Superiority Of Heterozygotes Research Articles

Dobzhansky’s concept of genetic coadaptation: Drosophila ananassae is an exception to this concept

Dobzhansky was the first to show that the inversion polymorphism in Drosophila pseudoobscura is subject to natural selection and is a device to cope with the diversity of environments. His extensive work on D. pseudoobscura has revealed interesting phenomena of population genetics. In continuation of his work on this species, he constructed a number of homozygous lines for different gene arrangements in the third chromosome, and while employing these lines in intrapopulation and interpopulation crosses, he quantified the fitness of inversion homokaryotypes and heterokaryotypes. Interestingly, his results showed that heterokaryotypes formed by chromosomes originating from the same geographic area exhibited superiority over the corresponding homokaryotypes. However, superiority of heterokaryotypes was lost in the crosses when chromosomes were derived from different localities. Based on these results, Dobzhansky suggested the concept of genetic coadaptation. According to this concept, 'in each locality, the chromosomes with different gene arrangements aremutually adjusted or coadapted to yield highly fit inversion heterozygotes through long continued natural selection. However, this adaptive superiority of inversion heterozygotes breaks down in interracial hybridization experiments when two gene arrangements are derived from different localities'. This concept has received experimental evidence in its favour on the basis of work done in other species of Drosophila, such as D. willistoni, D. paulistorum, D. pavani and D. bipectinata. In all these species, interracial hybridization led to the loss of superiority of inversion heterozygotes. Further, it has been suggested that coadapted polygenic complexes contained in the chromosomes are disrupted as a result of recombination in interstrain crosses. This concept was also tested in D. ananassae, a cosmopolitan and domestic species commonly found in India, while employing three cosmopolitan inversions exhibiting heterotic buffering. In interstrain crosses involving monomorphic and polymorphic strains due to three cosmopolitan inversions, the persistence of heterosis was observed, which does not support the above-mentioned hypothesis of Dobzhansky. Thus, evidence for coadaptation is lacking in natural populations of D. ananassae, which is considered as an exception to the Dobzhansky's concept of genetic coadaptation. Thus, heterotic buffering associated with the three cosmopolitan inversions in D. ananassae is not populational heterosis; rather, it appears to be simple luxuriance.

Protein co-evolution, co-adaptation and interactions

Co-evolution has an important function in the evolution of species and it is clearly manifested in certain scenarios such as host–parasite and predator–prey interactions, symbiosis and mutualism. The extrapolation of the concepts and methodologies developed for the study of species co-evolution at the molecular level has prompted the development of a variety of computational methods able to predict protein interactions through the characteristics of co-evolution. Particularly successful have been those methods that predict interactions at the genomic level based on the detection of pairs of protein families with similar evolutionary histories (similarity of phylogenetic trees: mirrortree). Future advances in this field will require a better understanding of the molecular basis of the co-evolution of protein families. Thus, it will be important to decipher the molecular mechanisms underlying the similarity observed in phylogenetic trees of interacting proteins, distinguishing direct specific molecular interactions from other general functional constraints. In particular, it will be important to separate the effects of physical interactions within protein complexes (‘co-adaptation') from other forces that, in a less specific way, can also create general patterns of co-evolution.

Temporal variation and gametic disequilibrium in Drosophila nasuta

The gametic disequilibrium between III-2 and 111-35 inversions of the third chromosome of Drosophila nasuta had been examined during different months at two localities. The results indicated that there was a substantial gametic disequilibrium between above inversions. The results also indicated the superiority of double inversion heterozygotes over both homozygotes (standard sequence homozygotes and inversion hotnozygotes) during different months at both localities. This suggested that the high frequency of double inversion heterozygotes might be due to their higher adaptive fitness. It had been suggested that natural selection acting through the suppression of recombination and intrachromosomal epistatic gene interaction providing fitness to the individuals were the main factors leading to above phenomenon in this species.

Genetic structure along a gaseous organic pollution gradient: a case study with Poa annua L.

The population genetic composition of Poa annua L. was studied by starch electrophoresis along a transect running NE from an organic reagents factory at Shanghai, China. Five enzyme systems were stained. We have reached the following preliminary conclusions: (1) Organic pollution has dramatically changed genotypic frequencies at some loci of Poa annua populations. At polluted sites, significant deviations from Hardy–Weinberg equilibrium were observed on loci Sod-1 and Me due to the excess of heterozygote. Especially in the two nearest sites to pollution source, all the individuals were heterozygous at locus Sod-1. The data suggests that heterozygotes were more tolerant to organic pollution than homozygotes, indicating the fitness superiority of heterozygotes. (2) A tendency towards clinal changes of allele frequencies was found at some polymorphic loci. Frequencies of the common alleles at loci Sod-1, Me and Fe-1 increased as the distance to the pollution source increased. (3) The effective number of alleles per locus, and the observed and expected heterozygosity were much higher in the pollution series than in the clear control site (Botanic Park population), but genetic multiplicity (number of alleles per locus) was lower than for the control. (4) Most genetic variability was found within populations, and only 2.56% were among populations of the polluted series. However, 9.48% of the total genetic variation occurred among populations when including the Botanic Park population. The genetic identity between populations of the pollution series (0.9869–1.0000, mean 0.9941) was higher than those between the pollution series and the Botanic Park population. UPGMA divided the five populations into two groups. One contained the four polluted populations, and the other only contained the Botanic Park population.

Effects of temperature change on ectotherm metabolism and evolution: Metabolic and physiological interrelations underlying the superiority of multi-locus heterozygotes in heterogeneous environments

1. 1. Metabolic sensitivity to environmental temperature change increases in positive exponential relation with initial acclimated rates of energy expenditure, and is influenced by the relative rate with which intracellular amino acids are being mobilised by protein turnover. 2. 2. Greater metabolic sensitivity results in greater physiological variability with fluctuating temperatures, higher energy costs incurred during the response to temperature change, and reduced viability upon exposure to extreme high temperatures. 3. 3. These interrelations help explain the faster production and greater viability of multi-locus heterozygotes experimentally exposed to increased temperature and other stressors, as well as ecogeographical associations indicating the superiority of multi-locus heterozygotes in heterogeneous environments. 4. 4. Implications are discussed for understanding the effects of temperature change on ectotherm metabolism and evolution.

Coadaptation Revisited

During the four decades or more since Dobzhansky introduced the term "coadaptation" to refer to the commonly observed selective superiority of inversion heterozygotes in populations of Drosophila pseudoobscura, the definition of the term has evolved, as have views concerning the rapidity with which coadaptation might occur. Indeed, the paucity of demonstrated instances of linkage disequilibrium in natural populations has led many to dismiss coadaptation as a factor in evolutionary change. The present article reviews the reasons why coadaptation (and the equivalent expression, "integration of gene pools") was proposed as a phenomenon occurring in local (or experimental) populations, offers supporting data obtained through a reanalysis of data on irradiated populations of D. melanogaster, and concludes that sound evidence supports coadaptation as a factor in the genetic change of populations.

Genotype X macro- and micro-environment interactions in barley populations (Hordeum vulgare L.)

SummaryTriple test cross progenies arising from F2 of three crosses, namely, C164 x EB1556 BG25 x NP21 and BH15 x RD103, were used for investigating their interactions with macro- and micro-environments for number of days to heading, plant height, number of tillers per plant, ear length, number of grains per ear, 100-grain weight and yield per plant in barley. The analysis indicated, in general, that the additive component of genetic variance was more sensitive to macro-environment than non-additive components. In order to have a more precise estimate of this component, an experiment repeated over a larger number of locations and years would be desirable. The ‘j and l’type of epistasis showed sensitivity equal to the dominance components in response to the change of soil conditions. Covariances between family means and variances were used to detect the interactions of genetic components with the micro-environmental differences. The additive and the dominance components showed sensitivity to microenvironmental changes and were not differentially affected by the adverse soil condition compared with normal soil. The additive gene effects were relatively more sensitive, indicating the probable superiority of heterozygotes over the corresponding homozygotes with regard to stability.Amongst the three populations, the population resulting from BH15 x RD103 was found to be most sensitive. Although the genotype x environment interaction for a character was population-specific, number of grains per ear and yield per plant were invariably the most sensitive.

Studies on the Superiority of Double Inversion Heterozygotes in a Laboratory Population ofDrosophila Nasuta

SUMMARYIn the natural populations of Drosophila nasuta, two independent gene arrangements, III-2 and III-35, in the third chromosome are very common and occur non-randomly. The double heterozygote combination is known to be superior in comparison to double homozygotes. A test has been carried out whether the double heterozygotes are also superior in the laboratory population kept at different temperatures. The present result demonstrates the superiority of double heterozygotes in the laboratory population. Further, it has been suggested that the low percentage of the double homozygotes is due to less viability of their carriers.

FERTILITY GENES IN NATURAL POPULATIONS OF DROSOPHILA MELANOGASTER. III. SUPERIORITY OF INVERSION HETEROZYGOTES.

Chromosomal polymorphism in natural populations is largely maintained by the adaptive superiority of inversion heterozygotes. Dobzhansky and his co-workers have carried out many studies on heterosis in Drosophila pseudoobscura populations (summarized in Dobzhansky, 1970). The superior fitness of heterokaryotypes has also been reported in other Drosophila species: D. persimilis (Spiess, 1958; Spiess and Langer, 1964); D. robusta (Carson, 1958; Prakash, 1967); D. willistoni (da Cunha et al., 1950; Pavan et al., 1957); D. ananassae (Tobari, 1962) and D. pavani (Brncic et al., 1969). Extensive chromosomal polymorphism in Japanese populations of D. melanogaster was demonstrated by Watanabe (1967). Two inversions on the second chromosome and four inversions on the third chromosome which had been found all over the world were highly polymorphic in Japanese natural populations. The second chromosome inversion In(2L)B was maintained in the population year after year at a frequency of about 35%. A higher adaptive value of the inversion heterozygotes was expected from estimates of the pre-adult viability. However, superiority was found only when In(2L)B was combined with another inversion, In(2R)C. The viability of the heterozygotes for In(2L)B and standard chromosomes was the same as that of the standard heterozygotes (Watanabe et al., 1964). A significant superiority of this inversion heterozygote in female

MAINTENANCE OF CHROMOSOMAL POLYMORPHISM IN A POPULATION OF DROSOPHILA PSEUDOOBSCURA. II. FECUNDITY, LONGEVITY, VIABILITY AND COMPETITIVE FITNESS.

Superiority of inversion heterozygotes over corresponding homozygotes is generally believed to be a major mechanism for maintaining chromosomal polymorphism in populations of D. pseudoobscura. Differential egg-to-adult survival in favor of the inversion heterozygotes has been postulated as a key component of this heterozygote superiority (Dobzhansky, 1947). Chromosomal polymorphism involving AR and CH gene arrangements has persisted in a cage population (No. 173) at 25 C since 1957 (Beardmore et al., 1960; Dobzhansky and Pavlovsky, 1961; Levine and Van Valen, 1964; Druger, 1966; Druger and Nickerson, 1972). Two independent analyses of this population revealed no significant advantage of inversion heterozygotes over homozygotes in egg-to-adult viability (Druger, 1966; Druger and Nickerson, 1972). This finding does not exclude heterozygote superiority as a mechanism for maintaining the polymorphism, since such superiority may operate in other ways. To investigate this possibility, studies of fecundity, longevity, and viability were conducted on AR/AR, CH/CH and AR/CH flies derived from AR and CH strains that were re-isolated from population No. 173. In addition to evaluating these individual components of fitness, a comparison of the fitness of polymorphic (AR + CH) and monomorphic (AR/AR) and (CH/CH) populations was obtained through competition with a standard population of a non-interbreeding species (Claringbold and Barker, 1961; Barker, 1963). D. nebulosa was used as a competitor for D. pseudoobscura (Barker, 1965).

Behavioural Dimorphism in Male Ruffs, Philomachus Pugnax (L.)

Behavioural Dimorphism in Male Ruffs, Philomachus Pugnax (L.)

MAINTENANTE OF CHROMOSOMAL POLYMORPHISM IN A POPULATION OF DROSOPHILA PSEUDOOBSCURA: VIABILITY UNDER CROWDED AND UNCROWDED CONDITIONS.

Numerous studies have shown that stable equilibria are usually attained when Drosophila pseudoobscura flies of similar geographic origin but having different gene arrangements in their third chromosomes are bred in population cages at 25 C (Dobzhansky, 1947a; Levene et al., 1954). Superiority of inversion heterozygotes over corresponding homozygotes has been cited as a major factor in attainment of such equilibria. Dobzhansky (1 947b) analyzed several experimental populations at equilibrium to determine the stage of the life cycle when selection was occurring. He found a significant excess of inversion heterozygotes and a deficiency of homozygotes among adult flies that developed under crowded cage conditions compared to Hardy-Weinberg expectations. This result indicated differential mortality between egg and adult stages favoring inversion heterozygotes. Druger (1966) analyzed an experimental cage population (No. 173) in which polymorphism for Arrowhead (AR) and Chiricahua (CH) inversions had persisted for more than eight years. Significant differential mortality in favor of inversion heterozygotes was not observed. In the present study, this analysis was repeated when population No. 173 was more than ten years old, using a larger sample, and deliberately overcrowding food cups with eggs. This was part of an attempt to determine those factors contributing to the longterm maintenance of polymorphism.

HETEROZYGOUS EFFECTS ON VIABILITY OF SPONTANEOUS LETHAL GENES IN <i>DROSOPHILA MELANOGASTER</i>

Heterozygous effects of lethal genes on viability in Drosophila melanogaster were examined using 81 second chromosomes carrying newly arisen lethal genes and 49 second chromosomes carrying lethal genes extracted from a Raleigh, N. C. population. The same number of lethal-free control chromosomes were employed. The following findings were obtained: (1) No evidence of superiority of lethal heterozygotes over the control was obtained in either almost completely homozygous or in intra- and interpopulational heterozygous genetic backgrounds. (2) The average degrees of dominance of lethal genes (h) are probably negatively correlated with the viabilities of their genetic backgrounds. The h values were 0.0076-0.038 for newly arisen lethals and 0.0057 for lethals from the natural populations. (3) The newly arisen lethals that showed more deleterious effects in one genetic background seemed to be also more detrimental in all the other genetic backgrounds employed. (4) Significant synergistic interaction could not be observed when single individuals had two lethal genes. For the natural lethals, a clear linear relationship was observed between the average viabilities of heterozygotes and the numbers of lethal genes per individual, while in newly arisen lethals, a tendency of synergistic interaction was observed but not significantly so.On the basis of the above findings and of the experimental results of Dobzhansky and Spassky (1968) and Oshima (1963), the behavior of lethal genes in natural populations was discussed.

Some consequences of selection by culling when there is superiority of heterozygotes.

Some consequences of selection by culling when there is superiority of heterozygotes.

LINKAGE DISEQUILIBRIUM AND HETEROSIS IN EXPERIMENTAL POPULATIONS OF <i>DROSOPHILA MELANOGASTER</i> WITH PARTICULAR REFERENCE TO THE <i>SEPIA</i> GENE

Population analysis was conducted for the adaptive superiority of sepia heterozygotes to either homozygote in Drosophila melanogaster. Four populations, which had different genetic backgrounds in the initial generations, were established. In early generations, where the magnitude of linkage disequilibrium (D of Lewontin and Kojima 1960) was large, and heterozygote superiority was seen. The frequencies of sepia homozygotes reached temporary equilibria (ca. 7-14 percent). However, as generation advanced, namely, the magnitude of linkage disequilibrium became small or disappeared, the frequency of sepia homozygotes decreased. However, in three sub-populations out of fifteen, permanent linkage disequilibrium was probably formed. Thus, intrinsic overdominance of the sepia gene was rejected.Mathematical models were set up for the explanation of the experimental data, and the necessary and sufficient conditions for the pseudo-overdominance in populations were obtained.

Fitness of the Genotypes at the B Locus Determining the Blood Group of Chickens

Relative fitness of the genotypes at the B locus determining blood groups in chickens was studied in two closed lines and their cross-breds. These lines, which were derived from one population and have been closed more than ten generations, were maintained by reciprocal recurrent selection at the Takikawa Livestock Breeding Station. Both lines possessed five alleles at the B locus, respectively. However, in one line one of them was extinct in 1959.Comparisons among the expected total, observed total and selected frequencies of the genes and the genotypes were performed for each generation. There was no obvious tendency in gene frequencies. On the other hand, the comparisons of geno-typic frequencies revealed apparent selective advantage of heterozygotes. The observed frequency of heterozygous genotypes in each total population was higher than the expected frequency. The proportion of heterozygotes increased again after selection. Thus, both natural and artificial selections favored the heterozygotes at the B locus. These results suggest that the B blood group in chickens is an example of balanced polymorphism.Direct comparisons of the characters under selection between the genotypes at the B locus showed, however, that the superiority of heterozygotes was not explicable by any single character studied here.

CYTOGENETIC AND EVOLUTIONARY STUDIES IN SECALE. I. SOME NEW DATA ON THE ANCESTRY OF S. CEREALE

Khush, G. S., and G. L. Stebbins. (U. California, Davis.) Cytogenetic and evolutionary studies in Secale. I. Some new data on the ancestry of S. cereale. Amer. Jour. Bot. 48(8): 723–730. Illus. 1961.—Cultivated rye, Secale cereale, was crossed with all 4 wild species of the genus. It crosses readily with S. montanum, S. africanum, and S. vavilovii, but crossability of S. cereale and S. silvestre is extremely low. Meiosis in the hybrid S. cereale × silvestre is characterized by high frequency of univalents at metaphase I, reduced chiasma frequency at metaphase I, high frequency of PMC's with unequal separations and laggards at anaphase I and II, high frequency of microspores with micronuclei, and extremely low pollen fertility. These abnormalities occurred less frequently in other hybrids, and consequently the pollen fertility is fairly high: 19.1% in the hybrid S. cereale × vavilovii and 31.3% and 31.8%, respectively, in the hybrids S. cereale × montanum and S. cereale × africanum. An interesting cytogenetic feature of all these hybrids, however, was the formation of a translocation configuration of 6 chromosomes at metaphase I of meiosis. In the hybrid, S. cereale × silvestre, a translocation configuration of 8 chromosomes was observed in a few cells. It is evident, therefore, that the genome of S. cereale differs from the genomes of wild species by 2 translocations. Another small translocation may differentiate S. cereale and S. silvestre, in addition. Thus, on the basis of chromosome arrangements, no particular wild species is most likely to be ancestral to S. cereale. Similarly, Stutz's hypothesis of hybrid origin of S. cereale seems highly improbable. After considering ecological preferences, breeding habits, geographical distribution, morphological and cytological affinities of wild species and cultivated rye, it is concluded that S. cereale evolved from S. montanum as a result of progressive cytological and morphological differentiation and that this differentiation was facilitated, probably, by adaptive superiority of translocation heterozygotes and rearrangement homozygotes.

Cytogenetic and Evolutionary Studies in Secale. I. Some New Data on the Ancestry of S. cereale

Khush, G. S., and G. L. Stebbins. (U. California, Davis.) Cytogenetic and evolutionary studies in Secale. I. Some new data on the ancestry of S. cereale. Amer. Jour. Bot. 48(8): 723–730. Illus. 1961.—Cultivated rye, Secale cereale, was crossed with all 4 wild species of the genus. It crosses readily with S. montanum, S. africanum, and S. vavilovii, but crossability of S. cereale and S. silvestre is extremely low. Meiosis in the hybrid S. cereale × silvestre is characterized by high frequency of univalents at metaphase I, reduced chiasma frequency at metaphase I, high frequency of PMC's with unequal separations and laggards at anaphase I and II, high frequency of microspores with micronuclei, and extremely low pollen fertility. These abnormalities occurred less frequently in other hybrids, and consequently the pollen fertility is fairly high: 19.1% in the hybrid S. cereale × vavilovii and 31.3% and 31.8%, respectively, in the hybrids S. cereale × montanum and S. cereale × africanum. An interesting cytogenetic feature of all these hybrids, however, was the formation of a translocation configuration of 6 chromosomes at metaphase I of meiosis. In the hybrid, S. cereale × silvestre, a translocation configuration of 8 chromosomes was observed in a few cells. It is evident, therefore, that the genome of S. cereale differs from the genomes of wild species by 2 translocations. Another small translocation may differentiate S. cereale and S. silvestre, in addition. Thus, on the basis of chromosome arrangements, no particular wild species is most likely to be ancestral to S. cereale. Similarly, Stutz's hypothesis of hybrid origin of S. cereale seems highly improbable. After considering ecological preferences, breeding habits, geographical distribution, morphological and cytological affinities of wild species and cultivated rye, it is concluded that S. cereale evolved from S. montanum as a result of progressive cytological and morphological differentiation and that this differentiation was facilitated, probably, by adaptive superiority of translocation heterozygotes and rearrangement homozygotes.

A FURTHER STUDY OF CHROMOSOMAL POLYMORPHISM IN DROSOPHILA WILLISTONI IN ITS RELATION TO THE ENVIRONMENT

Natural populations of many species of Drosophila are mixtures of chromosomal types which differ in inversions of blocks of genes. This polymorphism is known to be in the main adaptive, and most of the chromosomal variants are maintained in the populations owing to a superiority of heterozygotes over homozygotes (heterosis). The amount of polymorphism is, however, variable from species to species, as well as in populations of the same species. Some species are chromnosomally tiniform, while in Drosophila willistoni, D. paulistorum, and D. subobscura about 40 different gene arangements per species are known (Patterson and Stone, 1952; Dobzhansky, 1951; Mainx, Koske, and Smital, 1953). In some species and populations inversion heterozygotes are rare or absent, while in populations of D. willistoni which inhabit central Goyaz (Brazil) the average number of heterozygous inversions per individual is about 9, and one individual heterozygous for 16 inversions has been encountered (da Cunha, Burla, and Dobzhansky, 1950). The reasons for these spectacular variations in the degree of polymorphism are a challenging problem. Da Cunha, Burla, and Dobzhansky (1950) and Dobzhansky, Burla, and da Cunha (1950) have advanced a working hypothesis, according to wvhich the amount of adaptive polymorphism carried in a population is, other