Genetic adaptation in emergence time ofClunio populations to different tidal conditions

Summary 1. The emergence times of intertidalClunio-species (Diptera, Chironomidae) are correlated with special tidal conditions in such a way that the immediately following reproduction of the short-lived imagos can take place on the exposed habitat.2. If the habitat of aClunio-species is situated in the middle tidal region and exposed twice a day by the tidal cycle (T = 12.4 h), a tidal rhythm of emergence with an average period of 12.4 hours may result (example:Clunio takahashii).3. If the habitat is located in the lower tidal zone, exposed only at about the time of the spring tides, a semilunar rhythm of emergence is expected (examples:Clunio marinus andClunio mediterraneus). These semilunar rhythms are correlated with certain conditions of low tide which occur at the coastal locations every 15 days at about the same time of day. The semilunar rhythm is therefore exactly characterized by two dates: the lunar emergence time (a few successive days around full and new moon) and the diurnal emergence time.4. According to experimental investigations on the control of the emergence rhythm, the midges are able to determine both dates in advance.5. Coastal populations differ in their lunar and diurnal emergence times. These differences correspond to the time of low tide which exists at each location during the emergence days of the semilunar rhythm.6. Crossbreeding between stocks of different populations showed that the differences in diurnal emergence time are gene-controlled.

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Genetische Adaptation der Schlüpfzeiten vonClunio-Populationen an verschiedene Gezeitenbedingungen

Kurzfassung Die Schlüpfzeiten der in der Gezeitenzone lebendenClunio-Arten (Diptera, Chironomidae) sind mit bestimmten Wasserstandsbedingungen synchronisiert, und zwar derart, daß die unmittelbar anschließende Fortpflanzung der kurzlebigen Imagines auf dem trockengefallenen Habitat stattfinden kann. Wenn das Habitat einerClunio-Art in der mittleren Gezeitenzone liegt und parallel zu dem halbtägigen Gezeitenzyklus (T = 12,4 h) zweimal täglich auftaucht, dann kann sich eine 12,4stündige Schlüpfperiodik einstellen (Beispiel:Clunio takahashii). Wenn das Habitat in der unteren Gezeitenzone liegt und nur um die Zeit der Springtiden auftaucht, dann ist eine 15tägige (semilunare) Schlüpfperiodik zu erwarten (Beispiele:Clunio marinus undC. mediterraneus). Diese 15tägige Schlüpfperiodik ist synchronisiert mit bestimmten Niedrigwasserbedingungen, die an einem Küstenort alle 15 Tage jeweils um die gleiche Tageszeit auftreten. Sie wird daher durch zwei Daten eindeutig gekennzeichnet: (1) die lunaren Schlüpftage (wenige aufeinanderfolgende Tage um Voll- und Neumond) und (2) die tägliche Schlüpfzeit. Wie experimentelle Untersuchungen über die Steuerung der Schlüpfperiodik zeigten, können die Tiere beide Daten richtig vorausbestimmen. Die einzelnen Küstenpopulationen unterscheiden sich allerdings in Anpassung an die örtlichen Gezeiten- und Standortbedingungen recht auffällig in ihren lunaren und täglichen Schlüpfzeiten. Kreuzungsversuche zwischen Laboratoriumsstämmen verschiedener Populationen belegen, daß die Unterschiede in der täglichen Schlüpfzeit genkontrolliert sind.

     

Genetic variations in Tibetan populations and high-altitude adaptation at the Himalayas

Modern humans have occupied almost all possible environments globally since exiting Africa about 100,000 years ago. Both behavioral and biological adaptations have contributed to their success in surviving the rigors of climatic extremes, including cold, strong ultraviolet radiation, and high altitude. Among these environmental stresses, high-altitude hypoxia is the only condition in which traditional technology is incapable of mediating its effects. Inhabiting at >3,000-m high plateau, the Tibetan population provides a widely studied example of high-altitude adaptation. Yet, the genetic mechanisms underpinning long-term survival in this environmental extreme remain unknown. We performed an analysis of genome-wide sequence variations in Tibetans. In combination with the reported data, we identified strong signals of selective sweep in two hypoxia-related genes, EPAS1 and EGLN1. For these two genes, Tibetans show unusually high divergence from the non-Tibetan lowlanders (Han Chinese and Japanese) and possess high frequencies of many linked sequence variations as reflected by the Tibetan-specific haplotypes. Further analysis in seven Tibetan populations (1,334 individuals) indicates the prevalence of selective sweep across the Himalayan region. The observed indicators of natural selection on EPAS1 and EGLN1 suggest that during the long-term occupation of high-altitude areas, the functional sequence variations for acquiring biological adaptation to high-altitude hypoxia have been enriched in Tibetan populations.     

Limits to adaptation in asexual populations

In asexual populations, the rate of adaptation is basically limited by the frequency and properties of spontaneous beneficial mutations. Hence, knowledge of these mutational properties and how they are affected by particular evolutionary conditions is a precondition for understanding the process of adaptation. Here, we address how the rate of adaptation of asexual populations is limited by its population size and mutation rate, as well as by two factors affecting the fraction of mutations that confer a benefit, i.e. the initial adaptedness of the population and the variability of the environment. These factors both influence which mutations are likely to occur, as well as the probability that they will ultimately contribute to adaptation. We attempt to separate the consequences of these basic population features in terms of their effect on the rate of adaptation by using results from evolution experiments with microorganisms.     

Genetic similarities within and between human populations

The proportion of human genetic variation due to differences between populations is modest, and individuals from different populations can be genetically more similar than individuals from the same population. Yet sufficient genetic data can permit accurate classification of individuals into populations. Both findings can be obtained from the same data set, using the same number of polymorphic loci. This article explains why. Our analysis focuses on the frequency, omega, with which a pair of random individuals from two different populations is genetically more similar than a pair of individuals randomly selected from any single population. We compare omega to the error rates of several classification methods, using data sets that vary in number of loci, average allele frequency, populations sampled, and polymorphism ascertainment strategy. We demonstrate that classification methods achieve higher discriminatory power than omega because of their use of aggregate properties of populations. The number of loci analyzed is the most critical variable: with 100 polymorphisms, accurate classification is possible, but omega remains sizable, even when using populations as distinct as sub-Saharan Africans and Europeans. Phenotypes controlled by a dozen or fewer loci can therefore be expected to show substantial overlap between human populations. This provides empirical justification for caution when using population labels in biomedical settings, with broad implications for personalized medicine, pharmacogenetics, and the meaning of race.     

Genetic analysis of host-parasite coevolution in human malaria.

Recent twin studies of clinical malaria and immune responses to malaria antigens have underscored the importance of both major histocompatability complex (MHC) and non-MHC genes in determining variable susceptibility and immune responsiveness. By using a combination of whole genome genetic linkage studies of families and candidate genes analysis, non-MHC genes are being mapped and identified. Human leucocyte antigen (HLA) genotype was found to affect susceptibility to severe malaria in a large study of West African children. T lymphocytes that may mediate such resistance have been identified and their target antigens and epitopes characterized. Some of these epitopes show substantial polymorphism, which appears to result from immune selection pressure. Natural variant epitopes have been found to escape T-cell recognition in cytolytic and other T-cell assays. More recently a novel immune escape mechanism has been described in viral infections, altered peptide ligand antagonism, whereby variants of a T-cell epitope can downregulate or ablate a T cell response to the index peptide. The likely implications of such immune escape mechanisms for the population structure of malaria parasites, for HLA associations with malaria infection and disease, and for the design of new malaria vaccines, are discussed. The evolutionary consequences of such molecular interactions can be assessed by using mathematical models that capture the dynamic of variable host and parasite molecules. Combined genetic, immunological and mathematical analysis of host and parasite variants in natural populations can identify some mechanisms driving host-parasite coevolution.     

Genetic mapping in the human malaria parasite Plasmodium falciparum

The Plasmodium falciparum genome sequence has boosted hopes for a new era of malaria research and for the application of comprehensive molecular knowledge to disease control, but formidable obstacles remain: approximately 60% of the predicted P. falciparum proteins have no known functions or homologues, and most life cycle stages of this haploid eukaryotic parasite are relatively intractable to cultivation and biochemical manipulation. Genetic mapping based on high-resolution maps saturated with single-nucleotide polymorphisms or microsatellites is now providing effective strategies for discovering candidate genes determining important parasite phenotypes. Here we review classical linkage studies using laboratory crosses and population associations that are now amenable to genome-wide approaches and are revealing multiple candidate genes involved in complex drug responses. Moreover, mapping by linkage disequilibrium is practicable in cases where chromosomal segments flanking drug-selected genes have been preserved in populations during relatively recent P. falciparum evolution. We discuss the advantages and limitations of these various genetic mapping strategies, results from which offer complementary insights to those emerging from gene knockout experiments and/or high-throughput genomic technologies.     

Genetic resistance to malaria in mouse models

Murine models have proved to be excellent tools in the support of studies of the human genetic bases of malaria resistance and have enabled the mapping of 12 resistance loci, eight of them controlling parasitic levels and four controlling cerebral malaria. Further studies using this method have identified a Pklr variant that confers resistance to murine malaria, a result that shows the potential of this approach to aid the understanding of mechanisms of disease resistance. In the future, the use of murine models for genetic resistance to malaria could lead to the identification of relevant genetic factors that control this devastating disease.     

Genetic differentiation in life history between Daphnia galeata populations: an adaptation to local predation regimes?

Species of the water flea Daphnia exhibit constitutive as wellas phenotypically inducible anti-predator defence strategies,involving life history, morphological and behavioural traits.We explored the hypothesis of genetic differentiation in anti-predatordefence strategies using Daphnia clones originating from twodifferent water bodies: Tjeukemeer (the Netherlands) and FishPond (Belgium). Both water bodies are inhabited by zooplanktivorousfish. In contrast to Tjeukemeer, Fish Pond is also inhabitedby larvae of the phantom midge Chaoborus. The life history responsesof the two sets of clones to kairomones from fish (Perca), tokairomones from Chaoborus, and to a mixture of both were compared.Clones from Tjeukemeer and Fish Pond showed strong responsesto the presence of fish kairomone, with reductions in adultand neonate body length, in age at first reproduction and inthe total number of neonates produced during the first threeadult instars. Responses to Chaoborus kairomone were much lesspronounced, although there was a tendency towards an increasein the number of neonates in the first brood. Significant inter-populationgenetic differences were found for all the investigated traits.However, there was no indication for genetic adaptation of theFish Pond clones to negative size-selective predation by Chaoborus.Compared to Tjeukemeer clones, Fish Pond clones had a lowersize at first reproduction, produced smaller neonates and produceda higher number of neonates in their first brood. This suggestsadaptation to positive rather than to negative size selectivepredation. Genetic differences between populations were observedmainly for constitutive life history traits, rather than forphenotypic shifts in response to the presence of predator kairomones.     

Genetic susceptibility to malaria getting complex

The evolution of sickle cell disease illustrates the powerful selective pressure of malaria in Africa, and candidate gene association studies have identified more than ten putative susceptibility determinants involving erythrocytes or the immune system. Efforts at present are aimed at understanding the functional basis of known associations, and at developing both linkage- and association-based approaches of genome-wide screening for novel susceptibility factors.     

Local adaptation in rotifer populations

The adaptation of organisms to their environment has been a subject of study for a long time. One method to study adaptations in populations involves comparing contemporary populations of the same species under different selective regimes, in what is known as a ??local adaptation?? study. A previous study of the cyclically parthenogenetic rotifer Brachionus plicatilis found high heritabilities for some life-history traits. Some of these life-history traits significantly differed among six populations from Eastern Spain and data suggested some traits to have higher evolutionary rates than neutral genetic markers. Here, by studying the same B. plicatilis populations, we examine the variation and possible local adaptation of their main life-history traits, closely related to fitness, in relation to habitat salinity and temperature. These environmental factors have been shown to play a key role in the ecological differentiation among co-generic species of B. plicatilis. The results obtained in this study show that: (1) the seasonality of rotifer populations from Eastern Spain has profoundly influenced sexual reproduction strategies; (2) salinity is probably a key factor in the ecological specialization of some populations; and (3) rotifer populations harbour high variability in their fitness components.     

Genetic variation of recent Alu insertions in human populations

The Alu family of intersperesed repeats is comprised of ovr 500,000 members which may be divided into discrete subfamilies based upon mutations held in common between members. Distinct subfamilies of Alu sequences have amplified within the human genome in recent evolutionary history. Several individual Alu family members have amplified so recently in human evolution that they are variable as to presence and absence at specific loci within different human populations. Here, we report on the distribution of six polymorphic Alu insetions in a survey of 563 individuals from 14 human population groups across several continents. Our results indicate that these polymorphic Alu insertions probably have an African origin and that there is a much smaller amount of genetic variation between European populations than that found between other populations groups. Present address: Department of Pathology, Stanley S. Scott Cancer Center, Louisiana State University Medical Center, 1901 Perdido St., New Orleans, LA 70112 Correspondence to: M.A. Batzer     

Genetic variability in response to dietary calcium

Supplemental dietary calcium has been shown to reduce blood pressure in spontaneously hypertensive rats while restricted calcium diets cause an elevation in blood pressure. This latter nutrient effect has been enhanced by modest sodium restriction and is associated with a reduction in serum ionized calcium concentration. To determine whether alterations of dietary calcium and sodium have a similar influence on blood pressure in genetically normotensive rats, Fisher 344, Wistar Furth, and ACI rats were fed either a low (0.1%) calcium, low (0.25%) sodium diet or normal (1.0%) calcium, normal sodium (0.45%) diet from 4 weeks of age through 29 weeks of age. Indirect measurements of systolic blood pressure showed that only the Fisher 344 rats consistently responded to the low calcium/low sodium diets with an elevation of blood pressure. There was considerable variation in serum electrolytes across strains in the normal diets but all three strains experienced a reduction in ionized calcium and an elevation in phosphorus and magnesium on the restricted diets. In the Fisher 344 rats there were significant (p less than .05) inverse correlations among systolic blood pressure and serum ionized and total calcium concentrations and positive correlations among systolic blood pressure, phosphorus, and magnesium. There was no significant correlation between serum electrolytes and blood pressure in the other two strains. The data indicate that there is genetic variability in the blood pressure response to alterations in dietary calcium and sodium. The pattern of change in serum electrolytes across strains suggests that diet-induced alterations of serum electrolytes, specifically calcium, are not necessarily predictive of a pressor response. It would appear that some other calcium-sensitive physiological process involved in blood pressure regulation must respond differentially to calcium availability across strains.