Ultrastructural Evidence for Meiosis in a Nonheterothallic Isolate of the Myxomycete Didymium Iridis

Abstract

In a recent discussion of various aspects of the biology of the myxomycete Didymium iridis (Ditmar) Fries, Collins and Betterley (1982) considered the var? ious reproductive cycles of isolates of this species. According to these authors, of 43 isolates of D. iridis examined critically, only 12 have been reported to be heterothallic while 21 have been designated as nonheterothallic. While traditionally most workers have assumed nonheterothallic isolates to be homothallic (pos? sessing meiosis and karyogamy), Collins (1980) has stated that It now seems entirely possible that myxomycete life cycles are characteristically either hetero? thallic or apomictic, with homothallic cycles either rare or nonexistent. Most of the data supporting the idea of apomixis has, however, come from genetic studies and Feulgen nuclear DNA measurements made on myxamoebae, plasmodia, and young spores (see Collins and Betterley, 1982), and apparently no detailed attempt has been made to look for visual evidence of meiosis in thick-walled, maturing spores of supposed apomictic isolates. As a result, the present electron microscopic study was undertaken. Plasmodia ofthe nonheterothallic isolate (Mo-1) of D. iridis used in this study were kindly supplied by Dr. O. R. Collins ofthe University of California, Berkeley. This apparently apomictic isolate is reported not to have a haploid phase (Mulleavy, 1979). According to Collins (pers. comm.), this isolate has not exhibited the apomictic-heterothallic conversions reported in some isolates (Yemma et al, 1978; Mulleavy, 1979; Collins, 1980), even though attempts to convert it have been made. Single spore isolations done in conjunction with this ultrastructural study confirmed that the Mo-1 isolate remained nonheterothallic throughout the course of the investigation. Plasmodia ofthe Mo-1 isolate were maintained at 23 C on sterilized pieces of oat flakes on the surface of 2% plain agar plates. To induce fruiting, plasmodia were placed in total darkness for 3 da and then exposed to light. Sporangia in various stages of development (precleavage to 24 h past cleavage) were then prepared at hourly intervals for examination with transmission electron micros? copy according to the procedures of Glidewell and Mims (1979). The results of this study indicate that meiosis or at least an attempt at meiosis occurs in the Mo-1 isolate of D. iridis. The five necessary indicators of meiosis listed by Aldrich and Mims (1970) and reiterated by Aldrich and Blackwell (1976) were all observed in this study. These include (1) a precleavage nuclear division without synaptonemal complexes (SC's), (2) an intervening interphase with no synapsis, (3) SC's in the nuclei of spores 10-20 h old, (4) nuclear division figures within spores, and (5) a subsequent degeneration of nuclei within autophagic vacuoles.