Mutants affecting plasmodium formation in a homothallic strain of Physarum polycephalum.

A re-investigation of the strain Colonia of the Myxomycete Physarum polycephalum showed it to be homothallic. All amoebae can give rise to plasmodia within clones. Using genetic analysis it was shown that this ability was due to an allele of mating-type (mt), mt, which allowed (1) crossing of amoebae both carrying mt, (2) outcrossing with amoebae carrying heterothallic mating-types, Micro-spectrophotometric measurements on amoebal and plasmodial nuclei showed a change in DNA content consistent with a change in ploidy. Both results independently eliminate apogamy as an explanation for these results. Many interesting observations were made on amoebal growth during progress towards synthesising an axenic medium for amoebae. New mutagenic techniques devised to mutagenise amoebae all failed to increase mutagenic rate. An independent check on the work of Haugli using his method confirmed that (1) mutation rate could be enhanced from 10 to 100 fold by ultraviolet light with the synergistic effect of caffeine, (2) the maximum expression of mutations under these conditions occumed after 30 hours. Using this method 4 mutants affecting the amoebal-plasmodial transition were isolated. These amoebae fail to undergo the necessary developmental switch and do not produce plasmodia in clones. Genetic analysis showed them to be recessive, functionally different and, in the detailed analysis of one of these mutants, freely recombining with all other genetic markers including mt. This groundwork showed the feasibility of genetic analysis of a developmental process in great detail. A review of developmental genetics was made. The processes of cell fusion, nuclear fusion and mitosis in other organisms was discussed. Cell fusion seems to be a rare but important biological event. Nuclear fusion of interphase nuclei is restricted to gametic cells. All syncytial cells have a closed mitosis but not all cells with a closed mitosis are syncytial. The relevance of these processes to the amoebal-plasmodiel transition in other Myxomycstss was discussed.