K. Wöhrmann

Enzyme variability in populations of aphids.

SummaryFrom 1976 to 1978 nine aphid species were investigated electrophoretically. A low degree of polymorphism was found. Only 5 of the species were polymorphic, and only at most in 3 enzyme systems out of 19.

Phänotypenvariabilität und Umwelt bei Capsella bursa-pastoris (Cruciferae)

Summary Seed samples from 20—30 plants per population were randomly collected from natural populations of Capsella bursa-pastoris covering a wide range of different climatic conditions. Progenies were grown in random block experiments and up to 14 characters per plant recorded. Based upon statistical inference, population parameters were correlated both, with each other and with environmental parameters. Altogether, 48 environmental parameters have been taken into account. We found that even those characters were controlled by the environment, the adaptive value of which did not intuitively seem to be obvious (e.g. leaf shape). Of course, flowering time which is an extremely important fitness character, is strongly influenced by environmental parameters, mainly temperature and rainfall. However, these relationships were so complex that we often could not establish simple correlations. Depending upon the provenience of the sample, the same temperature values caused a widely spread spectrum of flowering times. In the light of these findings it is argued that adaptation within the common and widely distributed weed Capsella bursa-pastoris is highly influenced by locally different genotypes. There is no support for the “general purpose genotype” concept.

Experimental evidence for the adaptive value of sexual reproduction

It is generally believed that recombination by sexual reproduction is unfavourable in constant environments but is of adaptive value under changing environmental conditions. To test this theory, experimental populations of yeast (Saccharomyces cerevisiae) were set up and maintained at different levels of environmental heterogeneity. Recombination was estimated by determining sporulation rates. Sporulation rates first increased in populations living in highly variable environments, but after some time began to decrease. The decrease started last and was slowest in populations which were maintained under the same conditions for a sufficiently long time, to allow some adaptation of the gene pool to the respective environment. Patterns of genotypic variability could not be interpreted in such simple terms, but there was a statistically significant correlation between sporulation rate and genotypic variability. This correlation is to be expected because recombination generates genotypic variability. Summing up, recombination by sexual reproduction is advantageous in changing environments if the population can track the changes in the environment by changing its genotypic structure.

Das Blühverhalten vonCapsella bursa-pastoris (Brassicaceae)

Capsella bursa-pastoris (L.)Med. is a partially self-pollinating, autogamous plant. The selfing-rate depends on ecological factors. High atmospheric humidity at temperatures over 15 °C and low light intensity, i.e. cloudy and rainy weather, lead to almost exclusive self-pollination, while dry and sunny weather favours outcrossing. At low temperatures (about 4–10 °C) anthesis is prolongated up to five-fold, but allogamy is reduced. Day-length distinctly influences the beginning of flowering.Capsella bursa-pastoris is not a day-neutral plant (as was supposed up to now) but behaves as a quantitative long-day plant.

Untersuchungen zur Genetik eines Esterasepolymorphismus bei Saccharomyces cerevisiae

On the genetics of an esterase polymorphism of Saccharomyces cerevisiae. The inheritance of unspecific esterases of Saccharomyces cerevisae was investigated by tetrad analysis. It was shown that the four bands, found in the strains investibated, and distinguished by their electrophoretic mobility, are controlled by two loci with two alleles each. The loci E1 and E2 segregate independently whereas E2 is closely linked to the ad2 locus. The E1 locus could not be mapped; however, there was evidence for free recombination with the contromere.

Genetische Variabilität und relative Fitness in vorwiegend selbstbefruchtenden Populationen

SummaryRecent investigations on the structure of several experimental as well as wild populations of predominantly self-pollinated plant species revealed unexpectedly high amount of heterozygosity. The maintenance of a high proportion of heterozygotes despite heavy inbreeding could not be explained merely on the basis of natural outcrossing and mutation. The data were very suggestive of heterozygote superiority. In this review models for estimates of fitness values and their dependence on some other population parameters are considered. The impact of the present studies on plant breeding practices of self-pollinated crops are also dicussed.ZusammenfassungJüngere populationsgenetische Untersuchungen zeigen eine unerwartet hohe Heterogenität in Wildund Zuchtpopulationen von vorwiegend selbstbefruchtenden Pflanzen. Der hohe Anteil an Heterozygoten, der entgegen der Inzuchtwirkung in einer Population erhalten bleiben kann, ist nicht allein durch spontane Fremdbefruchtung und Mutation zu erklären, sondern läßt einen Selektionsvorteil der Heterozygoten vermuten. In vorliegender Arbeit werden Möglichkeiten zur Schätzung der „fitness“ und deren Abhängigkeit von verschiedenen Parametern aus der Literatur zusammengestellt. Die Bedeutung heterogener Populationen wird im Hinblick auf die Züchtung vorwiegend selbstbefruchtender Kulturpflanzen diskutiert.

[Population genetics in Saccharomyces cerevisiae : I. Yeast as a subject for population genetics].

The present paper gives a model of population genetics based on the life cycle of yeast (Saccharomyces cerevisiae). This model considers the most important factors influencing the frequencies of genes and genotypes in a yeast population and is the basis for the quantitative estimation of these parameters. The advantages and possibilities offered by the use of yeast as material for population genetics are discussed.SummaryThe present paper gives a model of population genetics based on the life cycle of yeast (Saccharomyces cerevisiae). This model considers the most important factors influencing the frequencies of genes and genotypes in a yeast population and is the basis for the quantitative estimation of these parameters. The advantages and possibilities offered by the use of yeast as material for population genetics are discussed.ZusammenfassungIn vorliegender Arbeit wird ein populations-genetisches Modell vorgestellt, das auf dem Lebenszyklus der Hefe (Saccharomyces cerevisiae) basiert. Das Modell berücksichtigt die wesentlichen Faktoren, die die Gen- und Genotypenfrequenzen in einer Hefe-population, beeinflussen, und ist die Grundlage für die quantitative Bestimmung dieser Parameter. Die Vorteile und Möglichkeiten, die die Verwendung von Hefe als populationsgenetisches Versuchsobjekt bietet, werden diskutiert.

Induction of sexual forms in the rose aphid Macrosiphum rosae

The rose aphid, Macrosiphum rosae (L.), was tested for its ability to produce sexual forms on rose leaves, after induction by environmental conditions in the field and in the laboratory. Clones were collected in Tübingen, Federal Republic of Germany, and in south‐eastern New South Wales, Australia. The German clones all produced sexual forms, while the response of the Australian clones was weak. There appeared to be a bias in the sex ratio towards females: This is discussed in developmental terms. The environmental and genetic factors influencing the response are considered.

The search for hidden enzymatic variation in the aphid Macrosiphum rosae (L.)

SummaryUsing two different buffer systems and up to 6 different electrophoretic polyacrylamide gel concentrations, hidden enzymatic variability was investigated in samples of the rose aphid Macrosiphum rosae. MDH1, PGM1, SDH, EST and LAP were found to be polymorphic. No additional variation was observed by changing the test conditions compared to those of earlier investigations using starch gel electrophoresis.

[Populationsgenetische Untersuchungen an Saccharomyces cerevisiae III. Theoretische Untersuchungen über die Bedeutung der Paarungsfähigkeit für die Populationsstruktur Population genetics of Saccharomyces cerevisiae. III. Theoretical investigations about the influence of the mating-ability on the population structure].

SummaryThis paper deals with the properties of a two-locus model constructed according to the lifecycle of the yeast Saccharomyces cerevisiae. In addition to the mating-type locus with two alleles, a second locus with two alleles was assumed. Further, we considered that the population can be composed of haploid and diploid cells.Of the population parameters involved in this model, there will be considered the influence of different mating-abilities of haploids on the population structure with regard to linkage and linkage disequilibrium.The main results are as follows: Assuming that more than two spores per ascus are formed which are able to mate, the frequencies of the mating-type alleles will adjust to a ratio of 1∶1. If the number of spores is less than two, the rare mating-type allele will be displaced by the common one. Under these conditions there is no change in the allele frequencies of the second locus unless the population is in a linkage disequilibrium. In this case the degree of the frequency change in the second locus depends on the difference in the ratio of the mating-types at the beginning, the linkage disequilibrium and the kind of linkage of the two loci. Differing mating-ability of the haploid genotypes has no influence on the equilibrium frequency of the mating-type locus. Under the assumption of multiplicative gene action the equilibrium allele frequencies of the second locus are strongly influenced by the epistatic component of the mating-ability. A permanent linkage disequilibrium can be maintained by distinct epistatic values, which increases with tighter linkage.If there is a permanent linkage disequilibrium, population fitness will not always reach its maximum.ZusammenfassungIn der vorliegenden Arbeit wurden die Eigenschaften eines Zwei-Locus-Modells untersucht, dem der Generationszyklus der Hefe, Saccharomyces cerevisiae, als Grundlage diente. Neben dem Paarungstyplocus mit seinen beiden Allelen a und α ist ein zweiter Locus mit zwei Allelen angenommen worden. Es wurde berücksichtigt, daß die Population sowohl aus haploiden als auch aus diploiden Individuen bestehen kann. Von den theoretisch möglichen wirksamen Populationsparametern wurden hier zunächst nur die Paarungsfähigkeit der Haploiden und die Koppelung in ihrem Einfluß auf die Populationsstruktur unter besonderer Berücksichtigung des Koppelungsgleichgewichtes untersucht. Es ergaben sich folgende Zusammenhänge: Unter der Voraussetzung, daß mehr als zwei Sporen pro Ascus entstehen, die die Fähigkeit zur Paarung besitzen, stellt sich ein Verhältnis der Paarungstypenallele von 1:1 ein.Ist die Anzahl der Sporen pro Ascus kleiner als zwei, so wird das seltene Paarungstypallel durch das häufigere verdrängt.Die Allelfrequenzen eines zweiten Locus ändern sich unter diesen Bedingungen nicht, es sei denn, daß in der Ausgangspopulation ein Koppelungsungleichgewicht bestanden hat. Das Ausmaß der Änderung ist dann davon abhängig, wie groß die Differenz der Frequenzen der Paarungstypallele in der Ausgangspopulation und der Gleichgewichtspopulation ist, wie groß das anfängliche Koppelungsungleichgewicht ist und wie eng beide Loci gekoppelt sind.Wenn die verschiedenen haploiden Genotypen eine unterschiedliche Paarungsfähigkeit besitzen, hat dies auf die Gleichgewichtsfrequenz der Paarungstypenallele keinen Einfluß. Die Frequenzen der Allele des zweiten Locus sind im Gleichgewicht bei Annahme multiplikativer Genwirkung jedoch stark von der epistatischen Komponente der Paarungsfähigkeit abhängig. Bei bestimmten Werten für E wird ein permanentes Koppelungsgleichgewicht aufrechterhalten, das mit zunehmender Kopplung größer wird.Die Populationsfitness erreicht bei permanentem Koppelungsgleichgewicht nicht immer ihr Maximum.