AN IMMUNOLOGICAL APPROACH TO THE STUDY OF EVOLUTION OF TRYPSINS

Antibodies were prepared in rabbits against trypsins obtained from four different species—namely bovine, porcine, spiny Pacific dogfish, and starfish (Evasterias trochelii). Each of the antisera, or the immunoglobulin G fraction thereof, was tested for its capacity to react with each of the four enzymes. The immunological reaction was assessed by three different techniques—precipitin reaction, antigen binding capacity, and inhibitory effect of the antibodies on the proteolytic activity of the enzymes. In each case, the homologous enzyme gave the strongest reaction with its antiserum but the heterologous enzymes were also capable of reacting to an appreciable extent. The cross-precipitation was the least sensitive method, and gave relatively low values, whereas cross-inhibition and cross-binding of radioactively labeled antigens indicated high extent of cross-reaction between the various trypsins. The relative capacity of interaction of the four enzymes with the four antibodies could be related to the order in which these enzymes developed during evolution. Thus the “order” of similarity was bovine > porcine > dogfish > starfish.     

GENIC VARIATION IN A NATURAL POPULATION OF Drosophila persimilis

The understanding of the speciation process can best be achieved by a knowledge of the genic differences between two closely related species and the correlation of such differences with the characteristics which demarcate the two species. D. persimilis and D. pseudoobscura are two sibling species differing from each other in morphology, ecological preferences, and behavior, and no hybrids exist in nature. I have compared the genetic changes in 24 loci of 25 strains of D. persimilis from Mather, California, with those in D. pseudoobscura by the method of acrylamide gel electrophoresis. The proportion of the genome heterozygous in individual D. persimilis is 10.5 per cent, which is comparable to the heterogeneity found in similar D. pseudoobscura populations. I did not find any locus which is monomorphic or polymorphic for entirely different allele(s) than in D. pseudoobscura. The different frequencies of shared alleles in the two species can only be explained by selection acting differentially in these species, since the frequencies of different alleles at various loci in several populations of D. pseudoobscura are very similar or show a very stable pattern of association with the third chromosome gene arrangements.     

A FURTHER ASSESSMENT OF THE ROLE OF FOUNDER EFFECTS IN THE OUTCOME OF Tribolium COMPETITION EXPERIMENTS

A novel experimental design was employed to further investigate the importance of founder effects in determining the outcome of competition between the flour beetles Tribolium castaneum and T. confusum. Each of 10 replicate cultures in seven competition series was started with 10 pairs of adults of each species that were, in turn, the progeny of 1, 2, 4, 8, 16, 32, or 64 pairs of adults, respectively, from the foundation population. Thus the founder effect occurred one generation prior to the initiation of competition. An analysis of variability among replicate cultures for the first eight monthly censuses showed a significant increase in variation with smaller numbers of original pairs, confirming the hypothesis previously proposed to explain the indeterminate outcome of replicate competition cultures initiated with small numbers of founders. In addition, there was a significant reduction in the rate of elimination of T. confusum with smaller numbers of original pairs. It is suggested that T. castaneum is more sensitive to inbreeding, and that the differential response of the two populations to inbreeding reflects a difference in their homeostatic properties.     

BODY WEIGHT AS A DETERMINANT OF CLINICAL EVOLUTION IN HAMSTERS (Mesocricetus auratus) INFECTED WITH Leishmania (Viannia) panamensis

SUMMARY

The clinical outcome of infection with Leishmania species of the subgenus Viannia in hamster model (Mesocricetus auratus) has shown to be different depending on experimental protocol. Body weight has been a relevant determinant of the clinical outcome of the infection in hamsters with visceral leishmaniasis but its importance as a clinical parameter in hamsters with cutaneous leishmaniasis is not known. In this study, the clinical evolution of infection with L. (V) panamensis was evaluated in juvenile and adult male hamsters during 11 weeks by comparing clinical parameters such as attitude, temperature, respiratory rate, appearance of the stool, and body weight between infected and non-infected groups. Results showed that body weight decreased in adult hamsters after infection by L. (V) panamensis; this observation supports the use of body weight as an additional parameter to define the management or treatment of cutaneous leishmaniasis in infected adult hamsters used as an animal experimental model for leishmaniasis.     

Founder Effects and Linkage Disequilibria in Experimental Populations of Drosophila

Six laboratory populations of Drosophila paulistorum were examined for changes in gene frequencies at an enzyme locus, tetrazolium oxidase (To). In some of the populations, the alleles were introduced on over 100 independently derived chromosomes. These populations showed considerable stability in gene frequencies although they were at widely different starting frequencies. Other populations were begun with only a few (about 6) independently derived chromosomes. These populations showed significant and somewhat erratic changes in To gene frequencies. The difference in behavior of the two sets of populations was almost certainly caused by linkage effects due to sample size. The implication of these studies in understanding the role of the founder effect in natural populations is briefly discussed.     

When does the good of the group override the advantage of the individual?

In a system of N populations of n reproductive individuals apiece, in which each population has constant variance v² and lasts L generations, group selection on a quantitative character has a reasonable chance of overriding selection within populations if (and only if) the populations never exchange migrants, each population is founded by colonists from a single parent population, and the number of populations exceeds the effective number of reproductive individuals per population. If each population derives from a single parent population, then the exchange of a single successful migrant per population per L generations can triple the strength of group selection required to overcome a given selection within populations. If populations exchange no migrants, then the derivation of one in every N populations from two equally represented parents (while the others all derive from a single parent) doubles the strength of group selection required to prevail. Group selection is accordingly likely to be effective only in certain categories of parasites.     

Trade-off between r-selection and K-selection in Drosophila populations

Density-dependent genetic evolution was tested in experimental populations of Drosophila melanogaster subject for eight generations to natural selection under high (K-selection) or low (r-selection) population density regimes. The test consisted of determining at high and at low densities the per capita rate of population growth of the selected populations. At high densities, the K-selected populations showed a higher per capita rate of population growth than did r-selected populations, but the reverse was true at low densities. These results corroborate the predictions derived from formal models of density-dependent selection. However, no evidence of a trade-off in per capita rate of growth was observed in 25 populations of D. melanogaster, each homozygous for a different second chromosome sampled from a natural population.     

First Report of Widespread Wild Populations of Triatoma infestans (Reduviidae, Triatominae) in the Valleys of La Paz, Bolivia

Wild populations of Triatoma infestans, the main vector of Chagas disease in the Southern Cone countries, may be involved in reinfestation of human dwellings, limiting the success of vector-control campaigns in Bolivia. Knowledge of the distribution of these populations remains incomplete. We report here the detection of T. infestans wild populations in large areas in the department of La Paz, Bolivia. Among 18 sylvatic areas investigated, 17 were positive with T. infestans specimens. The infection rate of captured T. infestans with Trypanosoma cruzi was 85.7% in adult specimens. These results expand the geographical distribution of wild populations of T. infestans; it may be distributed throughout the Inter-Andean Dry Forest eco-region of Bolivia. The current information allows us to propose the hypothesis that a sylvatic origin of the reinfestation is located in the valleys of La Paz.     

Natural Hybridization between the Cholla Cactus Species Opuntia spinosior and Opuntia versicolor

The sexually reproducing cholla cactus species, Opuntia spinosior and O. versicolor, hybridize naturally in Arizona to produce hybrid swarms and segregating introgressive populations with a high degree of individual variation. The pattern of variation in these hybrid populations can be compared with that in populations derived from crossing of O. spinosior and O. fulgida, where the hybrids reproduce vegetatively rather than by seeds. The latter hybrid combination, in marked contrast to the former, results in clonal colonies with little or no observable individual variation.     

Gene flow by pollen into small populations: Data from experimental and natural stands of wild radish

Gene flow can have an especially strong impact on the evolution of small populations. However, empirical studies on the actual rates and patterns of gene flow into small populations are few. Thus, we sought to measure gene flow into small populations of wild radish. Raphanus sativus. We found significant differences in gene flow receipt among experimental populations and within those populations over a season. A maximum-likelihood estimate revealed that almost all of the gene flow into these synthetic populations had its origin in relatively distant (>650 m), large natural populations rather than the proximal (255-400 m), small synthetic populations. We also estimated rates of interpopulation mating from simple paternity analysis of progeny produced by seven small (ca. 50 plants) natural populations. Again, we found significant heterogeneity in gene flow receipt. Although these populations varied 10-fold in their range of isolation distances (100-1000 m), gene flow rates did not vary with distance. The magnitude of gene flow rates estimated in all but one population was great enough for gene flow to play an important role in the evolution of these small populations.     

Worldwide increase in incidence of Type I diabetes – the analysis of the data on published incidence trends

Aims/hypothesis. Several reports on the incidence of Type I (insulin-dependent) diabetes mellitus have suggested that the incidence is increasing. The aim of this study was to find out whether the incidence is increasing globally or restricted to a selected populations only and to estimate the magnitude of the change in incidence.¶Methods. During 1960 to 1996 37 studies in 27 countries were carried out. To fulfil the inclusion criteria the study periods ranged from 8–32 years. The temporal trend was fitted by linear regression, with the logarithm of the age-standardized incidence as the dependent variable and the calendar year as the independent variable. Then, the regression coefficient ( × 100 %) is approximately the average relative increase in incidence per year (as percentage).¶Results. Results from the pooled data from all 37 populations showed that the overall increase in incidence was 3.0 % per year (95 % CI 2.6; 3.3, p = 0.0001). The statistically significant increase was found in 24 of 37 populations including all high incidence ( > 14.6 per 100 000 a year) populations. The relative increase was, however, steeper in the populations with a lower incidence. The correlation between logarithm of the incidence and the increase in incidence was r = –0.56, p = 0.0004.¶Conclusion/interpretation. The incidence of Type I diabetes is increasing worldwide both in low and high incidence populations. By the year 2010 the incidence will be 50 per 100 000 a year in Finland and also in many other populations it will exceed 30 per 100 000 a year. [Diabetologia (1999) 42: 1395–1403]     

Translocation experiments with butterflies reveal limits to enhancement of poleward populations under climate change

There is a pressing need to predict how species will change their geographic ranges under climate change. Projections typically assume that temperature is a primary fitness determinant and that populations near the poleward (and upward) range boundary are preadapted to warming. Thus, poleward, peripheral populations will increase with warming, and these increases facilitate poleward range expansions. We tested the assumption that poleward, peripheral populations are enhanced by warming using 2 butterflies (Erynnis propertius and Papilio zelicaon) that co-occur and have contrasting degrees of host specialization and interpopulation genetic differentiation. We performed a reciprocal translocation experiment between central and poleward, peripheral populations in the field and simulated a translocation experiment that included alternate host plants. We found that the performance of both central and peripheral populations of E. propertius were enhanced during the summer months by temperatures characteristic of the range center but that local adaptation of peripheral populations to winter conditions near the range edge could counteract that enhancement. Further, poleward range expansion in this species is prevented by a lack of host plants. In P. zelicaon, the fitness of central and peripheral populations decreased under extreme summer temperatures that occurred in the field at the range center. Performance in this species also was affected by an interaction of temperature and host plant such that host species strongly mediated the fitness of peripheral individuals under differing simulated temperatures. Altogether we have evidence that facilitation of poleward range shifts through enhancement of peripheral populations is unlikely in either study species.     

Microgeographic Genetic Variation of the Malaria Vector Anopheles darlingi Root (Diptera: Culicidae) from Córdoba and Antioquia,Colombia

Anopheles darlingi is an important vector of Plasmodium spp. in several malaria-endemic regions of Colombia. This study was conducted to test genetic variation of An. darlingi at a microgeographic scale (approximately 100 km) from localities in Córdoba and Antioquia states, in western Colombia, to better understand the potential contribution of population genetics to local malaria control programs. Microsatellite loci: nuclear white and cytochrome oxidase subunit I (COI) gene sequences were analyzed. The northern white gene lineage was exclusively distributed in Córdoba and Antioquia and shared COI haplotypes were highly represented in mosquitoes from both states. COI analyses showed these An. darlingi are genetically closer to Central American populations than southern South American populations. Overall microsatellites and COI analysis showed low to moderate genetic differentiation among populations in northwestern Colombia. Given the existence of high gene flow between An. darlingi populations of Córdoba and Antioquia, integrated vector control strategies could be developed in this region of Colombia.     

From the Cover: Cryptic sexual populations account for genetic diversity and ecological success in a widely distributed,asexual fungus-growing ant

Sex and recombination are central processes in life generating genetic diversity. Organisms that rely on asexual propagation risk extinction due to the loss of genetic diversity and the inability to adapt to changing environmental conditions. The fungus-growing ant species Mycocepurus smithii was thought to be obligately asexual because only parthenogenetic populations have been collected from widely separated geographic localities. Nonetheless, M. smithii is ecologically successful, with the most extensive distribution and the highest population densities of any fungus-growing ant. Here we report that M. smithii actually consists of a mosaic of asexual and sexual populations that are nonrandomly distributed geographically. The sexual populations cluster along the Rio Amazonas and the Rio Negro and appear to be the source of independently evolved and widely distributed asexual lineages, or clones. Either apomixis or automixis with central fusion and low recombination rates is inferred to be the cytogenetic mechanism underlying parthenogenesis in M. smithii. Males appear to be entirely absent from asexual populations, but their existence in sexual populations is indicated by the presence of sperm in the reproductive tracts of queens. A phylogenetic analysis of the genus suggests that M. smithii is monophyletic, rendering a hybrid origin of asexuality unlikely. Instead, a mitochondrial phylogeny of sexual and asexual populations suggests multiple independent origins of asexual reproduction, and a divergence-dating analysis indicates that M. smithii evolved 0.5–1.65 million years ago. Understanding the evolutionary origin and maintenance of asexual reproduction in this species contributes to a general understanding of the adaptive significance of sex.