GROWTH AND DEVELOPMENT OF THE MYXOMYCETE PERICHAENA VERMICULARIS. II. CHROMOSOME NUMBERS AND NUCLEAR CYCLES

Chromosome counts of dividing nuclei of the myxomycete Perichaena ver'micutlaris indicate a number of 25 ?z 2 in the amoebae and 50 4? 4 in the plasmodia, confirming earlier reports that amoebae are haploid and plasmodia diploid. Chromosome numbers obtained from nuclei durinlg sporangial development indicate a fluctuation in the location of meiosis influenced by environmental conditions. The implications of these observations are discussed in reference to past conflicting evidence of the location of meiosis. IN A PREVIOU,S PAPER (Ross, 1967a), the cultivation and vegetative nuclear divisions in living amoebae and. plasmodia of the myxomycete Perichaena vermicularis were described. Subsequent to that work, chromosome numbers were obtained during three phases of the life cycle, amoebae, plasmodia, and sporangia. These chromosome counts indicate a nuclear behavior pattern not gen.erally ascribed to the MIyxomycetes, but the pattern may form a basis for reconciling various conflicting reports of nuclear behavior in these organisms. MVIATERIALS AND METHODS-Amoebae and p asmodia of Perichaena vermicularis (Schw.) Rost. were grown in petri dishes on steer dung-oatmeal extract-agar srmeared with Escherichia coli and in microcultures as described previously (Ross, 1967a). The slime mold was grown in a room programmed for 14 hr light and 10 hr dark, with the temperature fluctuating from 29 C during the light cycle to 18 C during the dark cycle. Chromosorne numbers of amoebae and plasmodia were obtained from growing cultures that were actively feeding on the bacteria. Since the techniques for barvesting and examining slime molds have been. given in some detail in previous papers (Ross, 1.966, 1967a), they will not be repeated here. Maturing sporangia (plasmodiocarps) were harvested by removing small portions at 10-min intervals from the earliest recognizable stages of fruiting to the final maturation of the spores. Photographs were made on Kodak High Contrast Copy 35-mm film, through a Zeiss WL microscope e(quipped with phase-contrast optics. I Received for publication 7 February 1967. Supported by grant GB-4396 from the National Science Foundation. The technical assistance of Mrs. Frances Shrader is gratefully acknowledged. RESULTS AND OBSERVATIONS-Vegetative stages -The mature spores of P. vermicularis from all fruitings used in this study were uninucleate prior to germination (Fig. 1). The majority of spores (ca. 75 %) released a single amoeboid uninucleate protoplast on germination (Fig. 2); the rest divided just before or just after the spore wall cracked to release two amoeboid cells. Under appropriate conditions the amoebae would become flagellate for short periods (Ross, 1967a), but in the absence of free water on the agar they remain amoeboid. Cytological studies on rapidly growing populations of amoebae revealed a uniform chromosome count of 25 4 2 (Fig. 3, 4, 5) with no variations from this number in any of the 200 mitotic metaphases counted. The chromosome counts of plasmodial nuclei were obtained from several plasmodia, all of which were derived from different fruitings. Since mitosis in a plasmodium is essentially synchronous when it occurs, each slide bearing mitotic nuclei contained hundreds of nuclei in approximately the same stage of division and counts were made of 50 nuclei per plasmodium. Again the counts were uniform for all nuclei examined, 50 i 4 (Fig. 6, 7), the range representing the degree of accuracy obtainable when counting chromosomes of this size. Fruiting stages-The first morphological indication of the onset of fruiting is the gradual condensation of the transparent plasmodial strands into a pale pink translucent network, which slowly assumes the final form of the plasmodiocarp. The plasmodiocarp gradually becomes opaque pink, then pinkish-yellow, finally turning yellow-brown as the spore walls are formed 6-8 hr after fruiting began. Nuclear divisions were found 4-5 hr after the onset of fruiting, the time varying within this range among plasmodiocarps. Those examined during the first part of this