The hydra effect in predator–prey models

One-dimensional discrete-time population models, such as those that involve Logistic or Ricker growth, can exhibit periodic and chaotic dynamics. Expanding the system by one dimension to incorporate epidemiological interactions causes an interesting complexity of new behaviors. Here, we examine a discrete-time two-dimensional susceptible-infectious (SI) model with Ricker growth and show that the introduction of infection can not only produce a distinctly different bifurcation structure than that of the underlying disease-free system but also lead to counter-intuitive increases in population size. We use numerical bifurcation analysis to determine the influence of infection on the location and types of bifurcations. In addition, we examine the appearance and extent of a phenomenon known as the ‘hydra effect,’ i.e., increases in total population size when factors, such as mortality, that act negatively on a population, are increased. Previous work, primarily focused on dynamics at fixed points, showed that the introduction of infection that reduces fecundity to the SI model can lead to a so-called ‘infection-induced hydra effect.’ Our work shows that even in such a simple two-dimensional SI model, the introduction of infection that alters fecundity or mortality can produce dynamics can lead to the appearance of a hydra effect, particularly when the disease-free population is at a cycle.

The amount and distribution of genetic variation in seven introduced New Zealand populations of Carduelis chloris were assessed at 40 loci using starch gel electrophoresis and compared with those in native European populations. Fewer alleles (1.45) and fewer polymorphic loci (33 per cent) were detected across the introduced populations than across native populations (1.75, 55 per cent), reflecting the narrow geographical origin of the introduced populations. There was no evidence for severe inbreeding or genetic drift as the levels of average heterozygosity (H = 0.025), percentage polymorphic loci (P = 16.9) and average number of alleles per locus (A = 1.22) were indistinguishable from levels observed within European populations (77 = 0.025, P =14.1, A = 1.19). Furthermore, introduced populations were genetically less weakly differentiated (FST = 0.022) than native populations (FST = 0.041), indicating that little genetic drift has been involved in the colonization of the new range. Similar levels of genetic variability in native and introduced greenfinch populations are consistent with theoretical expectations as the founder population size was relatively large (>60 individuals) and a rapid increase in population size directly after colonization was documented. A review of earlier studies on introduced birds also revealed that reductions in levels of genetic variability seem to be inversely proportional to the size of the founder population, and that less variation has been lost if the rate of population growth directly after introduction was fast.

Gene arrangement frequencies of different chromosome types of three inversions, IIL-2, III-35 and III-2 are reported in some natural and laboratory populations of Drosophila nasuta. In general, the frequencies of inversion heterozygotes for these inversions are in excess of what is expected under Hardy-Weinberg equilibrium in both natural and laboratory populations. This suggests that these inversion heterozygotes are heterotic to the species as far as viability is concerned. Similar to what occurs in the cosmopolitan species, chromosomes with standard gene sequences are more frequent than inverted gene sequences in this species. The scarcity of inversion homozygotes is due to reduced viability of their carriers, which probably have lethal or sublethal combinations of alleles in the inverted sections of their chromosomes. Further, the inversion frequencies in laboratory populations follow the frequencies in natural populations.

The evolution of senescence in natural populations of guppies (Poecilia reticulata): a comparative approach.

In modern conditions, one of the most important tasks of the state is self-sufficiency in grain reserves. In this context, not least essential is the safety of grain while minimizing its losses and damage by insect pests. According to resent investigations results, one of the efficient means against pests of grain reserves is the phosphine gas. Phosphine has been widely used in Russia since the early 90s of the XX century. The article presents the study results on resistance to phosphine of the fusty flour beetle (Tribolium castaneum), larvae of the khapra beetle (Trogoderma granarium Ev.), imago of the wheat weevil (Sitophilus granaries L.) at every development stage. We have estimated a very close dependence of the mortality of Tribolium castaneum at any development stage in natural and laboratory populations on the concentration of phosphine. The Rostov population of the mace-eared crustacean is better resistant to phosphine than the laboratory population at any development stage. At the same time, imago of both laboratory and Rostov populations are not as resistant to the action of the insecticide as the egg stage. The study has identified a strong correlation between the mortality of active larvae in both natural and laboratory populations of Trogoderma granarium Ev. on one hand and the concentration of phosphine on the other. By the probit analysis results, the Petersburg population of active larvae of the khapra beetle is better resistant than the laboratory population, but their resistance will decrease as the exposure time increases. Experimentally studied and mathematically described is the dependence of adult mortality in natural and laboratory populations of Sitophilus granaries L. on the concentration of phosphine. The resistance index to phosphine in imago of the Stavropol, Krasnodar and Tambov populations is 1.5; 1.4 and 1.6, respectively.

Many important factors affect the spread of childhood infectious disease. To better understand the fundamental drivers of infectious disease spread, several researchers have estimated seasonal transmission coefficients in discrete-time models. In this paper, we build upon this previous work and also develop a framework for efficient estimation using continuous differential equation models. We introduce nonlinear programming formulations to efficiently estimate model parameters and seasonal transmission profiles from existing case count data for the childhood disease, measles. We compare results from discrete time and continuous time models and address several shortcomings of the discrete-time method, including removing the need for the data reporting interval to match the time between successive cases in the chain of transmission or serial interval of the disease. Using a simultaneous approach for optimization of differential equation systems, we demonstrate that seasonal transmission parameters can be effectively estimated using continuous time models instead of discrete-time models.

Population genetic variations of Helicoverpa armigera (Hiibner) over different major cotton growing regions were analyzed by DNA polymorphism amplified with four simple repetitive sequence primers. The results showed that the laboratory population had relatively lower genetic variation than natural populations. The genetic variation between natural populations was not significant and genetic variation existed in the same location from different years, indicating frequent migration among natural cotton bollworm populations. Cluster analysis indicated that individuals from Chaoyang of Liaoning Province (CY), Gaotang of Shandong Province (GT) and Dafeng of Jiangsu Province(DF) were more mixed each other, which suggested that CY population might have higher gene flow with GT and DF populations, especially with GT population. It supports the theory that the cotton bollworm in Northeast China came from Shandong and Hebei Provinces. This result also demonstrated that the molecular makers in this study are sensitive to detect population genetic structure changes.

Drosophila melanogaster from Australia, Europe and North America enter an adult ovarian dormancy in response to short days and low temperatures. The independent effects of temperature and day length in the determination of dormancy have been examined only in one long-established laboratory line (Canton-S). In all other studies of natural or laboratory populations, dormancy has been assessed at either a single short day or a single moderately low temperature. Herein, we determine the relative roles of temperature, photoperiod, and their interaction in the control of ovarian dormancy in D. melanogaster from two natural populations representing latitudinal extremes in eastern North America (Florida at 27 degrees N and Maine at 44 degrees N). In both natural populations, temperature is the main determinant of dormancy, alone explaining 67% of the total variation among replicate isofemale lines, whereas photoperiod has no significant effect. We conclude that ovarian dormancy in D. melanogaster is a temperature-initiated syndrome of winter-tolerant traits that represents an adaptive phenotypic plasticity in temperate seasonal environments.

Deterministic and stochastic evolutionary processes drive adaptation in natural populations. The strength of each component process is determined by the population size: deterministic components prevail in very large populations, while stochastic components are the driving mechanisms in small ones. Many natural populations, however, experience intermittent periods of growth, moving through states in which either stochastic or deterministic processes prevail. This growth is often countered by population bottlenecks, which abound in both natural and laboratory populations. Here we investigate how population bottlenecks shape the process of adaptation. We demonstrate that adaptive trajectories in populations experiencing regular bottlenecks can be naturally scaled in time units of generations; with this scaling the time courses of adaptation, fitness variance, and genetic diversity all become relatively insensitive to the timing of population bottlenecks, provided the bottleneck size exceeds a few thousand individuals. We also include analyses at the genotype level to investigate the impact of population bottlenecks on the predictability and distribution of evolutionary pathways. Irrespective of the timing of population bottlenecks, we find that predictability increases with population size. We also find that predictability of the adaptive pathways increases in increasingly rugged fitness landscapes. Overall, our work reveals that both the adaptation rate and the predictability of evolutionary trajectories are relatively robust to population bottlenecks.

Hermaphroditism is relatively common among coastal and reef-associated tropical marine fishes (Reinboth, 1970; Thresher, 1984; Sadovy and Shapiro, 1987). However, the western Atlantic mangrove killifish, Rivulus marmoratus, appears to be unique among hermaphrodites in its pattern of sexual allocation and reproduction. In laboratory populations, fish are reported to be either primary males that develop directly and produce only sperm, hermaphrodites that produce both sperm and ova, or secondary males that arise from hermaphrodites following loss of female function (Harrington 1975). In natural populations, primary males are reported to be rare (Harrington, 1975; but see Davis et al., 1990; Turner et al., 1992a), and secondary males have not been verified (Harrington, 1971). No pure females have been reported from either natural or laboratory populations (Harrington, 1975; Soto et al., 1992). On the basis of previous laboratory and field data, most individuals in this species are assumed to be obligate self-fertilizing hermaphrodites (Harrington, 1971; Davis et al., 1990; Soto et al., 1992). Results from studies of tissue transplants and allozymes confirmed this assumption (Harrington, 1975; Vrijenhoek, 1985). However, recent molecular studies (Turner et al., 1991, 1992b) have shown much richer genetic variation in natural populations of R. marmoratus than did earlier techniques. We examined gonadal development in R marmoratus to clarify the pattern of sexual allocation in this species and to confirm whether it is, in fact, an obligate selffertilizing species (Judson and Normark, 1996).

Thirteen laboratory populations (six mass cultures and seven isofemale lines) of Drosophila bipectinata were established from files collected from six different geographic localities (four in the north and two in the south) in India. These mass cultures and isofemale lines were maintained in food bottles in the laboratory for varying number of generations by transferring about 50 files (females and males in equal number) in each generation. After several generations, all the laboratory populations were analysed chromosomally to determine the frequency of different inversions. The results indicate that all the laboratory populations remained chromosomally polymorphic due to the persistence of inversions which were originally present ion the populations. The quantitative data on the frequency of inverted gene orders and the level of inversions heterozygosity in laboratory populations are compared with those in corresponding natural populations. This comparison clearly shows that there is a considerable increase in the frequency of inverted gene orders and the level of inversion heterozygosity in laboratory populations when compared with corresponding natural populations. This is most likely to be due to selective advantage of inversion heterozygotes under stringent competition in laboratory populations. Thus heterotic buffering is associated with chromosome inversion of Drosophila bipectinata.

The harlequin ladybird, Harmonia axyridis, is an important natural enemy of aphids throughout the world, but is now also considered an invasive alien species. We performed a meta-analysis of published life history data to address the question whether invading populations in Europe and North America have life history parameters that differ from native populations in Asia, explaining the beetle’s invasion success in new territories. In this meta-analysis, we accounted for important covariables that are often reported in published studies such as temperature, food source (aphids or eggs of Ephestia kuehniella), strain (laboratory or field populations) and photoperiod. Temperature was a key factor having consistent large effects on development rate, survival and reproductive characteristics of H. axyridis. Food source, strain, and photoperiod had effects on some, but not all characteristics, and their overall effect across characteristics was minor. Individuals of invasive populations had a shorter pre-oviposition period and higher fecundity at low temperatures than those of native populations, and a greater longevity across all temperatures. No differences in survival were found between native and invasive populations, while differences in development rate were not consistent, with opposing results obtained according to the way development rate was measured in trials reported in the literature. Results of this meta-analysis support the hypothesis that the life history of the beetle has changed during its invasion into North America and Europe. Invasive populations had a shorter pre-oviposition period and higher fecundity at low temperatures, as well as a greater longevity across all temperatures than native populations. These differences may partially explain the invasive success of H. axyridis.

Fitness interactions between loci in the genome, or epistasis, can result in mutations that are individually deleterious but jointly beneficial. Such epistasis gives rise to multiple peaks on the genotypic fitness landscape. The problem of evolutionary escape from such local peaks has been a central problem of evolutionary genetics for at least 75 years. Much attention has focused on models of small populations, in which the sequential fixation of valley genotypes carrying individually deleterious mutations operates most quickly owing to genetic drift. However, valley genotypes can also be subject to mutation while transiently segregating, giving rise to copies of the high fitness escape genotype carrying the jointly beneficial mutations. In the absence of genetic recombination, these mutations may then fix simultaneously. The time for this process declines sharply with increasing population size, and it eventually comes to dominate evolutionary behavior. Here we develop an analytic expression for Ncrit, the critical population size that defines the boundary between these regimes, which shows that both are likely to operate in nature. Frequent recombination may disrupt high-fitness escape genotypes produced in populations larger than Ncrit before they reach fixation, defining a third regime whose rate again slows with increasing population size. We develop a novel expression for this critical recombination rate, which shows that in large populations the simultaneous fixation of mutations that are beneficial only jointly is unlikely to be disrupted by genetic recombination if their map distance is on the order of the size of single genes. Thus, counterintuitively, mass selection alone offers a biologically realistic resolution to the problem of evolutionary escape from local fitness peaks in natural populations.

In varying lemmings from seven natural populations (from Bolzhezemel'skaya tundra to Chukotka Peninsula), the number of B chromosomes ranged from 0 to 15. In populations surveyed for several years, B chromosome frequencies were stable. Two laboratory colonies (founded by Dicrostonyx torquatus from the Polar Urals and D. t. chionopaes from Yakutia) produced more than 3000 animals, of which 1699 were karyotyped. A small excess of B chromosomes in the progeny over that in their parents was observed in each generation. Coefficients of transmission k1 of additional chromosomes were on average 0.519 in D. t. torquatus and 0.511 in D. t. chionopaes. In oocytes I and II, an accumulation of B chromosomes was observed as compared to somatic cells (k2 = 0.66). The reproductive output of animals from both laboratory colonies did not depend on size of the B genome. The reduction of body and scull sizes observed in animals carrying numerous B chromosomes may confer negative selective value in the conditions of the Extreme North. In general, the system of B chromosomes in D. torquatus is well balanced and very stable, being apparently under strong genetic control.

The frequency of chromosome damage was studied in the carriers of virus of the hemorrhagic fever with renal syndrome (Puumala virus) and in noninfected animals from two laboratory colonies and two natural populations of bank vole. In the laboratory colony, where Puumala virus persisted for three years, multiaberrant (“rogue”) cells were found in the bone marrow; the mean frequencies of both structural and numeral chromosome abnormalities were significantly enhanced. In the other laboratory colony, no Puumala virus was detected during all 30 years of its existence, but the mean frequencies of structural chromosome damage were increased to the same degree probably due to the prolonged breeding under laboratory conditions, which resulted in suppression of immunity and DNA repair. The voles from the natural populations were more resistant to the clastogenic viral effect, but they also had multiaberrant cells which served as indicators of viral infection. The data obtained support the hypothesis that viral infections increase mutation rate, contributing thereby to the evolution process.