Darwinian fitness and adaptedness in experimental populations of Drosophila willistoni

Fifteen second chromosomes were extracted from Drosophila willistoni flies collected in four natural populations. The adaptedness of populations homozygous for each chromosome was measured by average population size and productivity. Six control populations were established with mixtures of the wild second chromosomes. The Darwinian fitness of flies homozygous for each wild second ehromosome, and of flies carrying random combinations of these chromosomes, was measured relative to the fitness of flies heterozygous for a wild and a marker chromosome. The Darwinian fitness of homozygotes for each second chromosome relative to the fitness of flies carrying random combinations of the natural chromosomes was then inferred. The estimated loss of fitness on making the natural second chromosomes homozygous was substantial, ranging from 39 to 83 pereent, with an average reduction in fitness of 66 percent. These results with D. willistoni are consistent with those from similar experiments with other drosophila species, and they are compatible with a significant role for heterosis in the maintenance of genetic variability.Populations homozygous for wild chromosomes differ in their adaptedness to the experimental environment. Population size and productivity are correlated, although the correlation is far from complete. Some populations have high productivity and low population size, or vice versa. The control populations, with greater genetic variability, were superior in adaptedness to the average of the single-chromosome populations. The Darwinian fitness and the adaptedness of the genotypes in this experiment were not significantly correlated. It follows that certain measures used by population geneticists, such as genetic load and average Darwinian fitness, cannot be taken as general indices of how well adapted a population is to its environment.This work was supported by U.S. Public Health Service Grant RO1-HDO5055, NSF grant GB-20694 (International Biological Program). AEC contract AT-(30-1) 3096, and PHS Career Development Award K3 GM 37265. The collection of the flies was supported by the Fundacão de Amparo a Pesquisa do Estado de São Paulo, Brazil. The senior author's stay in New York, where the experiments were conducted, was financed in part by Research Fellowship 2-12861 from the Panamerican Union.     

Fitness costs of Cry1F resistance in fall armyworm,Spodoptera frugiperda

Transgenic corn, Zea mays L., expressing the Bacillus thuringiensis Berliner (Bt) protein Cry1F has been registered for Spodoptera frugiperda (J. E. Smith) control since 2003 in the USA. Unexpected damage to Cry1F corn was reported in 2006 in Puerto Rico, and Cry1F resistance in S. frugiperda from Puerto Rico was documented. The study of fitness costs associated with insect resistance to Bt insecticidal proteins is important for understanding resistance evolution and for evaluating resistance management practices used to mitigate resistance to transgenic corn. Currently, no studies have addressed the fitness costs associated with Cry1F resistance in S. frugiperda. In this study, susceptible and resistant strains with similar genetic background and their reciprocal crosses were used to estimate Cry1F resistance fitness costs. Comparisons between life‐history traits and population growth rates of homozygous susceptible, heterozygous and homozygous resistant S. frugiperda were used to determine whether the resistance is associated with fitness costs. Major fitness costs were not apparent in either heterozygotes or homozygous resistant insects. However, there was a slight indication of hybrid vigour in the heterozygotes. Additionally, two lines in which the frequency of the resistant alleles was fixed at 0.5 were followed for seven generations, after which the frequency of resistant alleles slightly decreased in both lines. The lack of strong fitness costs associated with Cry1F resistance in S. frugiperda indicates that initial allele frequencies may be higher than expected in field populations and will tend to remain stable in field populations in the absence of selection pressure (e.g. Puerto Rico).     

Genetic Load and Viability Distribution in Central and Marginal Populations of DROSOPHILA SUBOBSCURA

Relative viabilities of individuals homozygous or randomly heterozygous for wild O chromosomes derived from a marginal (Norwegian) and a central (Greek) population of D. subobscura were obtained by means of a newly prepared marker strain. In the central and marginal populations 20.8 and 28.8 percent of all chromosomes proved lethal or semilethal in homozygous condition. Mean viability was higher for +/+ random heterozygotes than for +/+ homozygotes. This remained the case for the marginal, but not for the central populations, after exclusion of the detrimental chromosomes from the calculations. The variances of viabilities were higher for homozygotes than for heterozygotes, but the test crosses with chromosomes from the marginal population had generally higher variances than those with chromosomes from the central population. No correlation was found in either populations between the action of a chromosome in homozygous condition with its action in heterozygous condition. This is interpreted as complete recessiveness of genetic load. The results are discussed in terms of the observed reduction of the inversion polymorphism which is not paralleled by a reduction in enzyme and, as shown here, by reduction in viability variation. It is thought that the heterotic effect of inversions is due to their homeostatic action, which depends less on structural genes than on higher orders of organization due to gene interaction or regulation. Whatever the causes, it is very likely that marginal populations differ from central populations with respect to their genetic system.     

Analysis of the relationship between allozyme heterozygosity and fitness in the rare Gentiana pneumonanthe L.

Especially for rare species occurring in small populations, which are prone to loss of genetic variation and inbreeding, detailed knowledge of the relationship between heterozygosity and fitness is generally lacking. After reporting on allozyme variation and fitness in relation to population size in the rare plant Gentiana pneumonanthe, we present a more detailed analysis of the association between heterozygosity and individual fitness. The aim of this study was to test whether increased fitness of more heterozygous individuals is explained best by the ‘inbreeding’ hypothesis or by the ‘overdominance’ hypothesis. Individual fitness was measured during 8 months of growth in the greenhouse as the performance for six life-history parameters. PCA reduced these parameters to four main Fitness Components. Individual heterozygosity was scored for seven polymorphic allozyme loci. For some of these loci (e.g. Aat3, Pgm1 and 6Pgdh2) heterozygotes showed a significantly higher relative fitness than homozygotes. To test the inbreeding model, regression analyses were performed between each Fitness Component and the number of heterozygous loci per individual. Multiple regressions with the adaptive distance of five loci as independent variables were used to test the overdominance model. Only the inbreeding model was a statistically significant explanation for the relationship between heterozygosity and fitness in G. pneumonanthe. The number of heterozygous loci was significantly negatively correlated with the coefficients of variation of three of the six initially measured fitness parameters. This suggests a lower developmental stability among more homozygous plants and may explain the higher phenotypic variation in small populations of the species observed earlier. The importance of the results for conservation biology is discussed.     

Organophosphorus insecticide resistance inDrosophila melanogaster populations

Natural variation inDrosophila melanogaster populations for mixed function oxidase activity and organophosphorus resistance was studied by sampling iso-chromosomal lines and laboratory selection. A 20-fold variation in malathion LC₅₀ was found among a sample of 25 third chromosomes from a Raleigh, North Carolina, population. These chromosomes were combined in a population that was selected for malaoxon (a toxic metabolite of malathion) resistance over 12 generations. Response to selection was rapid—within three generations—but small, less than two-fold increase in malathion LC₅₀. Mixed function oxidase activity, as assayed by 7-ethoxycoumarin-O-deethylase, increased in parallel with malathion resistance in the selected population. In spite of the fact that this population was initially formed from strains which were homozygous for chromosome III, after 12 generations of selection for malaoxon resistance only a minority of third chromosomes could be isolated as homozygous combinations. This suggests that selection favoured heterozygous combinations of alleles with decreased fitness in the homozygous state. In a second study, a sample of 39 iso-female lines were collected from a Vineland, Ontario, population. Imidan? (phosmet) LC₅₀ varied 20-fold among these iso-female lines and was strongly correlated with increased 7-ethoxycoumarin-O-deethylase activity. The distribution of 7-ethoxycoumarin-O-deethylase activity was bimodal and estimates of the effective number of segregating factors by Wright’s formula were consistent with a single gene controlling extreme 7-ethoxycoumarin-O-deethylase activity differences. Vineland flies responded rapidly to selection for imidan resistance, but as with malaoxon selection only to a small degree. The 7-ethoxycoumarin-O-deethylase activity increased in imidan-selected flies to the level of the most resistant iso-female line from the sampled population. The major part of selected imidan resistance and all of the increased 7-ethoxycoumarin-O-deethylase activity were attributed to third chromosomal genes. The results suggest that theseDrosophila populations contained a polymorphism for a major factor on chromosome III controlling elevated mixed function oxidase activity together with associated organophosphorus resistance. This polymorphism provided the immediate response to insecticide selection. Other genes have minor effects and combine to give a multifactorial response to selection over longer periods of time.     

An application of kinship process to the gene frequencies: linkage disequilibrium due to random drift in mendelian genetics with reversible mutation and in molecular genetics.

Selection at the colony level in social Hymenoptera with colonies containing single, once-mated queens is examined under a simple two-allele model. The condition for balanced polymorphism is $\text{X>}\text{2V}{}^2\text{(V + 1)}$, where V is the fitness of colonies with all workers homozygous and X that of colonies with both heterozygous and homozygous workers, relative to the fitness of colonies with all workers heterozygous. For certain fitness combinations satisfying the above relationship and characterized by values of V and X much lower than one, iteration reveals the development of stable limit cycles of allele frequencies rather than convergence to an equilibrium point. Addition of a third allele, or overlap between generations, eliminates these cycles. Queen-level overdominance is sufficient but not necessary for balanced polymorphism when V < 1, is both sufficient and necessary when V = 1, and is necessary but not sufficient when V > 1. Colony-level selection is a potentially powerful force maintaining genetic variation in populations of social insects, but does not imply correspondence between queen and worker genotype frequencies.     

The genetic conditions in heterozygous and homozygous populations of Drosophila. II. X-ray induced lethals in a homozygous and a heterozygous population

Strains of Drosophila melanogaster were made isogenic for their second chromosomes by means of the marker strain LCy/Pm. One of these strains was used as a founder for a homozygous experimental population (W). All other strains were mixed and established a heterozygous population (LKW). Both populations were free of lethals in the beginning with respect to their second chromosomes. After they had been exposed to an X-ray irradiation of 7000 r they contained about 26 per cent newly induced lethal chromosomes. Whereas in the heterozygous population the lethal frequency decreased rather fast to 10 per cent, that of the homozygous population remained rather constant at 25 per cent during a period of 135 days. After a year of continuation, however, both populations reached the same lethal frequency of about 10 per cent. Allelism tests carried out after 10 generations revealed that there was a highly heterotic lethal factor in the homozygous population. After excluding this heterotic lethal from the calculations, the lethal frequencies of the two populations remained significantly different. It was assumed that the relative mean fitness of lethal heterozygotes was generally higher in the homo-than in the heterozygous populations. The results indicate that homozygous populations are much more capable of incorporation new mutations than heterozygous.     

Inbreeding and fitness in captive populations: Lessons from Drosophila

The avoidance of inbreeding is a primary goal of endangered species population management. In order to fully understand the effects of inbreeding on the fitness of natural and captive populations, it is necessary to consider fitness components which span the entire life cycle of the organism. Using Drosophila melanogaster as a model organism for conservation genetics studies, we constructed 18 experimental lines derived from wild-type stocks which were homozygous for chromosome 2 (this chromosome constitutes 38% of the genome or is equivalent to F = 0.38). For six of these lines which exhibited a reduced homozygous fitness, we estimated the relative values of fitness components operating at both the juvenile stage (pre-adult viability) and adult stage (female fecundity and male-mating ability) of the life cycle. Males in these lines showed a markedly reduced mating ability, while viability and female fecundity were much less affected. Equilibrium values of the wild-type chromosomes in these lines were accurately predicted using a model that incorporated into it these independently estimated fitness components. These results emphasize the importance of studying all fitness components directly to determine overall fitness. A reduced mating ability among inbred males of a captive population can have serious consequences for its future sustainability, and can further jeopardize reintroduction efforts; consequently, a program to carefully monitor the reproductive success of individual males, as well as other fitness components, is recommended. © 1993 Wiley-Liss, Inc.     

Genetic load and viability variation in korean natural populations ofDrosophila melanogaster

Summary Natural populations of Drosophila melanogaster from Anyang and Susac (suburbs of Seoul) have been analyzed with respect to viability variation on the second chromosome. Homozygotes as well as random heterozygotes for wild chromosomes were studied. The frequency of lethal factors was about 16 per cent, that of drastics 26 per cent. The average viability of homozygotes was 0.650 including lethal lines and 0.858 for quasinormals; that for random heterozygotes was 1.125. Allelism tests have been performed for the lethals. The allelism rate turned out to be as high as 0.036 and 0.0214, respectively. Using a formula by Nei, the effective population size can be estimated from these data. Korean D. melanogaster populations proved as small as 2000 to 3000 individuals. No correlation between homozygous and heterozygous viabilities could be found. According to these observations, along with the fact that partly big clusters of identic lethals could be found in the allelism tests, it is concluded that in Korean populations quite a large part of the hard genetic load is balanced. The connection between population size, population structure and associative or genuine overdominance is discussed.     

Balanced polymorphisms of quasineutral alleles

When the average number of progeny born to the genotypes at a locus are linear functions of a random variable such that, on the average, the number of progeny produced by each genotype is the same (quasineutrality), then there will be a systematic pressure on gene frequency that tends to favor the genotype with the smallest variance in the number of progeny. In particular, there will be a stable equilibrium attained whenever the variance in the number of progeny produced by the heterozygous genotype is less than that of both homozygous genotypes. We assume that the random variable affecting fitness is some function of an underlying environmental variable that tends to smooth out the day-to-day fluctuations in the environmental variable, such as the mean or the range or the variance. The effect of the environment on fitness can, therefore, be assumed to be approximately constant for periods corresponding to one generation. If the logarithm of the average number of progeny is a linear function of a random variable, then in contrast to the foregoing conclusions, there will be no tendency for any change in gene frequency at the locus (Kimura, M. (1954), Process leading to quasi-fixation of genes in natural populations due to random fluctuations of selection intensities, Genetics39, 280–295; (1964), Diffusion models in population genetics, J. App. Prob.1, 177–232.).     

Long-term exhaustion of the inbreeding load in Drosophila melanogaster

Inbreeding depression, the decline in fitness of inbred individuals, is a ubiquitous phenomenon of great relevance in evolutionary biology and in the fields of animal and plant breeding and conservation. Inbreeding depression is due to the expression of recessive deleterious alleles that are concealed in heterozygous state in noninbred individuals, the so-called inbreeding load. Genetic purging reduces inbreeding depression by removing these alleles when expressed in homozygosis due to inbreeding. It is generally thought that fast inbreeding (such as that generated by full-sib mating lines) removes only highly deleterious recessive alleles, while slow inbreeding can also remove mildly deleterious ones. However, a question remains regarding which proportion of the inbreeding load can be removed by purging under slow inbreeding in moderately large populations. We report results of two long-term slow inbreeding Drosophila experiments (125–234 generations), each using a large population and a number of derived lines with effective sizes about 1000 and 50, respectively. The inbreeding load was virtually exhausted after more than one hundred generations in large populations and between a few tens and over one hundred generations in the lines. This result is not expected from genetic drift alone, and is in agreement with the theoretical purging predictions. Computer simulations suggest that these results are consistent with a model of relatively few deleterious mutations of large homozygous effects and partially recessive gene action.Subject terms: Quantitative trait, Inbreeding     

The incidental response to uniform natural selection

When populations are exposed to novel conditions of growth, they often become adapted to a similar extent, and at the same time, evolve some degree of impairment in their original environment. They may also come to vary widely with respect to characters which are uncorrelated with fitness, as the result of chance genetic associations among the founders, when these are a small sample from a large and variable ancestral population. I report an experiment in which 240 replicate lines of the unicellular chlorophyte Chlamydomonas were derived from primarily photoautotrophic ancestors and cultured as heterotrophs in the dark. All adapted to the dark and were impaired in the light after several hundred generations of culture. They also displayed a wide range of colony morphologies that were uncorrelated with fitness. This incidental response to selection probably arose through random variation in the initial composition of the lines. The differences between closely related species or varieties may likewise arise, in similar circumstances, by sampling error rather than natural selection.     

Genetic variation for preadult viability in Drosophila melanogaster

The extent of genetic variation in fitness and its components and genetic variation's dependence on environmental conditions remain key issues in evolutionary biology. We present measurements of genetic variation in preadult viability in a laboratory-adapted population of Drosophila melanogaster, made at four different densities. By crossing flies heterozygous for a wild-type chromosome and one of two different balancers (TM1, TM2), we measure both heterozygous (TM1/+, TM2/+) and homozygous (+/+) viability relative to a standard genotype (TM1/TM2). Forty wild-type chromosomes were tested, of which 10 were chosen to be homozygous viable. The mean numbers produced varied significantly between chromosome lines, with an estimated between-line variance in log(e) numbers of 0.013. Relative viabilities also varied significantly across chromosome lines, with a variance in log(e) homozygous viability of 1.76 and of log(e) heterozygous viability of 0.165. The between-line variance for numbers emerging increased with density, from 0.009 at lowest density to 0.079 at highest. The genetic variance in relative viability increases with density, but not significantly. Overall, the effects of different chromosomes on relative viability were remarkably consistent across densities and across the two heterozygous genotypes (TM1, TM2). The 10 lines that carried homozygous viable wild-type chromosomes produced significantly more adults than the 30 lethal lines at low density and significantly fewer adults at the highest density. Similarly, there was a positive correlation between heterozygous viability and mean numbers at low density, but a negative correlation at high density.     

Studies on the selective agents operating in experimental populations ofTisbe clodiensis (Copepoda,Harpacticoida)

In order to determine the biotic selective factors affecting the fitness of genotypes in the polymorphic speciesT. clodiensis, five types of experimental populations were utilized:

1. Populations started with equal numbers of virgin females and males of the three genotypes (pp, pP, PP) at f(p)=f(P)=0.5 in Hardy-Weinberg proportions;
2. Populations started with equal numbers of heterozygous females and males;
3. Populations started with equal numbers of recessive homozygous females and dominant homozygous females, each fertilized by males of the same genotype;
4. Populations with virgin females and males of the three genotypes at f(p)=0.2, f(P)=0.8 in Hardy-Weinberg proportions;
5. One population as in (4) but with f(p)=0.7 and f(P)=0.3.

The populations were sampled at about three weekly intervals in order to follow the gene frequencies for about fifteen generations. A comparison between group (3) and other groups suggests that interaction between genotypes may affect the survival of the genotypes concerned. Comparison of groups (1) and (2) seems to indicate that sexual selection is also involved. The estimated selective coefficients, which appear to vary in different periods and groups of populations, lead to the conclusion that fitness inT. clodiensis is a function of the genetic structure of the population as well as of the population density. The selection pressure is then determined by a great number of biotic components operating in different stages of the life cycle; and it seems reasonable to conclude that inT. clodiensis, at least under these experimental conditions, a frequency-dependent selective mechanism is involved.     

A population genetic analysis of self- and cross-incompatibility in sugar beet (Beta Vulgaris L.)

Summary A population genetic model is proposed for the reproduction of self-incompatible inbred lines in which incompatibility is controlled by 1–4 loci. From theoretical considerations it was expected that: a) with the random matings of lines I_n, (obtained by self-pollination of n generations), some lines would be cross-incompatible (all the plants within these lines would be homozygous for S-genes) and the rest would be cross-compatible (retain heterozygosity for one or more S-genes); b) in the case of random matings of Unes I_nG_m (obtained by self-pollination of n generations and by random pollination for m generations), some lines would be cross-incompatible (heterozygous for one S-gene) and the rest would be cross-compatible (retain heterozygosity for two or more S-genes); c) the relative proportion of sterile plants, obtained by random pollination of cross-compatible lines, would be related to the number of segregating S-loci and to the generation in which the lines are studied.Forty-four inbred lines of sugar beet derived from self-incompatible plants of a population were analysed. Comparisons of the observed values with the theoretically expected ones demonstrated that: a) of 18 I_n (I₁-I₄) lines, 6 were cross-incompatible (homozygous for S-genes) and 12 were cross-compatible having one S-locus segregating in 7 lines and two S-loci segregating in 5 lines; b) of 22 I_nG₁ (I₂G₁ and I₃G₁) lines, one line was self-fertile, 7 lines were cross-incompatible (heterozygous for one S-loci) and 14 lines were cross-compatible (heterozygous for two S-loci).No line was found to have three or more segregating S-loci. The results of this population genetics analysis of self- and cross-incompatibility in sugar beet comply with diallel analysis data on sugar beet incompatibility and indicates that it is under the gametophytic control of two basic S-loci.     

Simultaneous Estimation of Additive and Mutational Genetic Variance in an Outbred Population of Drosophila serrata

How new mutations contribute to genetic variation is a key question in biology. Although the evolutionary fate of an allele is largely determined by its heterozygous effect, most estimates of mutational variance and mutational effects derive from highly inbred lines, where new mutations are present in homozygous form. In an attempt to overcome this limitation, middle-class neighborhood (MCN) experiments have been used to assess the fitness effect of new mutations in heterozygous form. However, because MCN populations harbor substantial standing genetic variance, estimates of mutational variance have not typically been available from such experiments. Here we employ a modification of the animal model to analyze data from 22 generations of Drosophila serrata bred in an MCN design. Mutational heritability, measured for eight cuticular hydrocarbons, 10 wing-shape traits, and wing size in this outbred genetic background, ranged from 0.0006 to 0.006 (with one exception), a similar range to that reported from studies employing inbred lines. Simultaneously partitioning the additive and mutational variance in the same outbred population allowed us to quantitatively test the ability of mutation-selection balance models to explain the observed levels of additive and mutational genetic variance. The Gaussian allelic approximation and house-of-cards models, which assume real stabilizing selection on single traits, both overestimated the genetic variance maintained at equilibrium, but the house-of-cards model was a closer fit to the data. This analytical approach has the potential to be broadly applied, expanding our understanding of the dynamics of genetic variance in natural populations.     

Heterozygous carrier rate for type I–IV proximal spinal muscular atrophy in Chuvashes, Udmurts, and residents of the Moscow region

The first estimation of the heterozygous carrier rates for the SMN1 gene deletions and SMN2 gene duplications in populations of Russia has been performed. The numbers of SMN gene copies have been deter-mined in samples from Chuvash and Udmurt populations, as well the population of the Moscow region, by means of multiplex ligation-dependent probe amplification. The heterozygous carrier rates for the CMA gene were 2.7% (1: 37 people), 2.8% (1: 36 people), and 2.8% (1: 36 people) in Chuvashes, Udmurts, and residents of the Moscow region, respectively. The SMN2 duplication frequencies have been determined in the studied groups. It is 1.5, 4, and 2.5% in Chuvashes, Udmurts, and residents of the Moscow region, respectively. The high SMN2 duplication frequency in Udmurts may explain why the SMN1 heterozygous carriage frequency in this population was overestimated in earlier PCR-RFLP analyses.     

Dominance and overdominance of mildly deleterious induced mutations for fitness traits in Caenorhabditis elegans

We estimated the average dominance coefficient of mildly deleterious mutations (h, the proportion by which mutations in the heterozygous state reduce fitness components relative to those in the homozygous state) in the nematode Caenorhabditis elegans. From 56 worm lines that carry mutations induced by the point mutagen ethyl methanesulfonate (EMS), we selected 19 lines that are relatively high in fitness and estimated the viabilities, productivities, and relative fitnesses of heterozygotes and homozygotes compared to the ancestral wild type. There was very little effect of homozygous or heterozygous mutations on egg-to-adult viability. For productivity and relative fitness, we found that the average dominance coefficient, h, was approximately 0.1, suggesting that mildly deleterious mutations are on average partially recessive. These estimates were not significantly different from zero (complete recessivity) but were significantly different from 0.5 (additivity). In addition, there was a significant amount of variation in h among lines, and analysis of average dominance coefficients of individual lines suggested that several lines showed overdominance for fitness. Further investigation of two of these lines partially confirmed this finding.     

Slow Recovery from Inbreeding Depression Generated by the Complex Genetic Architecture of Segregating Deleterious Mutations

The deleterious effects of inbreeding have been of extreme importance to evolutionary biology, but it has been difficult to characterize the complex interactions between genetic constraints and selection that lead to fitness loss and recovery after inbreeding. Haploid organisms and selfing organisms like the nematode Caenorhabditis elegans are capable of rapid recovery from the fixation of novel deleterious mutation; however, the potential for recovery and genomic consequences of inbreeding in diploid, outcrossing organisms are not well understood. We sought to answer two questions: 1) Can a diploid, outcrossing population recover from inbreeding via standing genetic variation and new mutation? and 2) How does allelic diversity change during recovery? We inbred C. remanei, an outcrossing relative of C. elegans, through brother-sister mating for 30 generations followed by recovery at large population size. Inbreeding reduced fitness but, surprisingly, recovery from inbreeding at large populations sizes generated only very moderate fitness recovery after 300 generations. We found that 65% of ancestral single nucleotide polymorphisms (SNPs) were fixed in the inbred population, far fewer than the theoretical expectation of ∼99%. Under recovery, 36 SNPs across 30 genes involved in alimentary, muscular, nervous, and reproductive systems changed reproducibly across replicates, indicating that strong selection for fitness recovery does exist. Our results indicate that recovery from inbreeding depression via standing genetic variation and mutation is likely to be constrained by the large number of segregating deleterious variants present in natural populations, limiting the capacity for recovery of small populations.     

Viabilities of Third Chromosomes of DROSOPHILA PSEUDOOBSCURA Differing in Relative Competitive Fitness

Arrowhead (AR) third chromosome arrangements of Drosophila pseudoobscura, whose competitive fitnesses had been determined in population cages, were tested for their genetic loads in homozygous, heterozygous (homokaryotypic), and heterokaryotypic (AR/CH) combinations. The results showed that their competitive population cage performances were correlated to their viabilities as homozygotes but were not correlated to their viabilities as heterozygotes or as heterokaryotypes. However, the results do not fit in too simply with the mutational model of population structure, since the improvement of homozygous viability with increased competitive fitness was not accompanied by a significant degree of dominance as measured by the regression of viabilities of heterozygotes on homozygotes. Only the AR chromosomes derived from the population with poorest competitive fitness showed marked partial dominance (h=.35). The viabilities of heterokaryotypes were markedly uniform for all chromosomes tested and produced significantly greater numbers of flies per culture than the homokaryotypes. In general, the results show that the ranking of relative competitive fitnesses of these chromosomes is not a simple extrapolation of their viabilities, although marked changes in the populations tested have occurred. It is proposed that the differences in competitive fitness, homozygous viability, and degree of dominance observed among these chromosomes, arise from differences in genetic variability which enable different linkage relationships to be established for genes affecting these attributes.