HIGH LEVELS OF PHENOTYPIC VARIABILITY OF METAL AND TEMPERATURE TOLERANCE IN PARAMECIUM.

The selection of resistant individuals by a concentration of a chemical sufficient to kill sensitive individuals is the most easily understood and easily demonstrated type of natural selection. Yet at a polluted habitat it is clear that species differ in their capacity to respond to stress (e.g., Gartside and McNeilly, 1974). Are differences in tolerance between species and within species the product of previous interactions with environmental stress or do they have non-selective origins? How much phenotypic variation in tolerance is there within species and on what scale are populations spatially or temporally differentiated? What is the genetic basis of tolerance? We have a long-standing interest in these environmentally-important evolutionary problems, particularly in how the answers to these questions are affected by breeding systems (Nyberg, 1974). In this report we will describe the extent of phenotypic variation in tolerance to eight heavy metals and high temperature in two species of Paramecium. The two species, Paramecium primaurelia and P. triaurelia, are cryptic species in the inbreeding P. aurelia complex (Sonneborn, 1975). No gene flow is possible between these species, but they are morphologically similar and occur together (Landis, 1981), though P. primaurelia has a maximum abundance in lower latitudes than P. triaurelia (Sonneborn, 1957). All the species of the P. aurelia complex are considered to be inbreeders, because all stocks have the capacity to undergo an autogamous fertilization which results in a completely homozygous diploid. Within this inbreeding complex, P. primaurelia and P. triaurelia have similar breeding systems (Sonneborn, 1957). The interval from conjugation to autogamy has recently been estimated to be 49 fissions in P. triaurelia (Nyberg, 1979) and 44 fissions in P. primaurelia (Nyberg, 1982). We chose inbreeding species because Nyberg (1974) had proposed, and found some support for, the hypothesis that phenotypic variation in tolerance should be greater among stocks of inbreeding species than among stocks of outbreeding species. In P. primaurelia 26 stocks from 14 locations were studied, and in P. triaurelia 10 stocks from seven locations. All but one of the stocks were collected in 1975 and 1976. Some of the stocks were collected from a location known to be polluted by heavy metals, Sudbury, Ontario. The 48 h median tolerance limit (MTL) to cadmium, chromium (III), cobalt, copper, manganese (II), mercury, nickel, zinc, and high temperature has been determined for each stock. Heavy metals are a class of compounds whose environmental importance and evolutionary impact has been well documented, especially in plants (Antonovics et al., 1971). Correlations of tolerances to pairs of metals have been calculated to determine whether or not tolerances to different metals are independent. The variation among the stocks within each species has been divided into variation among locations and variation within a location. Our basic outlook is that tolerance limits are the result of selection on tolerance. We imagine that brief, highly localized episodes of selection for resistance are followed by longer periods of weak selection for sensitives (at most locales), followed by even longer periods