MEIOTIC BEHAVIOR OF THE MINICHROMOSOME IN THE PHYTOPATHOGENIC ASCOMYCETE LEPTOSPHAERIA-MACULANS

All Leptosphaeria maculans field isolates displayed a minichromosome ( MC) clearly separated from the overall electrokaryotype following puls ed-field gel electrophoresis. MCs exhibited a length polymorphism rang ing from 650 to 950 kb. Tetrad analyses revealed the parental inherita nce of MC length polymorphism (50% of the tetrads) or else the generat ion of novel-sized MCs (27%), which suggested that recombination occur red between MCs. Nineteen percent of the tetrads displayed a lack of t he MC band in the electrokaryotype for one or two of the four resultin g genotypes. Crosses between isolates carrying or lacking MCs revealed non-Mendelian segregation and suggested that some isolates could disp lay at least two copies of the MC. Only repeated sequences hybridising to all chromosomes were isolated from the MC, Finally, saprophytic or parasitic fitness was not modified when isolates apparently lacked th e MC. All these data suggested that the L. maculans MC behaves like a 'B' chromosome.