SYNGAMY AND PLASMODIUM FORMATION IN THE MYXOGASTRES

Abstract

IT IS GENERALLY acknowledged that the plasmodium of the Myxogastres is initiated by the fusion of motile gametes. Since Wilson and Ross (1955) demonstrated that meiosis in the Myxogastres occurs just before spore formation in the maturing sporangium, there is no doubt that the spores and the resulting swarm cells and myxamoebae are haploid. The positions of syngamy, karyogamy and plasmodium formation in the life cycle, however, are still unsettled. Reports of previous investigators on these aspects of the life cycle have presented conflicting evidence and no interpretation made up to the present has been wholly accepted. Martin (1940), in his review of the Myxomycetes, states that even though it is generally agreed that the swarm cells and myxamoebae are haploid, the position of nuclear fusion in the life cycle is in doubt, and that it is difficult to present a generalized account of the nuclear cycle of the Myxomycetes. Skupienski (1920, 1926) and Schiinemann (1930) believed that two to many myxamoebae fused to form a plasmodium, but that karyogamy was delayed until later in the development of the plasmodium. Von Stosch (1935), however, reported that syngamy was between flagellate swarm cells, not between myxamoebae. He stated that Didymium squamulosum, D. xanthopus, and Physarum cinereum were apogamous, since he had not observed fusion of swarm cells in these species. M. Wilson and Cadman (1928) described the fusion of swarm cells in Reticularia Lycoperdon and noted that karyogamy followed immediately after plasmogamy. They also found that several swarm cells would fuse with a zygote, after which the excess nuclei apparently disintegrated. Howard (1931) also reported the fusion of swarm cells and subsequent karyogamy in Physarum polycephalum. Much of the disagreement of previous investigators has been caused by the conflicting results obtained from studying different species. Most of the investigators have concentrated on members of the Physarales; Reticularia is one of the few genera belonging to other orders. The present study was undertaken in the belief that a more comprehensive and accurate picture of the life cycle could be obtained by using members of several families.