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Background  Bistability and hysteresis are increasingly recognized as major properties of regulatory networks governing numerous biological phenomena, such as differentiation and cell cycle progression. The full scope of the underlying molecular mechanisms leading to bistability and hysteresis remains elusive.  Nectria haemaotcocca , a saprophytic or pathogenic fungus with sexual reproduction, exhibits a bistable morphological modification characterized by a reduced growth rate and an intense pigmentation. Bistability is triggered by the presence or absence of σ, a cytoplasmic determinant. This determinant spreads in an infectious manner in the hyphae of the growing margin, insuring hysteresis of the differentiation.    Results  Seven mutants specifically affected in the generation of σ were selected through two different screening strategies. The  s  1  and  s  2  mutations completely abolish the generation of σ and of its morphological expression, the Secteur. The remaining five mutations promote its constitutive generation, which determines an intense pigmentation but not growth alteration. The seven mutations map at the same locus,  Ses  (for 'Secteur-specific'). The  s  2  mutant was obtained by an insertional mutagenesis strategy, which permitted the cloning of the  Ses  locus. Sequence and transcription analysis reveals that  Ses  is composed of two closely linked genes,  SesA , mutated in the  s  1  and  s  2  mutant strains, and  SesB , mutated in the  s * mutant strains.  SesB  shares sequence similarity with animal and fungal putative proteins, with potential esterase/lipase/thioesterase activity, whereas  SesA  is similar to proteins of unknown function present only in the filamentous fungi  Fusarium graminearum  and  Podospora anserina .    Conclusions  The cloning of  Ses  provides evidence that a system encoded by two linked genes directs a bistable and hysteretic switch in a eukaryote. Atypical regulatory relations between the two proteins may account for the hysteresis of Secteur differentiation.

Bistability and hysteresis of the 'Secteur' differentiation are controlled by a two-gene locus in Nectria haematococca.