cephalods from the Mediterranean Sea and the Atlantic Ocean. The aims are to obtain more elements for the comprehension of their systematics and phylogeny. Starting from a large sample of cephalopods from the Atlantic Ocean (Concarneau, France) and the Mediterranean Sea (La Goulette, Tunisia), fished during 3 consecutive years, we characterized the parasites from a morphological and molecular point of view. We realised a molecular analysis of the 18S rDNA marker in order to look at the notion of species, to precise their host specificity, to correlate the morphological and genetic diversity and to measure the genetic variability within one morpho-species.In our samples, the prevalence of the infection by chromidinids was revealed to be weak, probably biased by the position of the collection in the water column. The sampled hosts were predominantly benthic, while chromidinids mainly infect pelagic cephalopods. We predict that the diversity is underestimated, even if we were able to describe two new species. We obtained for the first time molecular data from the 18S rDNA marker. The robust phylogenetic analyses show that chromidinids are positioned within the apostome ciliates, belonging to the Oligohymenophorea.In our samples, the prevalence of the infection by dicyemids was shown to be more important (88%). A major problem in the dicyemid systematics is that a large number of named morpho-species are based on incomplete morphological descriptions, entailing ambiguities in their identification. We therefore redescribed all stages of the development of the species Dicyemennea eledones of our samples in detail. Phylogenetic analyses of the 18S rDNA marker sequences of our samples, combined with the sequences available in databases, allowed us to define 10 robust clades. The morpho-species are not distributed in a non ambiguous manner within these clades, as some can be found in different clades and some clades can regroup more than one morpho-species. This leads to the question of the relevance of the 18S rDNA marker for the identification of species and even genera. It also challenges the relevance of morphological criteria currently used for the dicyemid systematics.
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Dhikra Souidenne. Parasites Chromidina et Dicyémides des sacs rénaux de Céphalopodes : évaluation de leur biodiversité morphologique et moléculaire. Microbiologie et Parasitologie. Museum national d'histoire naturelle - MNHN PARIS; Université de Carthage (Tunisie), 2016. Français. ⟨NNT : 2016MNHN0009⟩. ⟨tel-01781044v2⟩

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