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R F Hoekstra

Publications and source records attributed to R F Hoekstra.

31 records · Page 2Linked to original sources
PubMed1994

Population genetics of filamentous fungi.

Population genetics aims to understand causes and consequences of the genetic structure of populations, i.e. distributions of genetic variants in space and time. Among the most important determinants of the genetic population structure is the genetic system itself, which is the collection of processes and mechanisms responsible for the transmission of genetic information. Filamentous fungi offer excellent opportunities for studying the effects of the genetic system on genetic population structure. Apart from their advantage as laboratory organisms, they exhibit a wide variety of genetic systems. In particular, their inherent capacity for anastomosis provides unique possibilities for investigating rates and consequences of horizontal gene transfer. Furthermore, the temporary confinement of the products of meiosis in a common structure (the ascus) enables the study of competitive and antagonistic interactions between the meiotic products. An intriguing example of the latter is the phenomenon of 'spore killing', resulting in distorted meiotic segregation. This paper concentrates on population level research of the occurrence of vegetative incompatibility in Aspergillus and Neurospora species and to what extent this will inhibit horizontal transmission of genetic information, and on spore killing in Podospora anserina.

Biological Evolution↗
PubMed1993

Genetic variability and relatedness in the complex group of black Aspergilli based on random amplification of polymorphic DNA.

A PCR-based technique, involving the random amplification of polymorphic DNA (RAPD), was used for assessing genomic variability among a wide range of culture collection strains of black Aspergilli and related species. The performance of this technique is compared with that of the two other genetic techniques most commonly used, namely restriction fragment length polymorphisms on rDNA and isozyme analysis. The eight main groups as assigned by RFLP were also distinguished by RAPD patterns. On the basis of 122 polymorphic RAPD products using six random primers, the 17 collection strains examined could be subdivided into 15 distinct sub-groups. We suggest that the RAPD method is a quick and reliable tool for establishing the amount of genetic variability in closely related species. Our study indicates that the complex group of black Aspergilli is characterized by a high degree of genetic differentiation. This is also apparent from the considerable karyotype variation present in the group.

Aspergillus↗
PubMed1993

Genetic maps of eight linkage groups of Aspergillus niger based on mitotic mapping.

This paper provides a genetic map of Aspergillus niger. At present 84 markers have been assigned to eight linkage groups. The chromosomal location of 60 markers is presented in this paper. The allocation of markers is based on recombination due to mitotic crossing over. Various methods for selection and analysis of homozygous recombinants were applied, using colour, auxotrophic and resistance markers. In addition, transformants carrying the heterologous Aspergillus nidulans gene coding for acetamidase (amdS) were used for mitotic mapping of markers in several linkage groups. In most of the transformants the amdS insert appeared to be centromere-distal to all known genetic markers, thus extending the genetic map. The linear order of the markers in the eight linkage groups has been determined. On the basis of these and earlier experiments tentative genetic maps for the eight linkage groups are presented. Genetic markers were found on both arms of the chromosomes, except for chromosomes II and IV. The genetic distance between markers and the centromere varies from about 10(-4) (LG I, II, V) up to more than 10(-2) (LG III, VI, VIII). The total frequency of mitotic recombination per genome in this fungus has been estimated to be at least 1.2 x 10(-1).

Amidohydrolases↗
PubMed1993

Evolutionary dynamics of spore killers.

Spore killing in ascomycetes is a special form of segregation distortion. When a strain with the Killer genotype is crossed to a Sensitive type, spore killing is expressed by asci with only half the number of ascospores as usual, all surviving ascospores being of the Killer type. Using population genetic modeling, this paper explores conditions for invasion of Spore killers and for polymorphism of Killers, Sensitives and Resistants (which neither kill, nor get killed), as found in natural populations. The models show that a population with only Killers and Sensitives can never be stable. The invasion of Killers and stable polymorphism only occur if Killers have some additional advantage during the process of spore killing. This may be due to the effects of local sib competition or some kind of "heterozygous" advantage in the stage of ascospore formation or in the short diploid stage of the life cycle. This form of segregation distortion appears to be essentially different from other, well-investigated forms, and more field data are needed for a better understanding of spore killing.

Biological Evolution↗
PubMed1992

On the developmental theory of ageing. II. The effect of developmental temperature on longevity in relation to adult body size in D. melanogaster.

Flies from a wild type strain of Drosophila melanogaster, previously kept at 25 degrees C, were reared at either 20, 25 or 29 degrees C. As expected, developmental time and adult body size decreased with increasing temperature. Adult longevity of flies reared at 25 degrees C was slightly greater than that of flies raised at 20 or 29 degrees C when measured at all three temperatures. This may reflect the laboratory history of the strain. On the whole, it appeared that longevity was independent of adult body size. These results support our previous conclusion (Zwaan et al., 1991) that developmental time and body size are not causally related to longevity in 'environmental' studies. It is stressed, that genetic analysis is needed to investigate the reputed correlation between development and ageing.

Aging↗
PubMed1992

Evolution of reproductive systems in filamentous ascomycetes. II. Evolution of hermaphroditism and other reproductive strategies.

The evolution of different reproductive systems in filamentous ascomycetes is studied in a population genetic model. These fungi differ essentially from higher plants and animals because mating types can exist in addition to male and female gametes, and the conidia serve as both male gametes and asexual spores; moreover, selfing is genetically equivalent to asexual reproduction in these haploid organisms. A variable fitness of ascospore production is predicted as the explanation for the evolution of two systems that abundantly exist in nature: hermaphroditism in heterothallic species and the formation of both asexual and sexual spores in homothallic species. Imperfect fungi will evolve if sexual spores do not show a remarkably higher fitness than asexual spores.

Ascomycota↗
PubMed1991

On the developmental theory of ageing. I. starvation resistance and longevity in Drosophila melanogaster in relation to pre-adult breeding conditions.

The developmental theory of ageing predicts a positive correlation between developmental time and adult longevity. Experiments that vary larval density and food level have been carried out to test this prediction. The results show differences in viability, developmental time, starvation resistance and adult longevity. It is concluded that pre-adult developmental time is not a causal factor for the determination of adult longevity in Drosophila melanogaster. The observed variation in adult longevity is discussed in relation to viability selection and changed adult physiology.

Aging↗
PubMed1987

The evolution of sexes.

It is very likely that sexual differentiation into two morphologically indistinguishable mating types has preceded the evolution of anisogamy. Therefore, the study of the evolution of mating types in an isogamous population is more informative for understanding the forces responsible for the evolution of different sexes than the study of the evolution of anisogamy; the latter represents the secondary problem of how, after the establishment of two sexes, an increasing degree of gamete dimorphism may evolve. Mating type evolution has been analyzed theoretically in population genetic models. These explorations show that mating types may evolve as a consequence of selection for more efficient gamete recognition, and also as a result of intragenomic conflict between nuclear and cytoplasmic DNA. However, in both cases the selection forces have to be very strong, which makes these possible explanations less convincing. Nearly all theories proposed for the evolution of anisogamy assume two conflicting selection forces to be relevant: selection for greater gamete productivity, and selection for greater zygote size. Although the explanation is intuitively plausible, the comparative evidence is a bit disappointing. Alternatively, anisogamy can be explained as a side-effect of selection for a greater efficiency in finding a mating partner by using sexual pheromones. Firm empirical evidence is lacking, however. In both problem areas--mating type evolution and anisogamy evolution--experimental work is badly needed.

Animals↗
PubMed1986

The cost of sex in hermaphrodite populations with variation in functional sex.

In this paper an analysis is made of a model of selection for asexual reproduction in hermaphrodite (or monoecious) populations in which variation occurs in relative female and male fertilities. It is shown that the advantage of an asexual mutant (the cost of sex) increases with increasing degree of differentiation in functional sex. This effect is very marked at low levels of selfing, but weak with a high selfing rate. In general, the advantage of an asexual mutant in a hermaphrodite population depends on the relative resource allocation to male and female gametes, and increases with increasing bias to femaleness. Thus the cost of sex in gynodioecious populations is (with a low level of selfing) as high as in a dioecious population. This applies, however, to a nuclear genetic determination of gynodioecy, which is presumably rare. In a more realistic model assuming nuclear-cytoplasmic determination of gynodioecy the cost of sex is considerably lower.

Animals↗