Leonard V. Polishchuk

Accumulation of slightly deleterious mutations in mitochondrial protein-coding genes of large versus small mammals.

After the effective size of a population, Ne, declines, some slightly deleterious amino acid replacements which were initially suppressed by purifying selection become effectively neutral and can reach fixation. Here we investigate this phenomenon for a set of all 13 mitochondrial protein-coding genes from 110 mammalian species. By using body mass as a proxy for Ne, we show that large mammals (i.e., those with low Ne) as compared with small ones (in our sample these are, on average, 369.5 kg and 275 g, respectively) have a 43% higher rate of accumulation of nonsynonymous nucleotide substitutions relative to synonymous substitutions, and an 8–40% higher rate of accumulation of radical amino acid substitutions relative to conservative substitutions, depending on the type of amino acid classification. These higher rates result in a 6% greater amino acid dissimilarity between modern species and their most recent reconstructed ancestors in large versus small mammals. Because nonsynonymous substitutions are likely to be more harmful than synonymous substitutions, and radical amino acid substitutions are likely to be more harmful than conservative ones, our results suggest that large mammals experience less efficient purifying selection than small mammals. Furthermore, because in the course of mammalian evolution body size tends to increase and, consequently, Ne tends to decline, evolution of mammals toward large body size may involve accumulation of slightly deleterious mutations in mitochondrial protein-coding genes, which may contribute to decline or extinction of large mammals.

Sensitivity to stress in the bivalve Macoma balthica from the most northern (Arctic) to the most southern (French) populations: low sensitivity in Arctic populations because of genetic adaptations?

The stress sensitivity, determined in copper exposure experiments and in survival in air tests, and the genetic structure, measured by means of isoenzyme electrophoresis, were assessed in populations of the Baltic clam Macoma balthica (L.) from its southern to its northern distribution limit, in order to test the hypotheses that near the distribution limit the clams would be more stress sensitive and would have a lower genetic variability. The populations in west and north Europe show a strong genetic resemblance. The populations in the sub-Arctic White Sea are genetically slightly different, and show a low stress sensitivity. The populations in the Arctic Pechora Sea are genetically very distant from the other populations, and show the lowest stress sensitivity. Near the southern distribution limit, in agreement with the hypotheses, genetic variability is low and stress sensitivity high. On the other hand, in contrast to expectation, near the northern distribution limit, in the populations of the Pechora Sea, the genetic variability was higher, thus not reduced, and the stress sensitivity was low compared to all other populations. Yet, it remains a question if such is due to gradual physiological acclimatization (and ongoing differential selection) or to genetic adaptation.

Contribution analysis of body mass dynamics in Daphnia

The concept of body mass dynamics can be viewed as part of life history theory, but its potential has remained largely untapped due to a lack of analytical methodology. We therefore propose a method, called contribution analysis, which enables us to decompose a change in body mass into contributions associated with variations in individual egg mass, clutch size, and standard somatic mass (somatic mass adjusted to body length). The advantage of contribution analysis is that various contributions are expressed in the same units (units of mass) and show the amount of resources committed to changes in the individual traits, while the traits themselves are measured in different units and thus incomparable on their own. The method is tuned to study zooplankton, and is applied to examine body mass dynamics in Daphnia galeata. We found that when recovering from a poor-resource environment just above the threshold food concentration, Daphnia primarily increase their standard somatic mass, that is, restore body condition. When the trophic environment improves further but remains below the incipient limiting level, resources are invested equally to enhance body condition and reproduction in terms of clutch size. Finally, when food is no longer a limiting factor, almost all resources are committed to increase clutch size. While individual egg mass also varies, it never attracts more resources than the shift in the most prioritized trait. We suggest that the significance of this shift in resource allocation priorities is to keep an adult female alive in a poor environment and thus to allow her to retain her reproductive potential for better conditions in the future. Contribution analysis of body mass dynamics may allow us to detect flexible allocation strategies in a changing natural environment.

Contribution analysis of disturbance-caused changes in phytoplankton diversity.

For an experimental system of marine planktonic algae, U. Sommer has published a set of regression equations linking species diversity to intensity and frequency of disturbance. I apply a simple version of contribution analysis to Sommer’s equations pursuing a twofold goal: (1) to reveal a mechanism responsible for changes in species diversity along the gradient of disturbance and (2) to evaluate the potential of the method used. The relative importance (i.e., contributions) of changes in disturbance intensity and frequency to the resultant change in diversity is estimated, as disturbance regime is gradually shifting from high frequencies and low intensities to low frequencies and high intensities. Changes in intensity are found to be of primary importance under a high-frequency and low-intensity disturbance regime, while changes in frequency are of primary importance under a low-frequency and high-intensity disturbance regime. Competitive interactions appear to shape the algal community at both ends of the disturbance gradient—not only when disturbances are weak though common, but also when they are strong and rare. The results suggest that contribution analysis may allow one to extract information on the underlying mechanisms from purely descriptive regression relationships.

Role of parthenogenetic natality and emergence from diapausing eggs in the dynamics of some rotifer populations

There are few quantitative data on the role of emergence from diapausing eggs in population dynamics of natural populations of zooplankton species; to our knowledge, all these concern copepods and ‘cladocerans’. We present here direct estimates of emergence from bottom resting eggs for another important category of freshwater zooplankton, namely rotifers. Three populations of rotifers of the genus Brachionus were studied in a lake. During the study period 10 population increases, each corresponding to an individual sampling interval, were detected. For each interval, emergence from immediately hatching, parthenogenetic eggs calculated on the basis of the Edmondson-Paloheimo model and emergence from diapausing bottom eggs determined in short-term experiments were estimated and compared to each other. We found that three of the population increases observed are entirely explained by parthenogenetic natality. In contrast, emergence from diapausing eggs can, on its own, account for none of population increases. For two population increases, however, it accounts for that part of population growth which remains unexplained by the parthenogenetic natality. For rotifer populations studied, emergence from diapausing eg eggs is generally less important than parthenogenetic births, when both are regarded as an immediate cause of population growth. This is in sharp contrast to the data available for some crustaceans (De Stasio, 1990) where the role of emergence from diapausing eggs in population dynamics has been clearly shown.

Growth in the bivalve Macoma balthica from its northern to its southern distribution limit: a discontinuity in North Europe because of genetic adaptations in Arctic populations?

The hypothesis was tested that towards a species limit of distribution its performance, such as growth or fitness, decreases. To this end, latitudinal changes in growth, maximum attainable length and genetic constitution were assessed for the Baltic clam, Macoma balthica (L.), at stations ranging from the most southern distribution limit (France) to its most north-eastern limit in the Arctic Pechora Sea (North Russia). Growth was analyzed by means of growth-rings on the shells, the genetic constitution by electrophoretic isoenzyme analysis. Growth patterns and the genetic constitution of populations from West Europe, North Europe and the White Sea were similar, whereas the populations from the Pechora Sea are distinct from the other populations. The performance of clams in the Pechora Sea populations, with a relatively low annual growth but high maximum length, was, in contrast to the hypothesis, not decreased. It is concluded that the Pechora Sea populations form a separate group, genetically different from other European populations and adapted to the Arctic conditions. [KEYWORDS: Arctic; adaptation; distribution limit; genetics; geographic cline; growth; Macoma balthica; shell-length Dutch wadden sea; electrophoretic data; recruitment; gironde; stress]

Scaling of population density on body mass and a number-size trade-off.

The energetic equivalence rule (EER). that is the negative relationship between population density and body mass with the body mass exponent of -0.75, is often observed for mammal species assemblages studied at regional scales. Little, however, is known about the mechanism that may generate it. Here we attempt to explain EER in terms of demography and life-history theory, namely on the basis of an interspecific trade-off between total lifetime female offspring and relative mass at birth. Based on data on 85 species of non-flying mammals from the territory and coastal waters of the former Soviet Union, we show that the number of offspring per lifetime is approximately inversely proportional to the relative mass at birth (the exponent is not significantly different from -1). Simple mathematics shows that given some other conditions, this minus-one trade-off entails the energetic equivalence rule. Two other consequences also follow. First, as an interspecific trade-off evolves from the intraspecific trade-off of the same type, the tendency for small-bodied species to be more abundant than large ones may have its evolutionary origin in the intraspecific trade-off between offspring number and size. Second, the trade-off may solve the paradox relating to EER: how could it be that in spite of probably unequal food availability for so different species as, for example, mice and elephants their populations consume, on average, equal amounts of energy, as the rule implies? We suggest that populations of different species consume the same amount of energy per unit area per unit time because their individual members require the same amount of energy per unit mass per lifetime, the latter being a somewhat different formulation of the minus-one trade-off between lifetime offspring number and relative mass at birth.