Negative sentences are considered to be marked vis-a-vis their positive counterparts. However, the markedness of sentence negation cannot be solely defined in terms of the presence or absence of a polarity particle, as shown for Gwenedeg, a Breton dialect spoken in south central Brittany, in the area known as Morbihan. Gwenedeg Breton has been ignored in theoretical works because of its low prestige and its phonological differences, which are reflected in its own spelling system respected in this work. Breton, a verb-second language (V2), displays both negative and positive sentence particles. The markedness of sentence negation is realized rather by structural and semantic/pragmatic asymmetries. Structural asymmetries (chapter two) are associated with Breton V2. They relate to the notion of Predicate Domain, which must be bound. Whereas the negative particle binds the predicate domain, its positive counterparts do not. Hence preverbal noun phrases (NPs) serve to bind the predicate domain in affirmative but not in negative sentences. Two of the three preverbal positions available in affirmative sentences remain accessible in negative sentences. Semantic/pragmatic asymmetries (chapter three) pertaining to the V2 order relate to referentiality. In Breton, referential NPs can bind the predicate domain and appear preverbally while non-referential NPs marked by the preposition ag 'of' cannot. Potential binders for the predicate domain depend also on auxiliary selection. The auxiliary 'to be' associated with states shows, in the present tense, four forms demanding subject or non-subject binders. They are sensitive to the position and definiteness of their subjects and two of them do not occur in negative sentences. The auxiliary 'to have', associated with events, demands a referential subject and has no preferred binders. However, this auxiliaryis used with eventive readings of state predicates obtained obtained only with referential subject. In negative sentences (chapter four), semantic asymmetries relate to aspect--event predicates are interpreted as stative--, and to the irrealis modality,-- indefinite NPs are interpreted as non-referential under the scope of negation. In Breton, this rule applies to the universal quantifier with a wide scope reading and to the existential quantifier with a narrow scope reading, being replaced in negative sentences by negative polarity items. Non-referential NPs marked by ag, which represent undefined substes of entities, must occur in postverbal position. Pragmatic asymmetries relate to the distinction presupposition versus assertion, and to metalinguistic negation, a marked kind of negation, which does not affect the aspect of event predicates nor the referentiality of NPs under its scope. Hence the universal quantifier with a wide scope reading, the existential quantifier with a narrow scope reading, and the eventive reading of state predicates can occur under the scope of metalinguistic negation. This analysis is extended to other languages.