Feeding Behaviors in Cellular Slime Molds: A Microbial System To Study Competiion Donna M. Bozzone O NE challengefaced in the teachingof introduc- goes a series of shape changes, specific genes are tory biology laboratory is in finding ecology turned on while others are turned off, and cells differ- exercises that are experimental, open-ended, entiate. The result is a fruiting body, a structure relatively inexpensive, and which can be completed consisting of a ball of spores borne atop a delicate and even repeated in a reasonable time-frame. A cellular stalk (Bonner 1944). The fruiting body is a survey of Introductory Biology laboratory manuals resting structure for the organism. Unless food be- shows exercises that undertake studies of pond eco- comes available, the amoebae will remain dormant Downloaded from http://online.ucpress.edu/abt/article-pdf/59/9/565/48121/4450384.pdf by guest on 26 September 2021 systems (Eberhard1990; Morgan & Carter1993), water within the spores. analysis (Abramhoff& Thomson 1994), forest ecosys- The problem investigated in this student project is tems (Morgan& Carter1993), and survivorship(Perry an ecological one. Examination of soil samples col- & Martin 1992). However, none takes full advantage lected from a variety of locations reveals that different of the ease with which microbial eukaryotes can be species of cellular slime molds exist very close to- used for laboratoryinstruction. Described in this pa- gether in nature and in all probability, inhabit the per is a laboratoryproject undertaken by our first year same physical space (Eisenberg 1976; Kuserk 1980; biology students. In this project,students study com- Buss 1982;Eisenberg et al. 1989;Ketcham & Eisenberg petition among various cellular slime molds. After a 1989). The question arises then, how do different brief introductionto the topic of competitionand basic species share or compete for resources to permit life history information about cellular slime molds, survival; how do these organisms partition their students either chose a suggested question, or posed niches?At least for some cases, it has been determined their own, and designed original experiments to an- that different species of cellular slime molds exhibit swer these questions. Without exception,the results of dissimilargrowth rates and competitiveabilities when student projectslead easily to additionaltestable ques- fed different species of bacteria (Horn 1971; Kuserk tions. 1980). Therefore,the similarity of the physical space occupied may not be the most critical factor; the different species avoid direct competition by having Background differentfood preferences.While this differentialuse If you were to isolate the organisms found living in of resourcesmight help to produce the microdistribu- the soil, some of the very interesting microbial eu- tion of cellular slime molds in nature, direct interfer- karyotesyou would encounterare the social amoebae, ence between species is another possible mechanism. or cellular slime molds (Raper 1984). As part of their Indeed, McQueen (1971a,b) studied competition be- unusual life cycle, cellular slime molds exhibit a tween two cellularslime mold species and argued that variety of cellular behaviors that has made them a direct interspecificcompetition is an important strat- very popular experimentalsystem for studying basic egy used by some cellular slime molds. The experi- questions about cell biology, gene expression, devel- ment outlined below describes a method to study opment, ecology, and evolution (Bonner1967; Loomis these behaviors. 1982;Spudich 1987).During the asexual component of the life cycle (Figure 1), spores germinate releasing amoebae. These feed on bacteria and produce more Methods amoebaeby binary fission. If the food supply becomes Student Instructions depleted, the amoebae enter their social phase. They aggregate, or come together, thus producing a multi- Setting Up Cultures. USE STERILETECHNIQUE cellular organism. This multicellularstructure under- FOR THE PROCEDURES DESCRIBED BELOW. 1. Each group of students will be provided with suspensions of various types of bacteria. For Donna M. Bozzone is Associate Professor and Chair of bacterial species #1, vortex or agitate the sus- Biology at St. Michael's College, Colchester, VT 05439; to the bacteria.Using a e-mail: [email protected]. pension gently suspend sterile loop, streak a thin line of the bacterial CELLULARSLIME MOLDS 565 into a sterile test tube. This suspension will be used by the entire class. 5. Once the bacterial streak is dry, deposit, at one end of the MORPHOGENESIS SLUG streak,10 ,lI of a cellularslime MIGRATION mold spore suspension. (You can use an automatic pipet, a sterile capillary pipet, or a sterile loop to do so.) Repeat this step for each bacterial streak. 6. If a sterile bench is available, return the petri dishes to it, and once again blow sterile air over the open dishes to facilitateevaporation of liquid Downloaded from http://online.ucpress.edu/abt/article-pdf/59/9/565/48121/4450384.pdf by guest on 26 September 2021 from the spore suspension. (If FRUITINGBODY AGGREGATION no hood is available, be care- ful when moving the dishes so that the liquid from the spore suspension won't spread. It will eventually dry on its own.) 7. Steps 1-6 can be repeated to study another cellular slime mold strain or species. 8. Determine the rate of feeding by measuring the movement SPORES of amoebae down the bacte- rial streak.The position of the amoebae on the bacterial streak is called the feeding front. Measurements should AMOEBAE be taken daily for 7-10 days. Figure 1. Life cycle of a cellular slime mold. 9. Examine the morphology of the different cellular slime mold species used in your ex- suspension on a petri dish of lactose-peptone periment to determine the identity of the cellu- (2LP) agar. lar slime mold fruiting bodies that develop at 2. Repeat Step 1 for bacterial species 2, 3, and so the far end of the bacterial streak (after the one. Markeach petri dish to indicate the type of streakhas been eaten). Comparetheir morphol- bacteriapresent. ogy to the fruiting bodies on the stock plates. 3. If a sterile bench equipped with a fan is avail- 10. Whateverquestion you decide to pursue, exam- able, place the petri dishes streaked with bacte- ine Figure 2 for possible ways to set up cultures ria within it. Turn the fan on, uncover the and choose or design one most appropriatefor dishes, and allow the liquid from the bacterial the question you are pursuing. Also, be sure to suspension to evaporate. design and set up your experiment so that you 4. Each group of students will also be provided have at least two replicates. with a petri dish containing cellular slime mold fruiting bodies that you will use to collect Ideas for Experiments spores. Each lab group will prepare a different spore suspension for general use. In order to 1. Does a given species of cellular slime mold show prepare the spore suspension, add 8 ml of any food preference? Bonner's Salt Solution (Bonner 1947; Table 1). 2. Does a given species of bacteria vary in its Using a sterile glass elbow, gently dislodge the suitability as a food source for differentcellular fruiting bodies. Pour the resulting suspension slime molds? 566 THEAMERICAN BIOLOGY TEACHER, VOLUME 59, NO. 9, NOVEMBER/DECEMBER1997 3. Will two or more species of cellular slime mold solutions. Table 1. Recipes for culture media and directly influence each other's feeding behavior Lactose-PeptoneAgar (double strength; 2LP) on a given bacterium? If so, is the "winner" determined by the type of bacterium? Lactose 2 g 4. Will two or more species of cellular slime mold Peptone 2 g KH2PO4 0.28 g indirectly influence each other's feeding behavior Na2HPO4 0.28 g on a given bacterium? If so, is the "winner" Agar 15 g determined by the type of bacterium? cellu- Add distilled or deionized H20 up to 1 liter. Autoclave 5. Will two opposite mating type strains of a 20-25 minutes, slow exhaust (liquid cycle). (Raper 1984). lar slime mold species show any difference in food preference? Sussman's Medium (half-strength;SM/2) 6. Will two opposite mating type strains of a cellu- Glucose 0.5 g lar slime mold species indirectly influence each Peptone 0.5 g other's feeding, and if so, is the "winner" deter- Yeast extract 0.05 g mined by the type of food? MgSO4 7H20 0.05 g 7. Can two species co-exist if provided with mixed KH2PO4 0.1 g Downloaded from http://online.ucpress.edu/abt/article-pdf/59/9/565/48121/4450384.pdf by guest on 26 September 2021 K2HPO4 0.05 g bacterial suspensions? 8. After approval by your laboratory instructor, Add distilled or deionized H20 up to 100 ml. Autoclave pursue a question of your own design. 20-25 minutes, slow exhaust (liquid cycle). (Sussman 1987). Bonner's Salt Solution Materials Available for Each Group CaCl2 0.3 g (Studentsdo not need to use everything.) KCI 0.75 g NaCl 0.6 g * 10 lactose peptone plates * Cellular slime molds: Add distilled or deionized H20 up to 1 liter. Autoclave 20-25 minutes, slow exhaust (liquid cycle). (Bonner 1947). Polysphondyliumpallidum Dictyosteliumdiscoideum NC4 Why spend hundreds or even thousands of dollars for a program that is r designed to do business graphs? GraphicalAnalysis for Windows! or Macintosh" Thisaward-winning program is an inexpensive,easy-to- usetool forproducing graphs in the sciencedass. As studentsenter their data, graphs are instantly updated. Datacan also be importedfrom data collection programs orTexas Instruments graphing calculators. The program is alsogreat for helping students find the relationship : ." 9 betweenvariables. Graphs, data tables, text, and histo- fI_ Iw1 1 _r . .. .Z. gramscan all be displayedin variouswindows. 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