Viabilities of originally natural O-chromosomal inversion homo- and heterokaryotypes in Drosophila subobscura

The influence of epistatic interactions of lethal and non-lethal genes upon viability of Drosophila inversion karyotypes is poorly known. In this paper we present comparative results for viabilities of 21 originally natural O-inversion homo- and 38 heterokaryotypes in a D. subobscura population. We observed strong heterotic effect in viability of O-lethal heterozygotes irrespective of different inversion backgrounds, which indicates a mechanism for protection of a great number of lethal genes. In O-non-lethal heterozygotes the heterotic effect in viability was absent irrespective of different inversion backgrounds. Our results showed a great extent of genetic load and high abundance of O-chromosomal arrangements in the population analyzed. It belongs to the set of central European populations. An epistasis of lethal genes present in O-inversion hetero- and not present in O-inversion homokaryotypes of moderate or low frequencies could be good example for co-adaptation of chromosomal inversions with regard to the genetic load. This represents a more efficient mechanism for limitation of genetic load than alternative mechanisms for protection of lethals. Except for lethal genes, possible epistatic interactions of mildly deleterious (subvital) genes, could also be responsible for limiting the extent of genetic load in natural D. subobscura populations. We demonstrated a non-random distribution of several combinations of viability classes among different O-inversion homo- and heterokaryotypes. As a consequence of that, the viabilities of the O-inversion homokaryotypes compared to heterokaryotypes were significantly higher at low frequencies than in moderate or high frequencies. This shows frequency-dependence as a mechanism of balancing selection for protection of O-chromosomal inversions in natural D. subobscura populations. In addition, the viabilities of the O-inversion homokaryotypes of lower frequency, compared to homokaryotypes of moderate or high frequency, were significantly higher. This again indicates the existence of supergene selection as another mechanism for protection of chromosomal inversions, as co-adapted complexes in natural D. subobscura populations.     

Chromosome-wide linkage disequilibrium caused by an inversion polymorphism in the white-throated sparrow (Zonotrichia albicollis)

Chromosomal inversions have been of long-standing interest to geneticists because they are capable of suppressing recombination and facilitating the formation of adaptive gene complexes. An exceptional inversion polymorphism (ZAL2^m) in the white-throated sparrow (Zonotrichia albicollis) is linked to variation in plumage, social behavior and mate choice, and is maintained in the population by negative assortative mating. The ZAL2^m polymorphism is a complex inversion spanning >100 Mb and has been proposed to be a strong suppressor of recombination, as well as a potential model for studying neo-sex chromosome evolution. To quantify and evaluate these features of the ZAL2^m polymorphism, we generated sequence from 8 ZAL2^m and 16 ZAL2 chromosomes at 58 loci inside and 4 loci outside the inversion. Inside the inversion we found that recombination was completely suppressed between ZAL2 and ZAL2^m, resulting in uniformly high levels of genetic differentiation (F_ST=0.94), the formation of two distinct haplotype groups representing the alternate chromosome arrangements and extensive linkage disequilibrium spanning ∼104 Mb within the inversion, whereas gene flow was not suppressed outside the inversion. Finally, although ZAL2^m homozygotes are exceedingly rare in the population, occurring at a frequency of <1%, we detected evidence of historical recombination between ZAL2^m chromosomes inside the inversion, refuting its potential status as a non-recombining autosome.     

Genetic load and coadaptation of chromosomal inversions. II. O-chromosomes in Drosophila subobscura populations

We have analysed the inversion polymorphism and genetic load of O-chromosomes in three populations of D. subobscura from southeastern Europe. As expected for a central populations the inversion polymorphism was extensive. In a like fashion, the genetic load, in particular the frequency of lethals, was heavy in all three populations. There were significant differences in the frequency of moderately deleterious genes. These differences in viability can be attributed to balancing selection. A comparison of these two kinds of genetic polymorphism indicates that there are differences in mean viability among different gene arrangements of O-chromosomes in the three populations. The differences observed are due to an unequal distribution of various viability classes among O-chromosome gene arrangements. We here show for the first time a specific distribution of lethal genes among these arrangements within the Palearctic distribution area of D. subobscura. The lethal allelism test showed lethals are non-randomly associated with the Ost gene arrangement. The amount of genetic load is heavy in gene arrangements with a high frequency, in comparison with the ones with a low frequency. Lethal genes may be protected in combinations of low and moderate frequency gene arrangements that harbor more lethal genes, as the Ost in the one population. Some arrangements that are less protected against recombination have a higher load than ones that are more protected against recombination. This can be taken as evidence for coadaptation.     

Recombination and selection in the maintenance of the adaptive value of inversions

A huge amount of data seem to confirm the adaptive value of inversions in Drosophila. The inhibition of recombination in heterokaryotypes mediated by inversions seems fundamental in maintaining their adaptive role. This study shows that recombination is highly suppressed in Drosophila subobscura because of chromosomal inversions, not only inside the inversions but also outside them. It seems that the region outside the inversion where recombination is inhibited is asymmetrical and independent of the inversion length. Despite the difficulty of crossovers taking place near inversion breakpoints, the only two recombination events detected inside inversions were located close to the breakpoint. Thus, selection could be largely responsible for the recombination reduction maintaining sets of adaptive alleles inside the inverted region. Heterokaryotype descendants were always in higher frequency than inbred or outbred homokaryotypes, regardless of the geographical origin of the chromosome, suggesting that chromosomes carrying the same arrangement, although with a different set of alleles for neutral markers, could be submitted to the same selection processes.     

Factors Influencing Fitness in Chromosomal Strains in DROSOPHILA SUBOBSCURA

Chromosomal polymorphism can be maintained by means of genic dominance, genic overdominance, recombination effects and karyotype selection. Experiments were carried out which allow us to distinguish among these factors as they effect egg hatch and larva-to-adult survival in D. subobscura. A strong karotype selection was observed. A small but highly significant recombination effect was also observed despite the fact that the material used in the experiment consisted of inbred, related strains and was therefore less likely to show such an effect than strains randomly chosen from a population. On the other hand, these strains, being inbred, were more likely to show genic dominance or genic overdominance than randomly picked samples. However neither genic dominance nor genic overdominance was detected.     

Simple inheritance,complex regulation: Supergene‐mediated fire ant queen polymorphism

The fire ant Solenopsis invicta exists in two alternate social forms: monogyne nests contain a single reproductive queen and polygyne nests contain multiple reproductive queens. This colony‐level social polymorphism corresponds with individual differences in queen physiology, queen dispersal patterns and worker discrimination behaviours, all evidently regulated by an inversion‐based supergene that spans more than 13 Mb of a “social chromosome,” contains over 400 protein‐coding genes and rarely undergoes recombination. The specific mechanisms by which this supergene influences expression of the many distinctive features that characterize the alternate forms remain almost wholly unknown. To advance our understanding of these mechanisms, we explore the effects of social chromosome genotype and natal colony social form on gene expression in queens sampled as they embarked on nuptial flights, using RNA‐sequencing of brains and ovaries. We observe a large effect of natal social form, that is, of the social/developmental environment, on gene expression profiles, with similarly substantial effects of genotype, including: (a) supergene‐associated gene upregulation, (b) allele‐specific expression and (c) pronounced extra‐supergene trans‐regulatory effects. These findings, along with observed spatial variation in differential and allele‐specific expression within the supergene region, highlight the complex gene regulatory landscape that emerged following divergence of the inversion‐mediated Sb haplotype from its homologue, which presumably largely retained the ancestral gene order. The distinctive supergene‐associated gene expression trajectories we document at the onset of a queen’s reproductive life expand the known record of relevant molecular correlates of a complex social polymorphism and point to putative genetic factors underpinning the alternate social syndromes.     

Hitchhiking of Deleterious Alleles and the Cost of Adaptation in Partially Selfing Species

Self-fertilization is generally seen to be disadvantageous in the long term. It increases genetic drift, which subsequently reduces polymorphism and the efficiency of selection, which also challenges adaptation. However, high selfing rates can increase the fixation probability of recessive beneficial mutations, but existing theory has generally not accounted for the effect of linked sites. Here, we analyze a model for the fixation probability of deleterious mutants that hitchhike with selective sweeps in diploid, partially selfing populations. Approximate analytical solutions show that, conditional on the sweep not being lost by drift, higher inbreeding rates increase the fixation probability of the deleterious allele, due to the resulting reduction in polymorphism and effective recombination. When extending the analysis to consider a distribution of deleterious alleles, as well as the average fitness increase after a sweep, we find that beneficial alleles generally need to be more recessive than the previously assumed dominance threshold (h < 1/2) for selfing to be beneficial from one-locus theory. Our results highlight that recombination aiding the efficiency of selection on multiple loci amplifies the fitness benefits of outcrossing over selfing, compared to results obtained from one-locus theory. This effect additionally increases the parameter range under which obligate outcrossing is beneficial over partial selfing.     

The Influence of Single Nucleotide Polymorphism Microarray-Based Molecular Karyotype on Preimplantation Embryonic Development Potential

In order to investigate the influence of the molecular karyotype based on single nucleotide polymorphism (SNP) microarray on embryonic development potential in preimplantation genetic diagnosis (PGD), we retrospectively analyzed the clinical data generated by PGD using embryos retrieved from parents with chromosome rearrangements in our center. In total, 929 embryos from 119 couples had exact diagnosis and development status. The blastocyst formation rate of balanced molecular karyotype embryos was 56.6% (276/488), which was significantly higher than that of genetic imbalanced embryos 24.5% (108/441) (P<0.001). No significant difference was detected in blastocyst formation rates in the groups of maternal age<30, 30–35 and >35 respectively. Blastocyst formation rates of male and female embryos were 44.5% (183/411) and 38.8% (201/518) respectively, with no significant difference between them (P>0.05). The rates of balanced molecular karyotype embryos vary from groups of embryos with different cell numbers at 68 hours after insemination. The blastocyst formation rate of embryos with 6–8 cells (48.1%) was significantly higher than that of embryos with <6 cells (23.9%) and with >8 cells (42.9%) (P<0.05). As for the unbalanced embryos, there was no significant difference of the distribution of abnormal molecular karyotypes in the subgroup of the arrest, morula and blastocyst. Thus, we conclude that embryos with balanced molecular karyotype have significant higher development potential than those with imbalanced molecular karyotype whilst maternal age, embryo gender and types of abnormal molecular karyotype have no significant influence on blastocyst formation. Compared with embryos with <6 and >8 cells, embryos with 6–8 blastomeres have higher rate of balanced molecular karyotype and blastocyst formation.     

Transgenic Increase in N-3/N-6 Fatty Acid Ratio Reduces Maternal Obesity-Associated Inflammation and Limits Adverse Developmental Programming in Mice

Maternal and pediatric obesity has risen dramatically over recent years, and is a known predictor of adverse long-term metabolic outcomes in offspring. However, which particular aspects of obese pregnancy promote such outcomes is less clear. While maternal obesity increases both maternal and placental inflammation, it is still unknown whether this is a dominant mechanism in fetal metabolic programming. In this study, we utilized the Fat-1 transgenic mouse to test whether increasing the maternal n-3/n-6 tissue fatty acid ratio could reduce the consequences of maternal obesity-associated inflammation and thereby mitigate downstream developmental programming. Eight-week-old WT or hemizygous Fat-1 C57BL/6J female mice were placed on a high-fat diet (HFD) or control diet (CD) for 8 weeks prior to mating with WT chow-fed males. Only WT offspring from Fat-1 mothers were analyzed. WT-HFD mothers demonstrated increased markers of infiltrating adipose tissue macrophages (P<0.02), and a striking increase in 12 serum pro-inflammatory cytokines (P<0.05), while Fat1-HFD mothers remained similar to WT-CD mothers, despite equal weight gain. E18.5 Fetuses from WT-HFD mothers had larger placentas (P<0.02), as well as increased placenta and fetal liver TG deposition (P<0.01 and P<0.02, respectively) and increased placental LPL TG-hydrolase activity (P<0.02), which correlated with degree of maternal insulin resistance (r = 0.59, P<0.02). The placentas and fetal livers from Fat1-HFD mothers were protected from this excess placental growth and fetal-placental lipid deposition. Importantly, maternal protection from excess inflammation corresponded with improved metabolic outcomes in adult WT offspring. While the offspring from WT-HFD mothers weaned onto CD demonstrated increased weight gain (P<0.05), body and liver fat (P<0.05 and P<0.001, respectively), and whole body insulin resistance (P<0.05), these were prevented in WT offspring from Fat1-HFD mothers. Our results suggest that reducing excess maternal inflammation may be a promising target for preventing adverse fetal metabolic outcomes in pregnancies complicated by maternal obesity.     

Evolutionary genetics of Drosophila ananassae. I. Effect of selection on body size and inversion frequencies

Adaptive importance of inversion polymorphism has been discussed in Drosophila species at several levels but no study has been carried out demonstrating the individual and combined effects of polymorphic inversions on the fitness of flies through bi‐directional selection. Therefore, artificial bi‐directional selection for thorax length in Drosophila ananassae was carried out for 10 generations. Both, Tukey test for selection difference and regression coefficients of offspring on mid‐parent are highly significant. The realized heritability (h²) in males of both high and low selection lines is more or less similar but is more pronounced in low line females, which suggests the asymmetrical response. This asymmetry in selection is discussed in the light of evidence provided by the study of chromosome inversion frequencies in different selection lines at different generations of selection. Interestingly, chromosome inversion frequency changes towards homozygosity for different gene arrangements in different selection lines. Tests of correlations at G₆ and G₁₀ among different gene arrangements as well as with mean thorax length suggest that 2L‐ST gene arrangement is negatively correlated, while 3L‐ST gene arrangement is positively correlated with thorax length. Furthermore, the present study shows the significant effects of 3L‐ST and 2L + 3L (positive correlation) on thorax length, while 3R‐ST and 2L + 3R show significant effect (negative correlation) on thorax length, which was not evident in the previous study. Present results also suggest how polymorphic inversions and their combinations affect the body size differently in different selection lines. These results suggest that thorax length in D. ananassae is under polygenic control and inversion polymorphism plays crucial role in maintaining body size by modifying genotypic frequency under various selection pressures.     

Association between Ocular Sensory Dominance and Refractive Error Asymmetry

Purpose

To investigate the association between ocular sensory dominance and interocular refractive error difference (IRED).

Methods

A total of 219 subjects were recruited. The refractive errors were determined by objective refraction with a fixation target located 6 meters away. 176 subjects were myopic, with 83 being anisometropic (IRED ≥ 0.75 D). 43 subjects were hyperopic, with 22 being anisometropic. Sensory dominance was measured with a continuous flashing technique with the tested eye viewing a Gabor increasing in contrast and the fellow eye viewing a Mondrian noise decreasing in contrast. The log ratio of Mondrian to Gabor’s contrasts was recorded when a subject just detected the tilting direction of the Gabor during each trial. T-test was used to compare the 50 values collected from each eye, and the t-value was used as a subject’s ocular dominance index (ODI) to quantify the degree of ocular dominance. A subject with ODI ≥ 2 (p < 0.05) had clear dominance and the eye with larger mean ratio was the dominant one. Otherwise, a subject had an unclear dominance.

Results

The anisometropic subjects had stronger ocular dominance in comparison to non-anisometropic subjects (rank-sum test, p < 0.01 for both myopic and hyperopic subjects). In anisometropic subjects with clear dominance, the amplitude of the anisometropia was correlated with ODI values (R = 0.42, p < 0.01 in myopic anisometropic subjects; R = 0.62, p < 0.01 in hyperopic anisometropic subjects). Moreover, the dominant eyes were more myopic in myopic anisometropic subjects (sign-test, p < 0.05) and less hyperopic in hyperopic anisometropic subjects (sign-test, p < 0.05).

Conclusion

The degree of ocular sensory dominance is associated with interocular refractive error difference.     

Efficiency of Generations for Estimating Marker-Associated Qtl Effects by Multiple Regression

Knowledge about the efficiency of generations for estimating marker-associated QTLs is needed for selection. The objective of this paper is to develop a theory to compare the efficiency of segregating generations and testcrosses from the cross of two inbred lines differing in value for a quantitative trait (P(1) X P(2)) for estimating additive, dominance and heterotic effects of QTLs by stepwise regression. An equation that predicts the smallest gene effect in genetic standard deviation units that can be detected with 50% chance at a significance level as a function of the heritability (h(2)) and the recombination frequency (r) of markers was developed for the segregating generations and testcrosses. For estimating additive effects, the most efficient generation was the doubled-haploid (DH) lines; the most inefficient was the North Carolina Design III (NCD III), followed by selfed backcrosses (SB); the selfed families from F(2) individual plants (F(2:3) lines) are inferior to the recombinant inbreds (RI) for low r, but are better than RI for high h(2) and r. Dominance effects are less efficiently estimated than additive effects. The NCD III is better than the SB and the F(2:3) lines for detecting dominance effects. The RI and DH do not estimate dominance effects. The differential heterotic QTL effects of lines P(1) and P(2) when crossed with tester T can be estimated by evaluating testcrosses of individual F(2) plants (F2T), recombinant inbreds (RIT) and double-haploid lines (DHT). The DHT is superior to the other generations. The F2T is better than the RIT for r >/= 0.20, but inferior for r </= 0.1 or low heritability.     

中国五种高山锄足蟾的核型、Ag-NORs和C-带的研究

作者用核型、Ag-NORs和C-带,对分布于川、滇两省的二属(齿突蟾、齿蟾)五种(胸腺齿突蟾、圆疣齿突蟾、凉北齿蟾、秉志齿蟾、疣刺齿蟾)锄足蟾作了属间和种间关系的比较分析,并讨论了它们的核型演化机制。结果表明:(1)齿突蟾和齿蟾两属间在核型和带型上都有明显的差异,演化途径主要的可能是含有重复DNA染色体片段的相互易位或臂间倒位;(2)属内不同种之间带型无显著差异,但某些对应染色体对间,其相对长度和臂比值差异明显,十分可能是常染色质片段的易位和臂间倒位所致;(3)凉北齿蟾有染色体数目变异多态现象;(4)五种锄足蟾均未发现异形性染色体。     

Nucleotide Variation, Linkage Disequilibrium and Founder-Facilitated Speciation in Wild Populations of the Zebra Finch (Taeniopygia guttata)

The zebra finch has long been an important model system for the study of vocal learning, vocal production, and behavior. With the imminent sequencing of its genome, the zebra finch is now poised to become a model system for population genetics. Using a panel of 30 noncoding loci, we characterized patterns of polymorphism and divergence among wild zebra finch populations. Continental Australian populations displayed little population structure, exceptionally high levels of nucleotide diversity (π = 0.010), a rapid decay of linkage disequilibrium (LD), and a high population recombination rate (ρ ≈ 0.05), all of which suggest an open and fluid genomic background that could facilitate adaptive variation. By contrast, substantial divergence between the Australian and Lesser Sunda Island populations (K_ST = 0.193), reduced genetic diversity (π = 0.002), and higher levels of LD in the island population suggest a strong but relatively recent founder event, which may have contributed to speciation between these populations as envisioned under founder-effect speciation models. Consistent with this hypothesis, we find that under a simple quantitative genetic model both drift and selection could have contributed to the observed divergence in six quantitative traits. In both Australian and Lesser Sundas populations, diversity in Z-linked loci was significantly lower than in autosomal loci. Our analysis provides a quantitative framework for studying the role of selection and drift in shaping patterns of molecular evolution in the zebra finch genome.     

Association with litter size of new polymorphisms on ESR1 and ESR2 genes in a Chinese-European pig line

The objective of this study was to search for polymorphisms in the coding region of the estrogen receptors 1 and 2 (ESR1 and ESR2 )and to analyze the effects of these variants and the well known intronic ESR1 Pvu II polymorphism on litter size in a Chinese-European pig line. We identified five silent single nucleotide polymorphisms (SNP) in the ESR1 cDNA: c.669T > C (exon 3), c.1227C > T (exon 5), c.1452C > T (exon 7), c.1665T > C and c.1755A > G (exon 8). One pair of these SNP (c.1665T > C and c.1755A > G) co-segregated in the analyzed line, and the SNP c.669T > C showed the same segregation pattern as the Pvu II polymorphism. These polymorphisms were tested in this study, although the c.1452C > T SNP within exon 7 was not analyzed due to its low informativeness. In the ESR2 cDNA, one missense SNP was found within exon 5, which caused an amino acid substitution in the coded protein: "c.949G > A (p.Val317Met)" and was tested on sow litter size. Information on 1622 litter records from 408 genotyped sows was analyzed to determine whether these SNP influenced the total number of piglets born (TNB) or the number of born alive (NBA). The polymorphisms ESR1: [Pvu II; c.669T > C], ESR1: [c.1665T > C; c.1755A > G] and ESR2: c.949G > A showed no statistically significant association with litter size. However, the ESR1: c.1227T allele was significantly associated with TNB. The additive substitution effect was estimated to be 0.40 piglets born per litter (P < 0.03), and no dominance effects were observed. This SNP could be useful in assisted selection for litter size in some pig lines, as a new genetic marker in linkage disequilibrium with the causative mutation.     

Inbreeding reveals interpopulation differences in inversion polymorphism of Drosophila subobscura

The adaptive significance of inversion polymorphism of Drosophila subobscura is well established. However, little is known about gene combinations within inversions which are coadapted because of population-specific effects. We studied this aspect of Dobzhansky's coadaptation hypothesis, using the systematic inbreeding method. Differences in magnitude and quality of inversion polymorphism in two ecologically and topologically distinct habitats were compared after several generations of continuous full-sib inbreeding. Populations from the two habitats differ in frequency of homokaryotypes after third and fifth generation of inbreeding and in the levels of homozygosity of different gene arrangements. The effect of homozygosity appears population and chromosome specific.     

Dexamethasone and Azathioprine Promote Cytoskeletal Changes and Affect Mesenchymal Stem Cell Migratory Behavior

Glucocorticoids and immunosuppressive drugs are commonly used to treat inflammatory disorders, such as inflammatory bowel disease (IBD), and despite a few improvements, the remission of IBD is still difficult to maintain. Due to their immunomodulatory properties, mesenchymal stem cells (MSCs) have emerged as regulators of the immune response, and their viability and activation of their migratory properties are essential for successful cell therapy. However, little is known about the effects of immunosuppressant drugs used in IBD treatment on MSC behavior. The aim of this study was to evaluate MSC viability, nuclear morphometry, cell polarity, F-actin and focal adhesion kinase (FAK) distribution, and cell migratory properties in the presence of the immunosuppressive drugs azathioprine (AZA) and dexamethasone (DEX). After an initial characterization, MSCs were treated with DEX (10 μM) or AZA (1 μM) for 24 hrs or 7 days. Neither drug had an effect on cell viability or nuclear morphometry. However, AZA treatment induced a more elongated cell shape, while DEX was associated with a more rounded cell shape (P < 0.05) with a higher presence of ventral actin stress fibers (P < 0.05) and a decrease in protrusion stability. After 7 days of treatment, AZA improved the cell spatial trajectory (ST) and increased the migration speed (24.35%, P < 0.05, n = 4), while DEX impaired ST and migration speed after 24 hrs and 7 days of treatment (-28.69% and -25.37%, respectively; P < 0.05, n = 4). In conclusion, our data suggest that these immunosuppressive drugs each affect MSC morphology and migratory capacity differently, possibly impacting the success of cell therapy.     

‘Nucleolar dominance’ as observed in barley translocation lines with specifically reconstructed SAT chromosomes

Summary Diploid homo- and heterokaryotypes of barley translocation lines with only one satellite chromosome pair containing two nucleolus organizer regions (NORs) in opposite arms were found to show repressed nucleolus formation by the transposed NOR as evident from the formation of only micronucleoli. The same was true for auto-tetraploid homokaryotypes and for translocation lines with all NORs tandemly arranged into the same chromosome arm. When NORs were transposed to chromosomes without NOR in the standard karyotype, the normal pattern of nucleolus formation remained unaffected. The modified mode of nucleolus formation after the combination of all NORs in one chromosome pair is interpreted to be due to intrachromosomal nucleolar dominance analogous to interchromosomal nucleolar dominance observed in certain interspecific hybrids.     

Evolutionarily stable mutation rate in a periodically changing environment

Evolution of mutation rate controlled by a neutral modifier is studied for a locus with two alleles under temporally fluctuating selection pressure. A general formula is derived to calculate the evolutionarily stable mutation rate μ(ess) in an infinitely large haploid population, and following results are obtained. (I) For any fluctuation, periodic or random: (1) if the recombination rate r per generation between the modifier and the main locus is 0, μ(ess) is the same as the optimal mutation rate μ(op) which maximizes the long-term geometric average of population fitness; and (2) for any r, if the strength s of selection per generation is very large, μ(ess) is equal to the reciprocal of the average number T of generations (duration time) during which one allele is persistently favored than the other. (II) For a periodic fluctuation in the limit of small s and r, μ(ess)T is a function of sT and rT with properties: (1) for a given sT, μ(ess)T decreases with increasing rT; (2) for sT </= 1, μ(ess)T is almost independent of sT, and depends on rT as μ(ess)T & 1.6 for rT << 1 and μ(ess)T & 6/rT for rT >> 1; and (3) for sT >/= 1, and for a given rT, μ(ess)T decreases with increasing sT to a certain minimum less than 1, and then increases to 1 asymptotically in the limit of large sT. (III) For a fluctuation consisting of multiple Fourier components (i.e., sine wave components), the component with the longest period is the most effective in determining μ(ess) (low pass filter effect). (IV) When the cost c of preventing mutation is positive, the modifier is nonneutral, and μ(ess) becomes larger than in the case of neutral modifier under the same selection pressure acting at the main locus. The value of c which makes μ(ess) equal to μ(op) of the neutral modifier case is calculated. It is argued that this value gives a critical cost such that, so long as the actual cost exceeds this value, the evolution rate at the main locus must be smaller than its mutation rate μ(ess).     

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.