W. Van Delden

The significance of genetic erosion in the process of extinction. I : Genetic differentiation in Salvia pratensis and Scabiosa columbaria in relation to population size

As part of a programme to determine the importance of the loss of genetic variation for the probability of population extinction, the amount of allozyme variation was determined in 14 populations of Salvia pratensis and in 12 populations of Scabiosa columbaria. Significant correlations were found between population size and the proportion of polymorphic loci (Salvia: r=0.619; Scabiosa: r=0.713) and between population size and mean observed number of alleles per locus (Salvia: r=0.540; Scabiosa: r=0.819). Genetic differentiation was substantially larger among small populations than among large populations: in Salvia GST was 0.181 and 0.115, respectively, and in Scabiosa 0.236 and 0.101, respectively. The results are discussed in relation to genetic drift, inbreeding and restricted gene flow.

THE EFFECTS OF POPULATION-SIZE AND PLANT-DENSITY ON OUTCROSSING RATES IN LOCALLY ENDANGERED SALVIA-PRATENSIS

Multilocus outcrossing rates were estimated in natural and experimental populations of Salvia pratensis, an entomophilous, gynodioecious, protandrous perennial. Male steriles were used to check the estimation procedure of outcrossing rates in hermaphrodites. Estimates of outcrossing rates in hermaphroditic plants ranged from 38.2% to 81.8% in natural populations and from 71.5% to 95.5% in experimental populations. No correlations were found between outcrossing rates and population size. However, outcrossing in hermaphrodites was promoted by high plant densities and low frequencies of male steriles. It is argued that effective management to preserve genetic variation in populations of S. pratensis should provide for the maintenance of high plant densities.

The significance of genetic erosion in the process of extinction. IV. Inbreeding depression and heterosis effects caused by selfing and outcrossing in Scabiosa columbaria

The effects of self‐fertilization, within‐population crosses (WPC) and between‐population crosses (BPC) on progeny fitness were investigated in the greenhouse for Scabiosa columbaria populations of varying size. Plants grown from field collected seeds were hand pollinated to produce selfed, WPC, and BPC progeny. The performance of these progenies was examined throughout the entire life cycle. The different pollination treatments did not significantly affect germination, seedling‐to‐adult survival, flowering percentage and the number of flower heads. But severe inbreeding depression was demonstrated for biomass production, root development, adult survival, and seed set. Additionally, multiplicative fitness functions were calculated to compare relative fitnesses for progeny. On average, WPC progeny showed a more than 4‐fold, and BPC progeny an almost 10‐fold, advantage over selfed progeny, indicating that S. columbaria is highly susceptible to inbreeding. No clear relationship was found between population size and level of inbreeding depression, suggesting that the genetic load has not yet been reduced substantially in the small populations. A significant positive correlation was found between plant dry weight and total fitness. In two out of six populations, the differences between the effects of the pollination treatments on dry weight increased significantly when seedlings were grown under competitive conditions. This result is interpreted as an enhancement of inbreeding depression under these conditions. It is argued that improvement of the genetic exchange between populations may lower the probability of population extinction.

The Alcohol Dehydrogenase Polymorphism in Drosophila melanogaster

In the late sixties the neo-Darwinian theory of evolution, hitherto generally accepted by biologists, was confronted with a new, revolutionary view: the theory of neutral or non-Darwinian evolution. In the neutralist view amino acid and nucleotide changes in the course of evolution are mainly due to random fixation of selectively neutral mutants (Kimura, 1968; King and Jukes, 1969). The approximately constant rate of evolution in terms of amino acid substitutions per site per year for various lineages, as claimed by neutralists, forms one of the main arguments for the neutralist theory. Enzyme polymorphisms in present-day populations are considered as a phase in molecular evolution (Kimura and Ohta, 1971; Kimura, 1977). The first estimates of the extent of enzyme polymorphisms in populations of Drosophila pseudoobscura (Lewontin and Hubby, 1966) and humans (Harris, 1966) based on electrophoresis of proteins were soon followed by many others, covering a wide range of animal and plant species. These surveys showed that most species are highly polymorphic [see reviews by Powell (1975), Nevo (1978), A. H. D. Brown (1979), and Hamrick et al. (1979)]. In the neutralist versus selectionist controversy the nature of these allozyme polymorphisms is disputed. Thw neutralist hypothesis states that the observed variation is mainly a product of mutation and drift of selectively neutral genes. The selectionist hypothesis claims that some from of balancing selection is resposible for the maintenance of allozyme polymorphisms.

GYNODIOECY IN PLANTAGO-LANCEOLATA L .1. POLYMORPHISM FOR PLASMON TYPE

SummaryStudies of the morphological polymorphism for the expression of male sterility in the gynodioecious species Plantago lanceolata revealed two separate series of stamen forms from sterile to fertile. The first type of complete male sterility (MS1) shows a disturbed development of the stamens, which are strongly reduced in size. The second type (MS2) has stamens which are petaloid. In the latter type the corolla and sometimes the pistil are also affected. Evidence is presented that these differences in expression of male sterility are cytoplasmically determined. The plasmon types are designated R and P respectively. In each plasmon type a series of intermediate sex forms occurs. Field counts showed that these intermediates are a non-negligible proportion of most populations. On the average MS1 reaches higher frequencies than MS2. Twenty two out of 27 populations appeared to be polymorphic for plasmon type. The remaining five populations are probably fixed for plasmon P and their distribution over the habitats studied suggests that the fitnesses of the different sex genotypes depend on environmental conditions.SummaryStudies of the morphological polymorphism for the expression of male sterility in the gynodioecious species Plantago lanceolata revealed two separate series of stamen forms from sterile to fertile. The first type of complete male sterility (MS1) shows a disturbed development of the stamens, which are strongly reduced in size. The second type (MS2) has stamens which are petaloid. In the latter type the corolla and sometimes the pistil are also affected. Evidence is presented that these differences in expression of male sterility are cytoplasmically determined. The plasmon types are designated R and P respectively. In each plasmon type a series of intermediate sex forms occurs. Field counts showed that these intermediates are a non-negligible proportion of most populations. On the average MS1 reaches higher frequencies than MS2. Twenty two out of 27 populations appeared to be polymorphic for plasmon type. The remaining five populations are probably fixed for plasmon P and their distribution over the habitats studied suggests that the fitnesses of the different sex genotypes depend on environmental conditions.

Spatially structured genetic variation in a broadcast spawning bivalve: quantitative vs. molecular traits

Abstract Understanding the origin, maintenance and significance of phenotypic variation is one of the central issues in evolutionary biology. An ongoing discussion focuses on the relative roles of isolation and selection as being at the heart of genetically based spatial variation. We address this issue in a representative of a taxon group in which isolation is unlikely: a marine broadcast spawning invertebrate. During the free‐swimming larval phase, dispersal is potentially very large. For such taxa, small‐scale population genetic structuring in neutral molecular markers tends to be limited, conform expectations. Small‐scale differentiation of selective traits is expected to be hindered by the putatively high gene flow. We determined the geographical distribution of molecular markers and of variation in a shell shape measure, globosity, for the bivalve Macoma balthica (L.) in the western Dutch Wadden Sea and adjacent North Sea in three subsequent years, and found that shells of this clam are more globose in the Wadden Sea. By rearing clams in a common garden in the laboratory starting from the gamete phase, we show that the ecotypes are genetically different; heritability is estimated at 23%. The proportion of total genetic variation that is between sites is much larger for the morphological additive genetic variation (QST = 0.416) than for allozyme (FST = 0.000–0.022) and mitochondrial DNA cytochrome‐c‐oxidase‐1 sequence variation (ΦST = 0.017). Divergent selection must be involved and intraspecific spatial genetic differentiation in marine broadcast spawners is apparently not constrained by low levels of isolation.

Microsatellite analysis of population structure and genetic differentiation within and between populations of the root vole, Microtus oeconomus in the Netherlands

Eight microsatellite markers for the root vole (Microtus oeconomus) were developed to assess the amount of genetic variation for nine Dutch root vole populations from four different regions, and to evaluate the degree of differentiation and isolation. All eight microsatellite loci were found to be highly variable with observed heterozygosity values ranging from 0.61 to 0.82. These values are similar to those observed for more distant populations from Norway, Finland and Germany. Therefore, the populations seem not particularly depauperate of genetic variation at the microsatellite level. Genetically, the Dutch populations were found to have diverged considerably. Pairwise comparisons of all populations studied revealed FST values significantly greater than zero for most comparisons. However, the magnitude of these values considerably depends on the compared population pair. The level of differentiation between local populations within Dutch regions is generally significantly lower than the differentiation between Dutch regions. The level of differentiation between Dutch regions, however, is not significantly different from that between populations of larger geographical distance. This implies that the regional Dutch populations are both isolated from each other and from other European populations. The observation that even local populations show low but significant genetic differentiation may be indicative for progressive isolation of these populations.

POLYMORPHISM AT THE ADH AND ALPHA-GPDH LOCI IN DROSOPHILA-MELANOGASTER - EFFECTS OF REARING TEMPERATURE ON DEVELOPMENTAL RATE, BODY-WEIGHT, AND SOME BIOCHEMICAL PARAMETERS

The role of developmental time in the world-wide cline of Adh and αGpdh allele frequencies of Drosophila melanogaster, and the relationship with weight and some biochemical characters, were investigated. Experimental strains were constructed with different combinations of Adh and αGpdh alleles but with similar genetic background. Developmental time, adult weight, protein-and triglyceride-content, and ADH and αGPDH enzyme activity were measured at a rearing temperature of 20, 25 and 29°C. Genotype effects were found in all studied characters. In general the developmental times of genotypes were: AdhFF < AdhFS < Adhss and αGpdhFF> αGpdhFS = αGpdhSS. Developmental time and adult weight were strongly affected by rearing temperature. Triglyceride content and ADH and αGPDH enzyme activity were slightly affected by temperature. Interactions between genotype and temperature effects were found for developmental rate, adult weight and protein content. No trade off was observed between developmental time on the one hand and adult weight, protein- and triglyceride-content, and ADH and αGPDH enzyme activity on the other hand. It is argued that developmental rate differences might be one of the underlying mechanisms of the world-wide cline of the Adh and αGPdh allele frequencies.

A reexamination of the negative assortative mating phenomenon and its underlying mechanism inDrosophila melanogaster

Averhoff and Richardson [(1974)Behav. Genet.4:207–225] reported a trend toward negative assortative mating inDrosophila melanogaster during the course of inbreeding. These authors proposed that the underlying mechanism was based on pheromone polymorphism and male selection. Mass mating experiments were carried out to verify their hypothesis, detailed behavior observations were made to identify the underlying mechanism, and sex pheromone composition and variation were examined by using gas chromatography-mass spectrometry analysis. The results showed that negative assortative mating is not a general phenomenon. Although male pheromones are probably polymorphic, female pheromones are not. We found no evidence for male selection as predicted by Averhoff and Richardson. It is argued that the most parsimonious mechanism underlying negative assortative mating is similar to one proposed by Bryant [(1979)Behav. Genet.9:249–256], which was based on interstrain differences in female reluctancy and male vigor.

Quantification of locomotion and the effect of food deprivation on locomotor activity in Drosophila

A new method to quantify locomotor behavior in Drosophila is presented, and compared with previous methods. It is based upon a radar wave, reflected by moving flies. A problem associated with the new apparatus is that its output is dependent on fly size. However, for the case the weight of the experimental flies has been determined, a correction is proposed. The method has been used by studying the effect of starvation upon locomotion in Drosophila melanogaster. It was found that starved flies are much more active than well fed flies. The importance of this effect under several conditions is discussed.