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N Ryman

Publications and source records attributed to N Ryman.

At least 19 recordsLinked to original sources
PubMed2003

Lack of molecular genetic divergence between sea-ranched and wild sea trout (Salmo trutta).

The supportive breeding programme for sea trout (Salmo trutta) in the River Dalälven, Sweden, is based on a sea-ranched hatchery stock of local origin that has been kept 'closed' to the immigration of wild genes since the late 1960s (about seven generations). In spite of an apparent potential for substantial uni directional gene flow from sea-ranched to wild (naturally produced) trout, phenotypic differences with a presumed genetic basis have previously been observed between the two 'stocks'. Likewise, two previous studies of allozyme and mitochondrial DNA variation based on a single year of sampling have indicated genetic differentiation. In the present study we used microsatellite and allozyme data collected over four consecutive years, and tested for the existence of overall genetic stock divergence while accounting for temporal heterogeneity. Statistical analyses of allele frequency variation (F-statistics) and multilocus genotypes (assignment tests) revealed that wild and sea-ranched trout were significantly different in three of four years, whereas no overall genetic divergence could be found when temporal heterogeneity among years within stocks was accounted for. On the basis of estimates of effective population size in the two stocks, and of FST between them, we also assessed the level of gene flow from sea-ranched to wild trout to be approximately 80% per generation (with a lower confidence limit of approximately 20%). The results suggest that the reproductive success of hatchery and naturally produced trout may be quite similar in the wild, and that the genetic characteristics of the wild stock are largely determined by introgressed genes from sea-ranched fish.

Animals↗
PubMed2001

Statistical power when testing for genetic differentiation.

A variety of statistical procedures are commonly employed when testing for genetic differentiation. In a typical situation two or more samples of individuals have been genotyped at several gene loci by molecular or biochemical means, and in a first step a statistical test for allele frequency homogeneity is performed at each locus separately, using, e.g. the contingency chi-square test, Fisher's exact test, or some modification thereof. In a second step the results from the separate tests are combined for evaluation of the joint null hypothesis that there is no allele frequency difference at any locus, corresponding to the important case where the samples would be regarded as drawn from the same statistical and, hence, biological population. Presently, there are two conceptually different strategies in use for testing the joint null hypothesis of no difference at any locus. One approach is based on the summation of chi-square statistics over loci. Another method is employed by investigators applying the Bonferroni technique (adjusting the P-value required for rejection to account for the elevated alpha errors when performing multiple tests simultaneously) to test if the heterogeneity observed at any particular locus can be regarded significant when considered separately. Under this approach the joint null hypothesis is rejected if one or more of the component single locus tests is considered significant under the Bonferroni criterion. We used computer simulations to evaluate the statistical power and realized alpha errors of these strategies when evaluating the joint hypothesis after scoring multiple loci. We find that the 'extended' Bonferroni approach generally is associated with low statistical power and should not be applied in the current setting. Further, and contrary to what might be expected, we find that 'exact' tests typically behave poorly when combined in existing procedures for joint hypothesis testing. Thus, while exact tests are generally to be preferred over approximate ones when testing each particular locus, approximate tests such as the traditional chi-square seem preferable when addressing the joint hypothesis.

Animals↗
PubMed1996

Demographic genetics of brown trout (Salmo trutta) and estimation of effective population size from temporal change of allele frequencies.

We studied temporal allele frequency shifts over 15 years and estimated the genetically effective size of four natural populations of brown trout (Salmo trutta L.) on the basis of the variation at 14 polymorphic allozyme loci. The allele frequency differences between consecutive cohorts were significant in all four populations. There were no indications of natural selection, and we conclude that random genetic drift is the most likely cause of temporal allele frequency shifts at the loci examined. Effective population sizes were estimated from observed allele frequency shifts among cohorts, taking into consideration the demographic characteristics of each population. The estimated effective sizes of the four populations range from 52 to 480 individuals, and we conclude that the effective size of natural brown trout populations may differ considerably among lakes that are similar in size and other apparent characteristics. In spite of their different effective sizes all four populations have similar levels of genetic variation (average heterozygosity) indicating that excessive loss of genetic variability has been retarded, most likely because of gene flow among neighboring populations.

Alleles↗
PubMed1995

Temporal allele frequency change and estimation of effective size in populations with overlapping generations.

In this paper we study the process of allele frequency change in finite populations with overlapping generations with the purpose of evaluating the possibility of estimating the effective size from observations of temporal frequency shifts of selectively neutral alleles. Focusing on allele frequency changes between successive cohorts (individuals born in particular years), we show that such changes are not determined by the effective population size alone, as they are when generations are discrete. Rather, in populations with overlapping generations, the amount of temporal allele frequency change is dependent on the age-specific survival and birth rates. Taking this phenomenon into account, we present an estimator for effective size that can be applied to populations with overlapping generations.

Age Factors↗
PubMed1990

Allele frequency estimation at loci with incomplete co-dominant expression.

Although allelic variants at a locus usually are expressed either dominantly or co-dominantly, there are many cases when a gene is dominantly expressed in some individuals and co-dominantly in others. We present a maximum-likelihood procedure for allele frequency estimation in such situations of "incomplete" co-dominant gene expression at an autosomal locus that segregates for two alleles. Our proposed estimator generally is less biased and has a smaller sampling variance than those previously described.

Alleles↗
PubMed1984

Silencing of duplicate genes: a null allele polymorphism for lactate dehydrogenase in brown trout (Salmo trutta).

A previously described isozyme polymorphism at one of two skeletal muscle LdhA loci in brown trout is due to a null allele, Ldh1(n), producing no detectable catalytic activity. Homozygotes for this allele have approximately only 56% of the LDH activity in skeletal muscle relative to homozygotes for the active allele. The remaining activity results from enzyme subunits produced by other LDH loci. The Ldh1(n) allele is common and widespread throughout brown trout populations in Sweden and is also found in populations from Ireland. The persistence of duplicate gene expression for the LdhA loci in almost all salmonid species is best explained by natural selection against individuals containing null alleles. However, there is no indication of natural selection against brown trout with the Ldh1(n) allele: We suggest that the selection against individuals containing null alleles that is apparently responsible for the persistence of duplicate LdhA loci in salmonids occurs only under certain environmental conditions.

Alleles↗
PubMed1984

The distribution of the paternity index as a basis for evaluation of sequential testing in paternity analysis.

Several procedures for evaluation of paternity testing data have been suggested in the literature, the majority of them being based on the paternity index statistic (L) or some transform of it. A major problem has been that the true distribution of the paternity index has not been known, making it difficult to perform quantitative evaluations of different procedures. We present an algorithm for computation of the distribution of the paternity index within the limits of a completely controlled amount of approximation. Using this algorithm we evaluate the power and the rate of erroneous classifications of a standard routine test based on a fixed number of genetic marker systems. The efficiency of this standard test procedure is compared to a stepwise (sequential) procedure where in each step one or several marker systems are scored for the mother-child-putative father trio. We suggest that a sequential strategy for testing may be more efficient than one that is based on a fixed number of systems. A sequential procedure can provide information about the accused man's state of paternity in a considerably larger fraction of cases without a substantial increase of the frequency of incorrect classifications. In addition, the cost measured as the average number of marker systems that has to be tested for each trio may be lower in the case of sequential testing than with a fixed number of systems.

Blood Group Antigens↗
PubMed1983

Genetic differentiation in four European subspecies of red deer (Cervus elaphus L.).

Red deer representing the four different European subspecies Cervus elaphus atlanticus, C. e. elaphus, C. e. germanicus, and C. e. scoticus were examined for allozyme variability at 35 enzyme loci. The proportion of polymorphic loci within populations (P) ranged from 0 to 13.8 per cent and the average heterozygosity (H) from 0 to 3.6 per cent. These estimates are within the range previously observed among mammalian species. Significant allele frequency differences were found both within and between subspecies. The mean genetic distance between subspecies (D = 0.0164) was smaller than the differentiation at similar taxonomic levels among other ungulates, probably because of a shorter time since divergence. Within subspecies the genetic differences between populations were similar to those reported between populations within closely related species in the same geographic region. Cluster analysis based on genetic distances indicated a major genetic dichotomy between the British C. e. scoticus and the Norwegian C. e. atlanticus on one hand and the Swedish C. e. elaphus and the continental C. e. germanicus on the other. Populations of pure C. e. elaphus were not found to differ genetically in any substantial way from Swedish populations of possible heterogeneous subspecific origin. An allele unique to C. e. scoticus was found in a Swedish enclosed population where imports of British deer are known to have taken place. A population established to preserve the genetic characteristics of the C. e. elaphus subspecies appeared to have lost 36 per cent of the electrophoretically measurable heterozygosity.

Alleles↗
PubMed1983

Differences in the relative distribution of human gene diversity between electrophoretic and red and white cell antigen loci.

Gene frequency data for 25 loci (2 HLA loci, 9 blood group loci, and 14 electrophoretically detectable loci) were collected from the literature of 18 human populations from all over the world. The data were subjected to a hierarchical gene diversity analysis to provide an estimate of the relative distribution of genetic variation between and within populations and population groups for different types of loci. Two different ways of grouping the populations, i.e., according to anthropological criteria and to a cluster analysis based on gene frequency data, gave essentially the same results. For all loci combined approximately 86% of total gene diversity was found within populations, 3% was associated with differences between populations within groups, and 11% related to group differences. These results are very similar to those obtained in previous studies based on fewer loci and different sets of populations. The distribution of genetic variation is different for different types of loci. The HLA loci give a picture very similar to that of the electrophoretic loci while the blood group loci have a substantially larger fraction of the total gene diversity distributed between populations or population groups.

Blood Group Antigens↗
PubMed1981

Use of odds of paternity computations in determining the reliability of single exclusions in paternity testing.

In parentage determination with genetic markers the concept of paternity index is used to evaluate the relative likelihood of fathership of an accused male individual. When a battery of genetic tests are performed, the event of inadvertant technical errors or the occurrence of suppressor genes that cause an apparent paternity exclusion of the male concerned reduces this likelihood to zero. However, through a study of statistical properties of the paternity index statistic based on the other genetic systems, it is possible to examine the reliability of one or a few such apparent exclusions. The details of this approach is examined here by analytical methods following the statistical principles of likelihood ratio test procedures. Some illustrative data are also analyzed to show the practical utility of this approach.

Alleles↗
PubMed1981

Incidence of Down's syndrome in Sweden during the years 1968-1977.

The incidence of Down's syndrome has been studied among children born in Sweden during the years 1968-1977. The risk for mothers of different ages of bearing such a child did not change during these years. This does not exclude that a change in incidence might have occurred in smaller areas of the country but escaped detection for statistical reasons. A higher than expected number of children with Down's syndrome were born in a few communities, which most likely is a chance event. No correlation could be detected between the incidence of Down's syndrome and a number of socioeconomic variables. The correlation with maternal age was studied in detail. There was a significant excess of males among both the newborn children with Down's syndrome and fetuses with trisomy 21 aborted after prenatal diagnosis. A similar tendency was found among the cases with a chromosome mosaicism but not among those with a translocation. Two hypotheses are put forward to explain the excess of males with trisomy 21.

Adolescent↗
PubMed1979

Reproductive isolation with little genetic divergence in sympatric populations of brown trout (Salmo trutta).

Two reproductively isolated demes of brown trout coexist in a small Swedish mountain lake, Lake Bunnersjörna. We electrophoretically examined 102 specimens from that lake for 27 enzymes encoded by 54 loci. The two demes are fixed for different alleles at a lactate dehydrogenase locus (LDH-1); statistically significant allele frequency differences at five other loci further support the complete lack of gene flow between these demes. There are significant differences in growth rates between fish in the two demes, but no further morphological differentiation h-s been detected.--In light of these findings, the genetic distance between these populations is surprisingly small (Nei's I = 0.975). These demes represent one of the least genetically divergent, reproductively isolated sympatric pair of vertebrate populations that have been identified. The results are discussed from both an evolutionary and ecological perspective.

Alleles↗