THE COLOR OF MYXOMYCETE PLASMODIA

ALTHOUGH THERE iS considerable evidence in the literature on myxomycetes that plasmodia of some species exhibit consistency of color on different substrata, there are also many conflicting reports concerning the plasmodial color of other forms. The Lister monograph (1925) and, to a much less extent, Macbride and Martin (1934) use plasmodial color as a taxonomic character. Pinoy (1908) postulated sexual dimorphism in Didymium nigripes on the basis of the occurrence of differently colored plasmodia. Skupienski (1934) reported a thermophilic race of Didymium xanthopus which is distinguishable, in part, by a white plasmodium. A. Lister (1888) was one of the first to describe a color change due to the ingestion of colored particles by a plasmodium. Cadman (1931) and Gray (1938) mention the same phenomenon. Color changes due to bacterial associates have been described by Pinoy (1903) (1915), Skupienski (1920), and Raper (1937). If plasmodial color is to hold any place as a taxonomic character or as an aid in distinguishing strains and races, information concerning the factors which influence color is essential. This paper reports observations on plasmodia grown in the laboratory, with particular reference to color variations caused by certain bacterial associates. MATERIALS AND METHODS.-The plasmodia used in the experiments described were collected in the field or obtained from spore sowings. Both sclerotia and active plasmodia were collected and placed in moist chambers in the laboratory. Cultures obtained from spores were started by sowing spores in distilled water in Syracuse watch glasses. The watch glasses were left on the laboratory table and no precautions taken to protect the swarm-cells from light. They were kept covered by another watch glass to reduce the risk of contamination by another myxomycete. The spores were observed from time to time with either a low power objective or a water immersion lens and after germination were transferred to nutrient agar with a clean pipette. The chief advantage of sowing spores in water rather than directly on the nutrient agar is that viability of the spores can be determined in water and that failure to obtain plasmodia because of non-viable spores may be ascertained. A further advantage is -that growth time on agar is shortened, and thus the amount of contamination is reduced. A 2.5 per cent carrot decoction agar proved to be a very satisfactory nutrient medium for obtaining plasmodia from spores. The amount of available nutrient material in this medium seemed to be small enough to discourage excessive mold growth and yet great enough to support the growth of a bacterial film over the surface of the agar. This nutrient agar was pre'Received for publication April 10, 1939. The writer expresses his appreciation to Prof. G. W. Martin for suggestions and criticisms during the progress of the work and in the preparation of this paper. pared by boiling the carrots for one and a half to two hours. Then agar was added, the carrot particles were removed by filtration, the pH adjusted to approximately 7, and the medium was tubed and autoclaved. Serratia marcescens, Chromobacterium violaceuni, and Sarcina lutea, grown on beef-peptone agar, and Phytomonas fascians2 grown on potato-dextrose agar were used as plasmodial associates. These organisms produced red, violet, yellow, and cadmium yellow pigments, respectively. In experiments performed with plasmodia collected in the field the plasmodia were first obtained in active condition upon plain agar plates and the bacteria added as a suspension in distilled water. Plasmodia obtained from spore sowings were kept on nutrient agar, and the bacterial suspensions were added. All cultures were maintained in the diffuse light of the laboratory at room temperatures which varied from 20?C. to 270C. The arrangement and nomenclature which follows is that of Macbride and Martin. DETAILED OBSERVATIONS.-Records of several Fuligo septica plasmodia collected in the field, indicate a variability in plasmodial color in this species. These plasmodia fruited and were positively identified. One specimen growing on apple bark had an ocher-gray plasmodium when collected, but transplants to plain agar plates yielded both white and yellow plasmodia. The yellow forms tended to become white in the laboratory. A different specimen produced only white transplants. Another plasmodium growing on very rotten wood was bright yellow when collected. Three days later part of this collection on plain agar was snow white. On the same day one culture plate containing wood on plain agar showed a greenish yellow plasmodium, and part of the same plasmodium in another Petri dish on plain agar was light yellow and "frothy." Six days later this light yellow plasmodium had fruited. Transplants of all three collections were subjected to suspensions of Serratia marcescens and always became pink within a few hours. None of these completed its life cycle, and sclerotia were formed in every case. Suspensions of Sarcina lutea produced no noticeable color change but the plasmodia remained active slightly longer than with Serratia. White plasmodia of Fuligo septica have been obtained from spore sowings on 2.5 per cent corn agar, but none grew to large size nor completed its life cycle in culture. Physarum polycephalum.-Since it was possible to collect the plasmodia of this species in the field, no attempts were made to obtain them from spore sowings. Several plasmodia found growing on old 2 Tilford, P. E. 1936. Fasciation of sweet peas caused by Phytomonas fascians n. sp. Jour. Agric. Res. 53: 383394. Culture obtained from Dr. E. M. Hildebrand of Cornell University.