Studies on the Colonia isolate of Physarum polycephalum.

Physarum polycephalum is a Myxomycete or true slime mould, the life cycle of which alternates between two growing phases; amoebae and plasmodia. Strains of the Colonia isolate of Physarum polycephalum form plasmodia within single clones of amoebae; this is in contrast to strains of other isolates which require the sexual fusion of two amoebae of different mating type for plasmodium formation to occur. The mode of plasmodium formation by Colonia strains enables them to be used for the isolation of mutant plasmodia after anoebal mutagenssis and direct sereening of plasmodia. This thesis reports the derivation of a Colonia strain, CL, (Colonia Leicester) for mutant isolation purposes together with the construction of isogenic strains of heterothallic mating type. These strains have been used to isolate and genetically analyse strains carrying auxotrophic mutations which can only be tested for in the plasmodial stage. Attempts have also been made to devise selective techniques for the isolation of strains carrying specific classes of mutation, in particular a strain deficient in thymidylate synthetase activity. A series of investigations employing microdensitometric estimation of nuclear DNA content, time-lapse microcinematography and genetic methods has indicated that plasmodium formation by Colonia amoebae is probably apogamous and not homothallic as was previously reported by other workers.