Dennis Nyberg

Sex in Ciliates

The existence of genetic variety affecting phenotype represents the presupposition for selection to operate and then for evolution to accomplish. Two means of achieving genetic variety can be basically distinguished: mutation and recombination. Both confer flexibility to the organisms enabling them to change as environmental conditions inevitably change. Understandably, variability has its hazards and its cost in terms of progeny inviability. Organisms have adopted quite different ecogenetic strategies to pursue the same aim: to manage in a balancing way the stability—variety dilemma.

Mammalian microevolution: Rapid change in mouse mitochondrial DNA

We have compared the sequences of mitochondrial DNA extracted from museum skins of white-footed mice caught in the Chicago area since 1855 and from modern mice trapped alive in the same locations. We found a consistently similar directional change of mouse genotype over this period at each of five collection sites that were separated by 10–70 km. The genotype most common 100 years ago is now extremely rare, indicating that the mammalian mitochondrial genome can undergo rapid evolution.

Museum collections of mammals corroborate the exceptional decline of prairie habitat in the Chicago region

Abstract The prairie deer mouse (Peromyscus maniculatus bairdii) was more common than the white-footed mouse (P. leucopus) in museum collections from the 6 Illinois counties of the Chicago region before 1920 but constitutes only 5% of specimens deposited since 1970. Because white-footed mouse prefers woody vegetation and because prairie deer mouse is limited to prairie or large open habitats, the change in proportion is likely driven by a disproportionate loss of prairie among remaining natural habitat and increases in woody vegetation within grasslands. The decline of the prairie vole (Microtus ochrogaster) relative to the meadow vole (M. pennsylvanicus) and the lack of recent specimens of Franklins ground squirrel (Spermophilus franklinii) corroborate the hypothesis that prairie habitats have declined much more so than wooded habitats in the Chicago region. Based on extinction models using museum records, it is probable that S. franklinii is already locally extirpated. Regression analysis suggests the white-footed mouse will be the only local Peromyscus in 0–140 years.

Rapid change in mouse mitochondrial DNA

We have compared the sequences of mitochondrial DNA extracted from museum skins of white-footed mice caught in the Chicago area since 1855 and from modern mice trapped alive in the same locations. We found a consistently similar directional change of mouse genotype over this period at each of five collection sites that were separated by 10–70 km. The genotype most common 100 years ago is now extremely rare, indicating that the mammalian mitochondrial genome can undergo rapid evolution.

Vitamin E supplementation and intense selection increase clonal life span in Paramecium tetraurelia

Vitamin E added to standard Cerophyl medium at 0.025 mg/ml significantly increased the mean clonal life span of 32 lines of Paramecium tetraurelia from 52.5 days and 187 fissions to 59.9 days and 209 fissions (p less than .05) when compared to unsupplemented, paired controls. The age-specific death rates increased exponentially. Regression analyses found a significantly lower rate of increasing mortality for the supplemented lines compared to controls. Weekly fission rates of supplemented lines declined linearly; more slowly than controls, but not significantly so. A second experiment tested two higher levels of supplemental vitamin E (0.10 and 1.00 mg/ml). Sublines receiving 1.00 mg/ml of supplemental vitamin E had higher rates of mortality and lower fission rates initially, but the mortality rates increased and the fission rates decreased more slowly than in sublines receiving 0.10 mg/ml supplemental vitamin E, resulting in maximum clonal life spans of 141 days and 330 fissions (1.00 mg/ml sublines), compared to 74 days and 271 fissions (0.10 mg/ml sublines). Survivorship of the last individual cells (nondividing) of each clone followed an exponential decline, with a significant increase in mean survival time for supplemented compared to unsupplemented cells (p less than .05).

The Species Concept and Breeding Systems

People group and organize entities or observations as a way of understanding the world. We also believe that the structures we generate doing this fundamental taxonomic activity approach the “true” organization of the natural world. Initially few characters, often of a single class, are available for organizing individuals into groups. As new characters, and especially those of a different nature or class, become available, the effectiveness of the original classification is tested and evaluated. An ideal taxonomy results in the same classification regardless of the nature of the characters used. In biology the species level was more effective than higher levels of classification in forming “natural” groups, i.e., those in which different classes of characters resulted in the same or similar groupings. In this century the greater effectiveness of the species level of classification could be rationalized through the gene pool shared by members of this evolutionary unit. Along with this new evolutionary definition of species came the discovery that morphologically similar Drosophila could sometimes be divided into more than one genetic species. These became known as sibling species.

Adolescence and the Reversibility of Maturity in Euplotes crassus

ABSTRACT. The clonal life history of ciliated protists is characterized by a sequence of phenotypes; sexual immaturity, maturity, and senescence. The distinctiveness of immaturity and maturity has been investigated. Standard assays of the onset of maturity of progeny clones from a cross between stocks EC1 and EC2 of Euplotes crassus demonstrated significant differences among clones and among testers within clones. They also revealed that the first positive test(s) of a progeny subclone were typically followed by at least one negative test. Special protocols were devised to investigate if maturity was reversible at the cellular level. In these experiments, the first mating pair of a progeny subclone was split before the consummation of mating. From these two cells as well as from control progeny and tester cells, subclones were established and every leftover cell was tested for maturity after each transfer. Both standard and split‐pair progeny subclones had immature and slow‐ to‐mate cells. The number of fissions before progeny exhibited sexual behavior indistinguishable from the testers was more than twice that to the first mating reaction of a subclone. At the first sign of maturity, progeny lines are a heterogeneous population of cells able and not able to mate, but remarkably, clonal descendants of those able to mate may become unable to mate. The development of maturity is progressive, quantitative and non‐monotonic rather than an instantaneous switch.

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

Evaluating the Predicted Local Extinction of a Once-Common Mouse

In an earlier paper (Pergams & Nyberg 2001) we found that the proportion of the prairie deer mouse (Peromyscus maniculatus bairdii), among all local Peromyscus museum specimens collected in the Chicago region, had significantly declined over time. This proportion changed from about 50% before 1900 to <10% in the last 25 years. Based on this proportion a regression model predicted the local extinction of the prairie deer mouse in 2009. To evaluate that prediction, we estimated current deer mouse abundance by live trapping small mammals at 15 preserves in Cook and Lake counties, Illinois (USA) at which prairie deer mice had previously been caught or that still contained their preferred open habitat. In 1900 trap nights, 477 mammals were caught, including 251 white-footed mice (P. leucopus), but only one prairie deer mouse. The observed proportion of Peromyscus that were prairie deer mice, 0.4%, was even lower than the 4.5% predicted for 2000. Here we also introduce a simple, new community proportions model, which for any given geographic region compares the proportions of species recently caught with the proportions of species in museums. We compared proportions of seven species collected in Cook and Lake counties and examined by Hoffmeister (1989) with proportions of these species that we caught. Ten percent of the museum community was prairie deer mice, but only 0.2% of our catch was. The current local scarcity of the prairie deer mouse is consistent with the regression-based prediction of its eminent local extinction. More conservation attention should be paid to changes in relative abundance of once-common species.

The effect of temperature on copper tolerance ofParamecium

SummaryThe copper tolerance ofParamecium tetraurelia decreases with increased temperatures over the range of 12 C to 34 C. The relationship is linear and the correlation=−0.98. The regression equation has an intercept of 16 μM Cu++ at 0 C, and tolerance is reduced by 0.33 μM for each degree increase in temperature.