Evidence for Mitotic Crossing-over During the Parasexual Cycle of the Cellular Slime Mould Dictyostelium discoideum

SUMMARY: The genes conferring white fruiting bodies (whi), growth-temperature sensitivity in strain x9 (tsgD12) and resistance to methanol or acriflavine (acrA1) are linked in a group designated chromosome II. A gene, present in strain ax2 and involved in the ability of this strain to grow axenically (axe2), has also been located on chromosome II. An analysis of diploid strains derived from a diploid initially heterozygous for all known genes of chromosome II has revealed that mitotic crossing-over occurs during the vegetative growth of diploid cells. An analysis of the classes of diploid thus isolated leads to a provisional ordering of the genes on chromosome II: centromere, whi, axe2, tsgD12, acrA1. The final element necessary to establish the existence of a parasexual cycle in Dictyostelium discoideum has thus been demonstrated.