CONTROL OF PLASMODIAL FUSION IN A PANAMANIAN ISOLATE OF DIDYMIUM IRIDIS

Plasmodial fusion in a Panamanian isolate (Pan. 1) of Didymium iridis is controlled by at least six fusion -loci. Only plasmodia with identical phenotypes for these six loci are capable of fusion. No evidence was obtained in this study to indicate that any one fusion locus more strongly affects the fusion process than any of the other loci. Control of plasmodial fusion in the Pan. 1 strain of D. iridis is similar to that found earlier for a Honduran strain (Hon. 1). THE PURPOSE of this report is to present a genetic basis for control of plasmodial compatibility in a Panamanian isolate (Pan. 1) of Didymium iridis (Ditmar) Fries. This is the third report of a series of studies which we intend to carry out on plasmodial compatibility in D. iridis. The first two reports were on a Honduran (Hon. 1) strain of D. iridis (Collins, 1966; Collins and Clark, 1968). Currently, there is considerable interest in the study of somatic or vegetative compatibility (Ebert, 1965; Palm, 1965; Wilson and Garnjobst, 1966; Collins and Clark, 1968). The plasmodial stage of many of the myxomycetes is well suited to asexual compatibility studies, since plasmodia are motile, coenocytic structures. Upon touching one another, compatible plasmodia fuse, whereas incompatible ones do not. The unique life cycle of a slime mold is shown in Fig. 1. In heterothallic forms, such as in certain isolates of Didymium iridis, spores from any sporangium are of two mating types, e.g., Al and A2 (Collins, 1961, 1963). A single isolated spore germinates by releasing one myxamoeba. Such an amoeba gives rise to a population of millions of identical haploid cells called a clone. Bacteria are usually supplied as a food source for myxamoebae in the laboratory. If free water exists, myxamoebae develop flagella and become swarm cells. Under drier conditions, swarm cells withdraw their flagella and resume amoeboid form. As cultures grow older or if there is a lack of moisture, myxamoebae encyst. These uninucleate microcysts germinate to form myxamoebae if plated out on fresh media. 1 Received for publication 24 June 1969. Part of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Wayne State University. the authors gratefully acknowledge financial support from a Wayne State UniversityFellowship and from N.S. F. Grants GB-5275 and GB-15770. The photographic assistance of Cecil B. King is also due special acknowledgment. 2 Present address: Department of Biological Sciences, University of Delaware, Newark. 8 Present address: Department of Botany, University of California, Berkeley. When cells from an A' clone are mixed with some from an A2 clone, zygotes are formed which develop into diploid plasmodia. Meiosis takes place as the plasmodium is transformed into sporangia. Under unfavorable conditions plasmodia may also form resistant structures called sclerotia which are composed of numerous macrocysts, each containing several nuclei. When favorable conditions return, sclerotia develop into plasmodia. In D. iridis it is known that multiple alleles exist at the mating locus, A. With regard to this locus, only heteroallelic crosses are fertile. (Collins, 1961, 1963; Collins and Ling, 1964). Brown and cream plasmodial color in D. iridis is under the control of two apparently nonlinked loci (Collins and Clark, 1966a; Collins, W6S), each with alternate alleles: bl, bl+; b2, b2 . In the strain used here, however, the b2 locus is occupied exclusively by wild type alleles. Color, therefore, depends only on alleles at the bl locus, i.e., bl+ bl+ and bl+ bl result in brown plasmodia, while bl bl yields cream plasmodia. Plasmodia can be kept in the vegetative condition 2-6 months and in some cases much longer. In addition, since the haploid myxamoebae, which act as gametes, may be maintained indefinitely in the laboratory, genetically identical plasmodia may be recreated over a span of many years. A combination of such advantages is lacking in other organisms in which asexual compatibility has been studied. REVIEW OF THE LITERATURE-Observations on plasmodial fusion extend back to some of the earliest work on slime molds. De Bary in 1887 stated that neither he nor his student, Cienkowski (1862), had ever observed plasmodia of different species fusing with one another. However, plasmodia of the same species may fuse (de Bary, 1887). Massee's (1892) report that the plasmodia of different species may fuse to form hybrids stands alone in the literature. Accordlng to Massee, plasmodia of Physarum leucopus and Physarum