Martin Ziehe

Reproductive systems in conifer seed orchards

SummaryIn order to quantify female and male fitness values of clones in a Pinus sylvestris L. seed orchard, multilocus-genotypes of parental clones were compared with those of open pollinated seeds in the bulked orchard crop. Female and male contributions to individual seeds were distinguished by observing enzyme gene loci active in both endosperm and embryo tissue. Seed probes from two successive flowering periods were surveyed. The female and male fitnesses of five parental clones measured relative to the population mean were derived. The contributions of four clones were found to be sexually asymmetric. One clone, for instance, made exclusively female contributions in one flowering period. Variations existed in fitness values between clones. Deviations in sex specificity occurred between flowering periods: one clone contributed asymmetrically in both periods, but in sexually reversed proportions. A method to comprehensively quantify and illustrate the observed phenomena is proposed.

The measurement of Darwinian fitness in human populations

Darwinian fitness of a biological trait refers to the contribution to successive generations made by individuals possessing the trait. This contribution depends on the age-specific fecundity and mortality of the individuals in the population. This paper gives, for human populations, an empirical study of two measures of Darwinian fitness: the Malthusian parameter, which describes the rate of increase of the actual population size, and entropy which describes the rate of increase of the effective population size. This empirical study indicates that Darwinian fitness is measured by entropy. This finding is in accord with the predictions of recent theoretical studies.

Directionality theory: an empirical study of an entropic principle in life‐history evolution

Understanding the relationship between ecological constraints and life-history properties constitutes a central problem in evolutionary ecology. Directionality theory, a model of the evolutionary process based on demographic entropy, a measure of the uncertainty in the age of the mother of a randomly chosen newborn, provides an analytical framework for addressing this problem. The theory predicts that in populations that spend the greater part of their evolutionary history in the stationary growth phase (equilibrium species), entropy will increase. Equilibrium species will be characterized by high iteroparity and strong demographic stability. In populations that spend the greater part of their evolutionary history in the exponential growth phase (opportunistic species), entropy will decrease when population size is large, and will undergo random variation when population size is small. Opportunistic species will be characterized by weak iteroparity and weak demographic stability when population size is large, and random variations in these attributes when population size is small. This paper assesses the validity of these predictions by employing a demographic dataset of 66 species of perennial plants. This empirical analysis is consistent with directionality theory and provides support for its significance as an explanatory and predictive model of life-history evolution.

Genetic structures as indicators for adaptation and adaptational potentials

Forests have become increasingly affected by human influence and forest genetics has gained substantial importance in view of the recently detected genetic implications of environmental changes. Conclusions drawn from genetic structures and their dynamics can help to evaluate the genetic processes and their consequences.

Gene resources and gene conservation in forest trees: General concepts

It is argued that the declaration of a collection of biological material as a gene resource must include the following basic specifications: (1) a genetic characterization of the collection, (2) the statement of the objectives to be pursued with the collection, (3) the methods and techniques of conservation, and (4) the methods and techniques for the utilization (regeneration) of the resource. If any of these specifications cannot be made or is incompatible with any other, the collection is either not suitable for declaration as a gene resource, or the specifications need correction. Various standard methods and techniques of conservation and regeneration of gene resources are discussed in relation to the objectives of conservation, and elementary consequences for declaration are drawn. The need for research centered on the field of gene conservation is emphasized, and the significance of legislation in this concern is addressed.

Selection caused by self‐fertilization

The generally held view that increased self‐fertilization should be advantageous in the absence of counteracting selective forces reducing viability or fertility is reexamined. It is pointed out that the models on which this view is based all imply a gain in male (pollen) fertility with increased selfing. Hence, the postulated advantage may equally well be due to increased fertility, a fact which reopens the discussion on the selective significance of differential selfing.

Selection caused by self-fertilization I. Four measures of self-fertilization and their effects on fitness

Abstract The standard models of selfing in seed plants consider only the ovules, which are assumed to have a constant selfing rate. It has recently become clear, however, that hermaphrodite or monoecious populations frequently show sexual asymmetry (nonconstant pollen:ovule fertilities among individuals). Such asymmetry usually results in pollen selfing rates which differ from those for the ovules and are frequency-dependent even for constant ovule selfing rates. A recent study of selfing rates for all gametes of an individual is extended here to include four selfing rates (for ovules, pollen, all gametes, and zygotes), and simple mathematical relationships linking the four rates are obtained. Unlike earlier models of selfing, it is not assumed that the ovule selfing rate is constant, but instead that this rate, like all the others, is determined by the mobility of the pollen, which in turn is determined by the floral biology and ecology. It is found that all four selfing rates are usually frequency-dependent. The selfing rate for all gametes (the combined selfing rate) is usually intermediate between those for the ovules and pollen, and the zygotic rate is usually the smallest of the four. The exceptions to the above statements occur for relatively extreme situations, such as complete selfing for pollen or ovules, no selfing, or sexual symmetry. Three modes of selfing are considered: prior (PS), competing (CS), and delayed (DS) self-fertilization. It is shown that if there are at least two types with different selfing rates in the population, then the ranking of their selfing rates may depend upon the frequencies of the types (for the combined and the zygotic rates), may be frequency-independent (ovule rate), or may be dependent or independent, according to the mode of selfing (pollen rate). The effects of the various influences on the amount of selfing are by no means negligible. Thus a numerical study shows pollen selfing rates for one type which vary from 0.09 to 0.96, according to its frequency. Another numerical result shows a change in combined selfing rate from 0.13 to 0.86, depending solely on the mode of selfing. Results for Scots Pine show that an ovule selfing rate of 0.5 was accompanied by a combined rate of 0.143. The population selfing rate is not the same as the mean of individual selfing rates, and can only be obtained if female fitnesses as well as ovule selfing rates are known for each type. Previous models of selfing have failed to distinguish between the effects of increased selfing and increased pollen fertility, with the result that increased selfing always resulted in greater fitness. In the present models the two effects are distinguishable, and it is found that increased selfing may result in increased or decreased fitness, depending also on population density and on a form of pollen density. Thus the old dogma that in the absence of viability and fertility selection increased selfing always results in increased fitness is finally refuted, and the importance of the influence of ecological parameters on selfing and fitness is emphasized, since population density and pollen density influence the selfing rates.

Reproductive systems in conifer seed orchards : 2. Reproductive selection monitored at an LAP gene locus in Pinus sylvestris L.

SummaryIn a Scots pine seed orchard the genetic structures at an enzyme gene locus active in pine seeds were compared among the parental clones and the allorchard seeds produced over a period of three years. The genotypes of the seeds were identified as ordered pairs consisting of the female and male contribution. The sexual reproductive function monitored at the studied locus differed significantly between the two sexes. This fact proves the necessity of taking sexual asymmetry into account in studies of reproductive selection. This is done by comparing the observed genotypic structures among the offspring with the corresponding multiplicative structure expected under random gametic fusion. Additionally, accounting for partial self-fertilization increased the conformity between the observed and the hypothesized model structures. The differences in female and male gametic contributions to the offspring were used to estimate female and male relative fitness components. Significant deviations between the allelic and/or genotypic structures of orchard clones and their seed, and between seed lots collected in different years, may reduce the efficiency of realizing breeding gains in seed orchards.

Zygotic genotypic frequencies under selection on female or male gamete production in partially self-fertilizing plant populations

SummaryIn order to improve our understanding of the combined influence of mating system components on genotypic structures in zygotes, it may be better to consider the selective effects in each of the two sex functions separately. In this paper, two diallelic selection models for plant populations with mixed self-fertilization and random gametic fusion are considered: (A) Selection on the production of free pollen (i.e., pollen not reserved for self-fertilization and thus subject to competition), and (B) selection on the production of ovules (all of which are fertilized). It is well known that sexually asymmetrical fertility selection produces a heterozygotic excess, whereas self-fertilization generally leads to a homozygotic excess relative to Hardy–Weinberg proportions. The combined influence of the two factors is investigated for the diallelic case with special emphasis on three topics: (i) Estimation of limits for the amount of genotypic equilibrium deviations from Hardy–Weinberg proportions, (ii) determination and illustration of the location of genotypic equilibria, and (iii) conditions for protectedness of polymorphisms and alleles. With respect to these aspects, the models A and B are compared with the classical symmetrical fertility model.The results and figures serve to interpret the combined influence of fertility selection and partial self-fertilization on zygotic genotypic structures. In addition, the frequently used argument that in predominantly self-fertilizing organisms only strong overdominance is capable of constituting a polymorphism turns out to require modification.

Target-diameter felling and consequences for genetic structures in a beech stand (Fagus Sylvatica L.)

At least in major parts of Germany, target-diameter felling has become an important measure of harvesting in naturally regenerating beech stands at ages of 120 years or older. Yet less is known about short and long-term consequences for the genetic variation in the subsequent generations.