HETEROTHALLISM AND HOMOTHALLISM: A STUDY OF 27 ISOLATES OF DIDYMIUM IRIDIS, A TRUE SLIME MOLD'

Single pore studies of 27 isolates of Didymium iridis from several geographical locations revealed that 11 are heterothallic, displaying a one-locus, multiple allelic mating system. Collectively, 9 of the 11 possess a total of 12 mating types in one multiple allelic series. Each of the remaining two isolates has a pair of mating types which are either incompatible with the 12 or they have not yet been tested against them. Sixteen isolates proved to be homothallic. THE MAIN PURPOSE of this paper is to report data obtained from an extensive survey of mating systems in isolates of Didymium iridis originating from several geographical locations. These data contribute to a better understanding of the evolution of mating systems and to a refinement of the species concept in the Myxomycetes. Moreover, as a result of this work several new isolates of known laboratory behavior are now available for future comparative studies. Existence of heterothallism in Didymium iridis was first reported by Collins (1961). Later, it was indicated that mating types of different isolates of the species belong to a multiple allelic series (Collins, 1963). These studies led to the use of D. iridis in a number of laboratory investigations. Consequently, it has become well-known to myxomycete researchers. The present work is the first in a series of comparative studies in the species which will focus upon plasmodial pigment composition, spore morphology at the scanning EM level, DNA cycle in heterothallic and homothallic isolates, and additional genetic studies. MATERIALS AND METHODS-The 27 isolates of D. iridis originated from five foreign countries and six states of the U.S.A. (Table 1 and 4). Sporangia were either collected directly from the field or they were allowed to develop on plant debris in the laboratory using the moist chamber technique of Gilbert and Martin (1933). In either case, fresh specimens containing viable spores served as the starting material for the present study. Each isolate was tested for capacity to complete its life cycle in agar culture. This was done by plating out the spore contents of a whole sporangium on half-strength corn meal agar and observing the cultures periodically to determine I Received for publication 2 August 1974. Supported in part by a National Science Foundation grant, GB-40149, the Miller Institute for Basic Research and a biomedical research grant. I express special thanks to Hsi-Chang Tang and Janelle Chiu for laboratory assistance with this work. performance. When new sporangia developed, they were assigned laboratory code names and spores from these were subsequently analyzed to determine whether or not the isolates displayed mating types. Those which did were designated as heterothallic, while all others were labelled homothallic. The possibility that at least some of the latter may represent non-sexual forms is not ruled out, but in the absence of unequivocal evidence for this in the specimens involved, homothallism is assumed. Samples of the 27 specimens are stored in my laboratory. Details of the life cycle of heterothallic forms of D. iridis have been summarized recently (Clark and Collins, 1973), and earlier the author (Collins, 1961, 1963, 1969a, 1973) described techniques for making mass-spore and single-spore cultures of the organism and for inducing sporulation (Ling, 1968; Collins, 1969a, 1973). Therefore, the diplohaplontic life cycle and techniques will not be described in detail here, but the outline below explaining the life cycle and filial designations is provided to insure that there is no confusion regarding genetic terminology. In addition, a photograph of a typical plasmodium and one showing sporangia are presented (Fig. 1, 2). sporulation PARENT PLASMODIUM > PARENT SPORES (2N) (N) single-spore cross P ARENT CLONES F F1 PLASMODIUM isolations (N) (2N) sporulation snl-pr i F1 SPORES single-spore F1 CLONES (N) isolations (N) RESULTs-Heterothallic isolates-Of the 27 isolates investigated, 11 have been shown to be heterothallic. Mating types of four of these have been referred to in previous publications (Collins, 1961, 1963, 1969b) while the remaining seven are being studied for the first time. Because an