THE DISTINCTIVE FOOTPRINTS OF LOCAL HITCHHIKING IN A VARIED ENVIRONMENT AND GLOBAL HITCHHIKING IN A SUBDIVIDED POPULATION

Loci with higher levels of population differentiation than the neutral expectation are traditionally interpreted as evidence of ongoing selection that varies in space. This article emphasizes an alternative explanation that has been largely overlooked to date: in species subdivided into large subpopulations, enhanced differentiation can also be the signature left by the fixation of an unconditionally favorable mutation on its chromosomal neighborhood. This is because the hitchhiking effect is expected to diminish as the favorable mutation spreads from the deme in which it originated to other demes. To discriminate among the two alternative scenarios one needs to investigate how genetic structure varies along the chromosomal region of the locus. Local hitchhiking is shown to generate a single sharp peak of differentiation centered on the adaptive polymorphism and the standard signature of a selective sweep only in those subpopulations in which the allele is favored. Global hitchhiking produces two domes of differentiation on either side of the fixed advantageous mutation and signatures of a selective sweep in every subpopulation, albeit of different magnitude. Investigating population differentiation around a locus that strongly differentiates two otherwise genetically similar populations of the marine mussel Mytilus edulis, plausible evidence for the global hitchhiking hypothesis has been obtained. Global hitchhiking is a neglected phenomenon that might prove to be important in species with large population sizes such as many marine invertebrates.     

POPULATION GENETICS OF A PARASITIC CHROMOSOME: EXPERIMENTAL ANALYSIS OF PSR IN SUBDIVIDED POPULATIONS

Nasonia vitripennis is a parasitoid wasp that harbors several non-Mendelian sex-ratio distorters. These include MSR (Maternal Sex Ratio), a cytoplasmic element that causes nearly all-female families, and PSR (Paternal Sex Ratio), a supernumerary chromosome that causes all-male families. As in other hymenoptera, N. vitripennis has haplodiploid sex determination. Normally, unfertilized (haploid) eggs develop into males and fertilized (diploid) eggs develop into females. The PSR chromosome violates this normal pattern; it is inherited through sperm, but then causes destruction of the paternal chromosomes (except itself), thus converting diploid fertilized eggs (normally females) into haploid eggs that develop into PSR-bearing males. PSR is an extreme example of “parasitic” or “selfish” DNA. Because N. vitripennis has a highly subdivided population structure in nature, population-level selection may be important in determining the dynamics of PSR in natural populations. A theoretical analysis shows that subdivided population structure reduces PSR frequency, whereas high fertilization proportion (such as produced by the MSR element) increases PSR frequency. Population experiments using two deme sizes (3- and 12-foundress groups) and strains producing two fertilization proportions [wild-type (LabII)–57–67% female, and MSR (MI)–90–93% female] confirm these predictions. PSR achieved frequencies over 0.90 in 12–foundress group MSR populations in contrast to 0.20–0.40 in wild-type 12–foundress populations. PSR was selected against in wild-type populations composed of three-foundress groups. In MSR populations with three-foundress groups, presence of PSR selected against the MSR cytoplasmic element, eventually leading to low frequencies of both PSR and MSR. Complicated dynamics may occur when these two sex-ratio distorters are both present in highly subdivided populations. The existence of PSR in natural populations may depend on the presence of MSR. Results indicate that population subdivision could be important in determining the frequency of sex ratio distorters in N. vitripennis.     

MACRO- AND MICROGEOGRAPHIC STRUCTURE OF A SPATIALLY SUBDIVIDED BEETLE SPECIES IN NATURE

Spatial subdivision of species can affect their population structure by allowing processes such as limited dispersal, spatial heterogeneity in selective pressures, small population sizes, and random events to operate. By studying species restricted to islands or “island” habitats, one can attempt to determine which of these factors have affected the current structure of the population. Collops georgianus (Coleoptera: Melyridae), a beetle species endemic to the “island” habitat of granitic rock outcrops, was chosen to see how its spatially subdivided distribution has affected its genetic structure. Its genetic structure was examined on both a macrogeographic and a microgeographic level using protein electrophoresis. Macrogeographically, 12 populations throughout its range were sampled. The discontinuous distribution of outcrops, and thus populations, throughout its range, has determined the connectivity of the populations. Significant variation in allele frequencies and substructuring (F_ST = 0.192) was found throughout the range, but there was no spatial autocorrelation. Microgeographically, in the central part of the range, where outcrops are denser and more continuously distributed in space, there was evidence of isolation by distance. Very little variation in allele frequencies was found, but a low but significant level of substructuring occurred among the populations. Comparison of disjunct and continuous populations microgeographically revealed no effect of disjunct distributions, although a significant effect of distance was detected. Effective population size variation among populations and between years, compounded with the effects of local extinctions, suggest that random processes such as drift and founder effects are important determinants of the population's genetic structure.     

INBREEDING AS A STRATEGY IN SUBDIVIDED POPULATIONS

A generalized expression for coefficients of consanguinity and relationship with previous inbreeding is presented to examine various breeding strategies in subdivided populations. Conditions that would favor inbreeding are developed for: 1) nonfamilial inbreeding within a deme versus outbreeding; 2) altruistic inbreeding by females versus outbreeding; 3) sib-mating versus outbreeding; and 4) sib-mating versus nonfamilial breeding within a deme. Inbreeding behavior is advantageous under certain conditions but depends on the types of mating, the previous breeding history of the deme, the rate of accumulation of inbreeding depression, and the cost of migration. In polygynous mating systems it is genetically more advantageous for males to migrate, because female emigration may 1) leave a related male with no mate or one fewer mate, or 2) force both male and female to risk the cost of migration. Nonfamilial breeding is always a better strategy than sib-mating given previous inbreeding within the deme. Even when the cost of migration is zero, inbreeding is favored if the coefficient of relationship among relatives is greater than the ratio of the probabilities of offspring inviability to offspring viability. Although high inbreeding coefficients are probably not adaptive unless the costs of migration are great or inbreeding depression constants are small, low levels of inbreeding are advantageous in many situations. Therefore, increased genetic representation by way of inbreeding and inclusive fitness is a major component of the evolutionary process.     

EVOLUTION OF ALTRUISM IN KIN-STRUCTURED AND RANDOM SUBDIVIDED POPULATIONS

A. population structure favorable to the evolution of an altruistic trait is studied by Monte Carlo simulation. The model is based on a small-scale nonindustrial human society but seems generalizable to other highly social mammals. Three hierarchical levels are recognized: 1) the ecologically isolated local group (hamlet) which may be composed of kin and/or unrelated individuals; 2) the deme (settlement) comprising several such groups which interbreed; and 3) the set of demes (metapopulation) among which gene flow occurs. The first two levels of the model are based on D. S. Wilson's structured deme concept; the third allows for gene flow among demes in the metapopulation and for the structured diffusion of alleles across a wider area than might be included within the scope of a single deme. The simulation models genetic drift by a process of hamlet formation which may be random, or variously kin-structured. Hamlets may then become extinct based on a probability function of their gene frequencies. Individual selection within settlements is modeled deterministically, and gene flow among settlements is modeled as two-dimensional steppingstone migration of random or kin-structured groups. Results of the simulations show that, with realistic values for group sizes, moderate extinction rate, and high rates of migration (m > 27%), disadvantageous alleles (s = 10% and 25%) may increase markedly due to differential hamlet extinction over the course of 50 generations. The greater the degree of kin-structuring of founder groups, the higher the variance among hamlets and the faster the rate of increase of the allele for altruism. Nonetheless, even in some randomly founded groups, a clear increase in the altruism gene frequency occurred. It is also notable that kin-structured group selection by hamlet extinction may be effective when the initial frequency of altruism genes is very low (average of one per deme) and among a relatively small number of demes (25). Thus the process of group extinction in a hierarchically structured population allows rapid increase of an allele for altruism under plausible demographic conditions.     

NONSYMBIOTIC AND SYMBIOTIC NITROGEN FIXATION IN A WEAKLY MINEROTROPHIC PEATLAND

The acetylene reduction assay was used to measure nonsymbiotic and symbiotic nitrogen fixation in a weakly minerotrophic peatland throughout the ice-free season. Nonsymbiotic nitrogen fixation was found in surface materials and subsurface peat. In surface materials, nitrogenase activity measured in the field contributed about 0.6 kg N ha^-1 yr^-1, was closely associated with Sphagnum, but was not correlated with temperature between 12 and 27 C. No cyanobacteria were found in association with Sphagnum. In subsurface peat, nitrogenase activity measured in situ contributed no more than 0.4 kg N ha^-1 yr^-1 and was closely correlated with temperature between 7 and 21 C. There were uncertainites in these measurements due to presence of ethylene oxidizing activity and a long time lag. Symbiotic nitrogen fixation was found only in actinomycete-induced root nodules of Myrica gale L. Legumes were absent and the few lichens present lacked nitrogenase activity. Based on acetylene reduction assays, Myrica gale fixed about 35 kg N ha^-1 yr^-1. Nitrogenase activity in Myrica gale showed a strong seasonal pattern which varied little during three consecutive years even though water levels varied substantially. Nitrogen input to the peatland from nonsymbiotic nitrogen fixation was only 15% the amount contributed by bulk precipitation. Symbiotic fixation, in contrast, contributed approximately six times the amount in bulk precipitation.     

CARBONIC ANHYDRASE AND CARBON FIXATION IN COCCOLITHOPHORIDS1

Rates of carbon fixation in coccolithophorids in culture, unlike many other algae, are carbon limited at ambient levels of dissolved inorganic carbon (DIC). Apparently, plants often rely on activity of carbonic anhydrase (CA) to raise the level of CO₂ in cells and achieve carbon saturation. However, CA activities in the coccolithophorids, Coccolithus (= Emiliania) huxleyi Lohmann and Hymenomonas (=Cricosphaera) carterae Braarud, were either not detectable or very low compared to activities in other systems, including other algae, higher plants, and representative animals. Furthermore, additions of CA to medium with 2 mM DIC at pH 8.1 resulted in nearly 30% enhancement of photosynthesis, but not coccolith formation. Although carbon fixation in coccolithophorids can be suppressed by the CA inhibitor acetazolamide, studies of CaCO₃ nucleation revealed a non-specific effect of the inhibitor. Using a 30 min assay based on pH decreases accompanying loss of dissolved. CO₃^2-, inhibition of crystal formation in the absence of CA at 1 mM acetazolamide was demonstrated for decalcified crab carapace, a tissue with which normal CaCo₃ deposition in vitro has been shown. The results suggest only a minor role for CA in coccolithophorids.     

FLUCTUATING SELECTION AND RESPONSE IN A POPULATION OF FRESHWATER COPEPODS

The timing of springtime production of diapausing eggs by a population of the freshwater copepod, Diaptomus sanguineus, has been shown previously to be consistent with avoidance of seasonally intense fish predation. Natural selection acting on the timing of diapause fluctuates between years depending upon the population density of fish. Here we show that, in the field, the mean timing of diapause shifts between years in response to fluctuations in selection. Diapause is earlier in years following high predator density, and is later in years following low predator density. Although selection intensity in individual years may be large, the mean intensity over the decade of fluctuating selection investigated here is close to zero. Photoperiod sensitivity of the diapausetiming trait is heritable in the laboratory. The combination of fluctuating selection and multi-generation storage of genotypes as diapausing eggs in lake sediments may contribute to the maintenance of the genetic variation that permits the rapid selection response seen in the field.     

DISPERSAL AND PLANT MATING SYSTEMS: THE EVOLUTION OF SELF-FERTILIZATION IN SUBDIVIDED POPULATIONS

Intermediate rates of self-fertilization can be evolutionarily stable when the progeny of self-fertilization events are less successful migrants than those of outcrossing events, unless self-fertilization reduces an individual's contribution to the pollen pool by an amount equal to the rate at which it self-fertilizes. This result holds regardless of whether pollen or diaspores are more widely dispersed. The differential migration of selfed and outcrossed progeny may be a result of differential establishment with comparable rates of dispersal, or it may be a result of differential dispersal rates. In the first case, detailed predictions concerning the evolutionarily stable selfing rate can be made. In the second case, only qualitative predictions are possible in the absence of specific assumptions about how the migration rate is affected by the average selfing rate in each subpopulation.     

粒单系造血祖细胞(CFU-GM)琼脂半固体培养的原位固定及鉴定染色

用粒单系造血祖细胞（CFU－GM）体外琼脂半固体培养法,琼脂块原位固定,Wright染色与细胞化学和免疫细胞化学技术,对培养人的骨髓CFU－GM进行集落性质鉴定,结果表明：Wright染色能区别集落的性质及细胞的分化程度：CFU－GM的细胞中PAS、POX呈阳性反应;CD14、CD15分别在单核细胞和粒细胞的胞浆或胞膜上表达。该方法对鉴定细胞集落类型、集落中细胞的分化与成熟程度是简便可靠的手段。     

NITROGEN FIXATION BY LEGUMES AND BLUE-GREEN ALGAL-LICHEN CRUSTS IN A COLORADO DESERT ENVIRONMENT

Potential sources of fixed nitrogen in a Colorado desert environment were examined by the acetylene reduction method at the Deep Canyon Desert Research Center, near Palm Desert, California. In field and greenhouse studies all members of the genera Astragalus, Dalea, Lotus, Lupinus, Melilotus, and Prosopis examined formed active nodules (acetylene reduction) with indigenous soil bacteria. No evidence of nodulation was found for Acacia greggii, Cercidium floridum, or Hoffmannseggia microphylla. Lotus tomentellus was estimated to fix 0.1 kg N ha⁻¹ by the time of flowering under field conditions. Several members of the genus Dalea showed substantial rates of acetylene reduction in the greenhouse: D. emoryi, 16.1 + 3.5, D. mollissima, 11.4 + 3.7, D. schottii 2.9 + 1.7, D. spinosa 2.5 + 0.4 μmoles ethylene plant⁻¹ hr⁻¹. In greenhouse assays where water was supplied continuously, blue-green algal-lichen crusts reduced acetylene at an average rate of 11.0 + 5.7 nmoles ethylene cm⁻² hr⁻¹ with a maximum of 57.1. But when in situ assays were done following irrigation of a field plot with 2.3 cm of water, much lower activities were observed with a maximum activity of only 6.4 nmoles cm⁻² hr⁻¹.     

HERITABLE VARIATION IN RESOURCE UTILIZATION AND RESPONSE IN A WINERY POPULATION OF DROSOPHILA MELANOGASTER

It has been found that Drosophila melanogaster lines from the “Chateau Tahbilk” winery cellar had higher larval ethanol resistance than lines originating from outside the cellar. Because the adaptive significance of this trait is unclear, we have reexamined genetic microdifferentiation at Tahbilk with other resources and different tests for ethanol adaptation. Cellar stocks tended to be more resistant to starvation after exposure to wine seepage than stocks originating from orchard traps outside the cellar. Lines from a grape residue pile were also more resistant to starvation after seepage exposure than orchard stocks even though these collection sites were a few meters apart. Cellar and orchard stocks did not differ in ethanol resistance as measured by larval viability tests on low sucrose medium. However, stocks from the grape residue pile showed an increase in adult longevity over ethanol vapor compared to those from the cellar or orchard stocks. These differences were not due to maternal effects. In laboratory tests of behavioral responses, cellar stocks were relatively more attracted to seepage than orchard stocks in one year but not in the other two years. The findings suggest some adaptive differentiation related to resource heterogeneity at Tahbilk.