Genetic Basis Of Variation Research Articles

Biochemical phylogeny of the eight species in the Drosophila melanogaster subgroup, including D. sechellia and D. orena.

SummaryThe phylogenetic relationships of the eight species of the Drosophila melanogaster subgroup are examined on the basis of genetic variation at 33 putative enzyme loci. Values of Nei's genetic distance (ds) range from 0·28 to 1·74. D. sechellia appears closer to D. simulans than to D. mauritiana, the two former being the most closely related. D. orena is quite distantly related to D. erecta (ds = 1). Genetic differentiation supports the existence of three main lineages within the melanogaster subgroup and the yakuba-teissieri pair appears to be closer to the melanogaster lineage than to the erecta-orena one. Inferences of the times of species divergence from allozyme data are made and their agreement to other estimates is discussed.

Genetic basis of variation in reproductive performance. I. Ewe lambs treated with fluorogestone acetate and PMSG in a controlled environment

This study involved records on fertility, prolificacy, multiple birth, fecundity, body weight and age at breeding of 2380 ewe lambs from 432 sires of three synthetic strains developed at the Animal Research Centre, Ottawa, Canada. Ewes were divided into two equal flocks which were bred to lamb alternately at 4-month intervals under an 8-month breeding cycle. Lambs were housed indoors, in a controlled environment, on expanded metal mesh floors in windowless barns and were bred in January, May and September at the end of a period of short days. Ewes that lambed were generally heavier and slightly older at breeding as were ewes that lambed to mating at the synchronised estrus. Though not significant, trends demonstrating seasonal differences in the heritabilities of all reproductive traits were observed. Paternal half-sib estimates showed fertility, prolificacy, multiple births and fecundity were non-heritable in ewes lambing to matings at the synchronized estrus. Heritability estimates for prolificacy and multiple birth were 0.22 and 0.13, respectively, in ewes lambing to matings at the synchronized plus follow-up estrus, suggesting that slow genetic progress can be achieved in these traits from selection. Heritability estimates ranged from 0.03 to 0.22 for body weight and from 0.28 to 0.54 for age at breeding. Ewes that lambed showed lower values than all ewes exposed. Phenotypic correlations between reproductive traits and either body weight or age at breeding were low and ranged from 0.04 to 0.09. There were higher estimates of genetic correlation of prolificacy (and multiple birth) with either body weight or age at breeding in ewes lambing to matings at the combined estruses. Indirect selection of ewe lambs based on age and body weight at breeding may be an effective alternative to direct selection for genetic improvement of prolificacy and multiple birth.

Natural selection on the plant-water relations of Cleome serrulata growing along natural moisture gradients.

I investigated the extent and adaptive importance of genetically-based variation in plant water relations in two populations of the annual plant Cleome serrulata found growing along relatively short (<30 m) and mild soil moisture gradients. Field measurements of predawn plant water potentials showed that plants at the dry end of the moisture gradients had consistently lower ψPlant in May and June of 1984; differences up to 0.9 MPa were seen along the gradients. Seeds were collected from maternal plants growing along the moisture gradients and then grown under well-watered conditions in the greenhouse. Pressure-volume curves were constructed for a total of 92 seedlings from 25 maternal plants when the seedlings were four weeks old.Considerable genetic variation in the four highly correlated water potential components was seen in both populations, suggesting relatively high heritabilities (h2≈0.5). A partial correlation analysis revealed that cell wall elasticity was higher in seedlings from maternal plants which grew in the dry portions of each site. This suggested that natural selection had acted on this character during one or more previous generations. It appears that slight variations in the physiological genotype can significantly affect overall fitness in C. serulata.

Genetic basis of variation in morphometric characters as implied by hybrids between subspecies of Esox americanus (Pisces: Esocidae)

Artificial hybrids between Esox americanus americanus and Esox americanus vermiculatus, reared in a common laboratory environment, scored intermediately in discriminant analysis to the distribution of scores of reference parental samples. This implies a genetic basis for the variables that differ between the subspecies. Such intermediacy also provides a test of the utility of discriminant function analysis in such analyses. Scores of putative intergrade specimens from Alabama overlapped those of E. a. vermiculatus, indicating our incomplete understanding of the intergradation between these two forms.

Variation in and Heritability of Aspects of Pheromone Production in the Pink Bollworm Moth, Pectinophora gossypiella (Lepidoptera: Gelechiidae)

The genetic basis of variation in pheromone production by pink bollworm moths, Pectinophora gossypiella (Saunders), was examined by GLC analysis of pheromone gland extracts from females in a laboratory colony. The genetic correlation between the amounts of the two pheromone components, (Z,E)- and (Z,Z)-isomers of 7,11-hexadecadienyl acetate, was 0.99, suggesting that genetic factors controlling their production are identical or closely linked. The mean percentage of the (Z,E)-isomer was 44.2%. The limited variation found in component blends (CV = 5.3%) may reflect the importance of low signal variability for efficient communication. Heritability of total amount of pheromone in the gland was 0.41, and heritability of pheromone blend was 0.34. One generation of inbreeding had no detectable effect on pheromone blend, but total pheromone production was decreased by 26%. Mean component blends produced by females in a laboratory strain and in a strain incorporating recently collected wild males were not significantly different, but the amount of pheromone was significantly lower in the laboratory colony.

Studies on the Stellaria longipes Complex (Caryophyllaceae)-Biosystematics

All populations proved to be self-compatible but most required pollination by in- sects. Cross-pollination led to a slightly higher seed production than selfing. Self-pollinating pop- ulations were found in two localities. Clonal material grown under different environmental con- ditions produced very different phenotypes; leaf shape, inflorescence development, and stature are particularly plastic. Pubescence and capsule characters are not influenced by environment; how- ever, most wild populations were heterozygous for these characters. No correlation was found between polyploidy and morphology or taxonomy, though there were differences in pollen di- ameter between the main cytotypes. Crosses were produced between major cytotypes, the main variational entities, and widely separated populations. All proved successful and the progeny were fertile. The interacting roles of reproductive biology, genetic variability, and phenotypic plasticity are presented. Stellaria longipes Goldie s.l. is a highly poly- morphic circumpolar species complex. In most current floras (e.g., Fernald 1950; Hitchcock et al. 1964; Tutin et al. 1964), several species are recognized within the complex, following the monographic studies of Hulten (1943, 1968), Bocher (1951), and Porsild (1963). These studies were, however, based on field and herbarium observations; information on the genetic basis of variation was not available, and that on cy- tology was very incomplete. Philipp (1972), working on the Greenland populations of this species complex, and Chinnappa and Morton (1974), using data from the whole range of the

MACROGEOGRAPHIC VARIATION IN LITTER SIZE IN ANTECHINUS (MARSUPIALIA: DASYURIDAE).

The best documented cases of geographic variation in brood size are the tendency of avian clutches to increase with latitude (Lack, 1947, 1954; Klomp, 1970), and mammalian litters to increase with both latitude (Lord, 1960), and altitude (Dunmire, 1960; Fleming and Rauscher, 1978; Bronson, 1979). There is no consensus on the cause of this phenomenon (Stearns, 1976; Ricklefs, 1980). Lack (1947) proposed that parents produce broods which correspond with the maximum number of young they can nourish. Numerous modifications to this hypothesis have been suggested (Ashmole, 1963; Cody, 1966; Owen, 1977; Hogstedt, 1980), and a number of theoretical models have been developed that suggest brood size should be reduced below the most productive size (Stearns, 1976). Two classes with wide application can be recognized among the latter models. The first includes those based on the tradeoff between fecundity and adult mortality resulting from increased reproductive effort (Charnov and Krebs, 1974). According to this approach adults should limit the size of their brood in certain environments to extend their opportunities of breeding again (Ricklefs, 1977a). This viewpoint has been criticized because it fails to predict what size smaller broods should be to ensure iteroparous reproduction (Ricklefs, 1977b). The second includes those models which predict tradeoffs between demands for resources on the part of reproduction and other functions, particularly avoidance of predation (Cody, 1966; Skutch, 1967; Perrins, 1977). Ricklefs (1977a) presented evidence that predation is not sufficiently important to account for the magnitude of variation commonly observed along geographic gradients. Several methodological difficulties have hindered investigation of these hypotheses. The genetic basis of variation in brood size has rarely been established (Krohne, 1980, 1981). Further, the influence of predation and the proposed trade-off between reduced clutch size and increased adult longevity have been difficult to quantify in field populations. While it is possible to distinguish between semelparous and iteroparous species, an index of the extent of iteroparity remains elusive and is confounded by difficulties in ageing wild individuals and the production of more than one brood within a season. A study of variation in brood size within semelparous species might be informative, but brood sizes in semelparous invertebrates and lower vertebrates and monocarpic plants are enormous and difficult to quantify. In this study we report macrogeographic variation in the litter size of Antechinus, a genus of mammals in which the frequency of iteroparous versus semelparous reproduction can be precisely quantified, and in which we believe litter size differences are genetically determined. Litter size is manageably low and variable (6-13), and only one litter is produced each year. This combination of characters provides a tractable system for resolution of the separate factors which might influence litter size.

Response of 19 cultivars of soybeans to ultraviolet‐B irradiance

Nineteen soybean cultivars were grown for four weeks in controlled environmental chambers with artificial daylight supplemented by five UV‐B irradiance regimes to determine the range of growth and development responses of seedlings. Data from nine plant characteristics were assessed: leaf area, dry weight of leaves, stems and roots, total plant dry weight, height, ratio of roots to shoots and leaf area to weight and rating of leaves for damage. Significant differences were observed in the responses noted. Stunting, leaf chlorosis and loss of apical dominance were three general symptoms apparent on all cultivars which received UV‐B irradiance. Varying degrees of reduced leaf area and dry weight of the plants and altered ratios of weights of leaves per unit area and weight of roots to shoots were also found. It was concluded that different soybean cultivars demonstrate a marked difference in sensitivity to UV‐B radiation under the artificial conditions of controlled environmental growth chambers and this may indicate a genetic basis for variability in sensitivity of soybean cultivars to this waveband. However, the sensitivity to UV‐B radiation was increased by the lower than normal photon fluence of photosynthetically active radiation (225 μE m−2 s−1).

Evolution of Herbicide Resistance in Senecio vulgaris: Variation in Susceptibility to Simazine Between and Within Populations

SUMMARY (1) Inter-population variation in susceptibility to simazine in Senecio vulgaris was investigated in forty-six different populations collected from several different fruit farming locations in England. Significant population differences occurred in susceptibility to simazine indicating a genetic basis for variation in susceptibility. (2) There was a positive, linear relationship between percentage survival of a population and the number of consecutive years of simazine application indicating the possibility that natural selection had occurred. (3) Intra-population variation in susceptibility to simazine was investigated in one population originating from an apple orchard in Suffolk and a selfing series procedure was used to obtain the genetic and environmental components of variance and an estimate of broad sense heritability. The latter was low.

SEXUAL DIMORPHISM, SEXUAL SELECTION, AND ADAPTATION IN POLYGENIC CHARACTERS.

Conspicuous sexual dimorphism is a feature of many species of higher animals. The genetic basis of variation in metrical characters, including that in sexual dimorphism between families or lines, is usually polygenic (Falconer, 1960; Frankham, 1968b; Wright, 1968, 1977; Bird and Schaffer, 1972; Ehrman and Parsons, 1976). Genetic experiments on mice, birds and Drosophila flies indicate that artificial selection practiced on a character of one sex causes not only a direct response of the character in the selected sex, but also a correlated response of the homologous character, if any, in the opposite sex (Shaklee et al., 1952; Harrison, 1953; Korkman, 1957; Becker et al., 1964; Eisen and Legates, 1966; Frankham, 1968a, 1968b; Eisen and Hanrahan, 1972). Such correlated selective responses are attributable to pleiotropy (and linkage) of genes affecting the characters of both sexes, that is, correlations between the additive effects of genes as expressed in males and females. The genetic correlation between homologous characters of the sexes is often quite high (op. cit.). As will be shown, this greatly restricts the rate of evolution of sexual dimorphism relative to that for the average phenotype of the two sexes. Sexual dimorphism may result from natural and/or sexual selection. Darwin (1874, Part 2) elucidated how natural selection operating differently on males and females arises from their distinctive roles in reproduction, or from competition between the sexes for resources such as food, leading to adaptive sexual dimorphism. He also reasoned that intrasexual contests for mates and intersexual mating preferences exert sexual selection, usually on the males, producing sexual dimorphism which is maladaptive with respect to natural selection. Comparisons within and between closely related species led Darwin to conclude that adult males typically are more modified than adult females or juveniles of either sex, but that females have often acquired male characters by transference. It was difficult for Darwin to believe that sex-limitation of characters could evolve by selection, but Fisher (1958, Ch. 6) outlined how divergent selection on the two sexes could accumulate genes with different effects in males and females, causing a character at first expressed equally in both sexes to become sexually dimorphic and finally sex-limited. The strength of sexual selection is enhanced by a polygamous mating system, but the possibility of sexual selection in monogamous systems of mating exists due to male competition for early-breeding females, and mate choice exercised by these females (Darwin, 1874; Fisher, 1958; O'Donald, 1977). Systems of mating are often thought to evolve in response to ecological pressures (reviewed by Selander, 1972; Brown, 1975; Emlen and Oring, 1977), although mating preferences may be self-reinforcing (Fisher, 1958; O'Donald, 1967, 1977; Lande, unpubl.). Darwin and Fisher described qualitative methods by which an observed sexual dimorphism could be attributed mainly to either natural or sexual selection. To assign natural selection as the primary cause requires ecological observations that males and females follow different ways of life and employ the dimorphic character(s) adaptively in their distinct modes of survival or reproduction. Darwin presented several such examples, mostly among the lower classes of animals. Selander (1972) 292

Trichostrongylus colubriformis infection of guinea pigs: genetic basis for variation in susceptibility to infection among outbred animals.

Two lines of guinea-pigs, whose susceptibilities to infection with the parasitic nematode Trichostrongylus colubriformis differ significantly, have been established. This difference is probably based on genetically determined differences between the ability of members of each line to bring about immune expulsion of the parasite.

Biochemical genetic variation in populations of golden trout, Salmo aguabonita: evidence of the threatened Little Kern River golden trout, S.a. whitei.

Eight wild populations of the High Sierra golden trout, Salmo aguabonita, and one domestic stock of rainbow trout, Salmo gairdneri, were examined for biochemical-genetic variation in eight protein systems. Variation within the eight systems was determined by at least 10 loci in both golden and rainbow trout and all the alleles identified in rainbow trout were observed as electro-phoretically identical phenotypes in golden trout. Variation was observed at an average of 51 percent of the loci in the golden trout samples and for five of the 10 loci in the rainbow trout. Average heterozygosity ranged from 12.6 to 13.9 percent for seven of the golden trout populations with one showing a low value of 5.4 percent. A comparable estimate of 12.1 percent was found for the rainbow stock. On the basis of genetic variation and allele frequencies at three loci, the eight golden trout populations were divided into two distinct groups. Three populations sampled from the Little Kern River basin tended to be genetically distinct from two additional Little Kern River basin populations and from three geographically distinct populations sampled from the eastern Kern River area. The former three populations were hypothesized to be of a recent rainbow-golden hybrid origin. Trout in the other two Little Kern River basin populations, sampled in head-waters of a stream tributary to the Little Kern River, were considered to be the threatened Little Kern golden trout, S. a. whitei Evermann, due to their high degree of genetic similarity to the geographically distinct subspecies S. a. aguabonita sampled from the eastern Kern River area. The finding of substantial genetic variation in the wild golden trout populations indicates that this threatened species is not at present genetically impoverished and thus does not appear to be in immediate danger of extinction through lack of adaptive capability.

Quantitative genetic analysis in Phalaris tuberosa III. Maternal effects on seedling growth and development

SUMMARYThe genetic basis of variation in rate of seedling growth, and development has been examined in the Australian commercial population of Phalaris tuberosa. A model of additive genetic maternal effects has been used, with seed weight of the female parent as an index of maternal ability. Rate of leaf appearance, rate of tillering and growth per tiller are all genetically variable in the population, with estimated heritabilities of 0·36, 0·23 and 0·34 respectively on an individual seedling basis. Total seedling growth has a lower heritability (0·17), due to a negative genetic correlation between tiller production and growth per tiller ( − 0·46). These two components have also been shown to be subject to qualitatively different seed size maternal effects. Genetic differences in seed size in the female parent have been found to influence growth per tiller, while environmental differences in seed size affect primarily the rates of leaf appearance and tiller production.

The genetic basis of variation in factor 8 levels among haemophiliacs.

The genetic basis of variation in factor 8 levels among haemophiliacs.

Science and Liberal Education

ONE of the chief characteristics of the present age is the ascendency of science. Never before in history has any agency of peace or war exerted so profound an influence during an equal interval of time as has science within the past five or ten decades of human existence. In all areas of the world, but especially in the Western World, science has contributed immeasurably to our material well being, our intellectual development, to our happiness and our health. Our knowledge of the physical world and of the cosmic organization of which it is a part has been greatly extended; control of the atom has brought a source of power whose dimensions stagger the imagination. In chemistry, progress can be assessed by the tremendous developments in organic chemistry-in textiles, synthetic drugs, fuels, and plastics, and also in our understanding of protein chemistry and enzyme action. In biology, we have a consistent and logical account of the origin and development of life on this planet. We know the genetic basis for variation and how variations with adaptive significance have led, through evolution, to the origin of species. Progress has also been striking in applied biology. Our food supply has been increased to a remarkable extent by the development of hybrid crops and through genetic selection of strains of domestic animals capable of greater meat, or milk, or egg production. Control of unwanted vegetation and selective nurturing of useful plant growth by hormones and chemical agents represent other areas in which progress has been made. Artificial photosynthesis of carbon dioxide and water into carbohydrate, a process on which green plants have thus far enjoyed exclusive patent rights, may be just around the corner. In medicine, likewise, progress has been truly phenomenal. Many epidemic diseases, which only a few years ago exacted heavy toll from children and young people, have been effectively controlled. At present, therefore, the average life expectancy in America is only a fraction of a year short of the Biblical three score years and ten.