Bengt Olle Bengtsson

The barrier to genetic exchange between hybridising populations.

Suppose that selection acts at one or more loci to maintain genetic differences between hybridising populations. Then, the flow of alleles at a neutral marker locus which is linked to these selected loci will be impeded. We define and calculate measures of the barrier to gene flow between two distinct demes, and across a continuous habitat. In both cases, we find that in order for gene flow to be significantly reduced over much of the genome, hybrids must be substantially less fit, and the number of genes involved in building the barrier must be so large that the majority of other genes become closely linked to some locus which is under selection. This conclusion is not greatly affected by the pattern of epistasis, or the position of the marker locus along the chromosome.

Avoiding inbreeding: at what cost?☆

Abstract Animals reproducing sexually are faced with a dilemma. They can mate in the local area where they were born, but must then take the risk that they will mate with a close relative and produce offspring suffering from inbreeding depression. Or they can migrate to a new area and mate with an unrelated animal, but then there is a chance that they will die or lose valuable reproductive time during the migration. This dilemma is here described by some simple models. The condition for spreading of a new reproductive strategy in a population is given. The results are discussed using data from the Great Tit, Man and the Japanese Quail on the effect of inbreeding.

Genetic variation in organisms with sexual and asexual reproduction

Abstract The genetic variation in a partially asexual organism is investigated by two models suited for different time scales. Only selectively neutral variation is considered. Model 1 shows, by the use of a coalescence argument, that three sexually derived individuals per generation are sufficient to give a population the same pattern of allelic variation as found in fully sexually reproducing organisms. With less than one sexual event every third generation, the characteristic pattern expected for asexual organisms appear, with strong allelic divergence between the gene copies in individuals. At intermediary levels of sexuality, a complex situation reigns. The pair‐wise allelic divergence under partial sexuality exceeds, however, always the corresponding value under full sexuality. These results apply to large populations with stable reproductive systems. In a more general framework, Model 2 shows that a small number of sexual individuals per generation is sufficient to make an apparently asexual population highly genotypically variable. The time scale in terms of generations needed to produce this effect is given by the population size and the inverse of the rate of sexuality.

Chiasma and recombination data in plants: are they compatible?

It has long been considered that the number of chiasmata formed during meiosis corresponds to the number of crossovers indicated by the genetic map. However, recent investigations in plants show an unexpected discrepancy in the results obtained when calculating the total number of crossover events per meiosis by these two methods. Is this discrepancy due to methodological difficulties? Or is there something fundamentally wrong with our understanding of crossovers and chiasmata?

Growth and cation uptake in spruce (Picea abies Karst.) grown in sand culture with various aluminium contents

Three clones of spruce (Picea abies Karst.) were grown in sand culture for 9 weeks, whereafter they were transferred to water culture for 3 days. The plants were supplied with or grown in nutrient solution containing 0, 0.1, 1.0 respectively 10 mM AlCl3. The monomer Al fraction (including Al3+) and the Al3+ fraction were determined in the fresh nutrient solutions. The shoot/root dry weight ratio and the water content in shoots and roots were determined, and the main cationic composition (Al, K, Ca, Mg, Mn) in needles, stems and roots were analysed. A short term uptake experiment was performed 3 days after the plants were transferred from sand culture to water culture. The K+(86Rb+) and Ca2+(45Ca2+) uptake rates in the roots were determined in absence of external AlCl3.A high proportion of the supplied Al was taken up by the plants; 57% of total supply in the 0.1 mM Al treatment and 20% in the 1.0 mM Al treatment. Between 11% and 28% of the total Al taken up in the 0.1–10 mM Al treatments was exported to the shoot, indicating that Al not only was bound in the root free space, but also crossed the plasmalemma. Al decreased the Ca and Mg concentrations in the roots. The short term uptake experiment also showed that Ca2+(45Ca2+) uptake in the roots decreased with increasing external Al concentration. However, Ca and Mg concentrations were not lower in the shoots, except at the 10 mM Al treatment. This indicates that a higher percentage of the ions taken up was exported to the shoots in presence of Al. Concentrations of Mn and K in the roots were about the same in the Al treatments as for the control, except that the K concentration decreased at the 10 mM Al treatment. K levels in stems and needles increased with increasing Al treatment probably as a result of a decreased efflux from the roots combined with a higher export/uptake ratio. However, at the end of the cultivation period, K+(86Rb+) uptake, in the roots, decreased with increasing Al treatment, probably mainly due to a reduced ATP level. The shoot/root dry weight ratio was highest for the two highest Al treatments (1.0 and 10 mM) due to a reduced root growth.

Towards a genetic theory for the evolution of the sex ratio III. Parental and sibling control of brood investment ratio under partial sib-mating☆

Abstract Models of sex ratio evolution under partial sib-mating are investigated in haplodiploids and diploids. In the cases of parental and sibling control of the brood investment ratio between the sexes in diploids, we find that the “unbeatable” investment ratio obtained by W. D. Hamilton ( Science 156 , 477–488) for his local mate competition model corresponds in our inbreeding models to a weak form ESS (evolutionary stable strategy) fixation state and also to the population investment ratio at certain internal equilibria of our models. For haplodiploids, “strong form ESS” values exist under inbreeding in models involving father and sister control. Under brother and mother control, however, the ESS derived from local mate competition models is unstable in our inbreeding models to the introduction of any other investment ratio. We stress important qualitative differences between models involving local mate competition and inbreeding.

The balance between sexual and asexual reproduction in plants living in variable environments

The balance between sexual and asexual propagule production is studied in an evolutionary model where plants produce the two kinds of propagules in genetically determined proportions. The male function of plants producing asexual propagules can be varied, and the sexual and asexual propagules carry different probabilities to turn into new reproductive individuals. These fitnesses may vary over years. The evolution of the population’s reproductive system is studied assuming modifier alleles with small effects.

Towards a genetic theory for the evolution of the sex ratio II. Haplodiploid and diploid models with sibling and parental control of the brood sex ratio and brood size

A genetical model is formulated in which the sex ratio in broods and the relative size of broods are determined by the genotype at an autosomal locus. The results also apply to the case in which the sex-ratio locus is sex linked and expressed in the homogametic sex and to the case in which the locus is expressed in the diploid sex of a haplodiploid organism. Fisher (1930) argued that the sex ratio evolves under natural selection to a value such that parental expenditure is equalized between the sexes. Shaw and Mohler (1953) and MacArthur (1965) proposed that the sex ratio evolves to increase a certain expression for fitness. The sex ratio suggested by Fisher (1930) is in fact identical to the sex ratio specified by these maximization principles. Further, in our model, the Fisherian sex ratio corresponds exactly to the sex ratio at certain equilibria that are approached whenever they exist.

The introgression of a functional nuclear gene from Poa to Festuca ovina.

In sheeps fescue, Festuca ovina, genes coding for the cytosolic enzyme phosphoglucose isomerase, PGIC, are not only found at the standard locus, PgiC1, but also at a segregating second locus, PgiC2. We have used PCR-based sequencing to characterize the molecular structure and evolution of five PgiC1 and three PgiC2 alleles in F. ovina. The three PgiC2 alleles were complex in that they carried two gene copies: either two active genes or one active and one pseudogene. All the PgiC2 sequences were very similar to each other but highly diverged from the five PgiC1 sequences. We also sequenced PgiC genes from several other grass species. Phylogenetic analysis of these sequences indicates that PgiC2 has introgressed into F. ovina from the distant genus Poa. Such an introgression may, for example, follow from a non-standard fertilization with more than one pollen grain, or a direct horizontal gene transfer mediated by a plant virus.

Biased conversion as the primary function of recombination.

Recombination is hard to understand in darwinian terms when the process is identified with the production of crossover chromosomes. As an alternative explanation I propose instead that biased conversion is the primary function of meiotic recombination. In particular I show that a conversion process directed against the most common type of genetic damage can substantially reduce the mutational load, even if the conversion force is weak and if the conversion process occasionally creates new mutations.