R. van Treuren

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 significance of genetic erosion in the process of extinction

SummaryThe amount of genetic variation within a population is, among other things, related to population size. In small populations loss of genetic variation due to high levels of genetic drift and inbreeding may result in decline of individual fitness and increase the chance of population extinction. This chain of processes is known as genetic erosion. In this study we tested the genetic erosion hypothesis by investigating the relation between morphological variation and population size in two perennial, outbreeding plant species, Salvia pratensis and Scabiosa columbaria. To relate phenotypic variation to genetic variation the experiments were performed under common environmental conditions. For both species a positive correlation was observed between the amount of phenotypic variation and population size (Salvia r=0.915; Scabiosa r=0.703). Part of this variation is likely to have a genetic base, although maternal effects were present in the seedling and juvenile life stages. Differences between populations could in both species be attributed to parameters related to fitness, i.e. growth rate in Salvia and reproductive effort in Scabiosa. Discriminant functions reflecting these parameters did not however discriminate between large and small populations.Results are discussed in relation to the common environment approach and to electrophoretic results obtained earlier (Van Treuren et al. 1991).

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.

Relationships between plant density, outcrossing rates and seed set in natural and experimental populations of Scabiosa columbaria

Outcrossing rates were estimated in both natural and experimental populations of Scabiosa columbaria, a self‐compatible, entomophilous, gynodioecious, protandrous perennial. In natural populations, estimates of the outcrossing rate in hermaphrodites were near to one and ranged from 0.84 ± 0.07 to 1.12 ± 0.11. The effect of plant density on outcrossing rates was studied in two experimental populations of 27 individuals. Contrary to expectation the estimates of the outcrossing rate in hermaphrodites were about 100% for both densities. However, in the sparse population, the fraction of developed seeds of plants used to estimate outcrossing rates was significantly lower than of plants in the dense population (0.41 ± 0.06 and 0.68 ± 0.08, respectively). Artificial pollinations of these plants in the greenhouse showed that the fraction of developed seeds was 0.60 ± 0.01 and 0.83 ± 0.05 after self‐ and cross‐pollination, respectively. The combined results suggested that the differential success of self‐ and cross‐pollination might have caused equalization of the outcrossing rates in the experimental populations, despite different plant densities. The implications of the results for conservation biology are discussed.

Estimating null allele frequencies at a microsatellite locus in the oystercatcher (Haematopus ostralegus)

A significant heterozygote deficiency was found for microsatellite locus 20H7 among adult breeding birds in four populations of the oystercatcher (Haematopus ostralegus). Genotype frequencies at seven other loci were according to Hardy–Weinberg equilibria. Deviations between observed and expected genotype numbers decreased substantially when the data were corrected based on the estimated frequency of a putative null allele at locus 20H7. However, no null homozygotes were observed in the total sample of 378 individuals. The probability that, because of chance effects, null homozygotes were not represented in the sample (n=230) from the most intensively studied population (Schiermonnikoog) was estimated to be less than 1%. Parent–offspring comparisons from Schiermonnikoog showed that observed genotype numbers in the offspring were in accordance with expected values based on the estimated frequency of the putative null allele in the population. Moreover, a null homozygote was observed among the nestlings. The combined results indicated that a null allele is present at locus 20H7 in oystercatchers and that the inheritance is according to normal Mendelian segregation. If the absence of null homozygotes among adult animals cannot be ascribed to statistical effects, null homozygotes may suffer a selective disadvantage during the juvenile stage.

ON GENETIC EROSION AND POPULATION EXTINCTION IN PLANTS - A CASE-STUDY IN SCABIOSA-COLUMBARIA AND SALVIA-PRATENSIS

Due to human activities, populations of many species have become small and isolated. In this situation they become subject to genetic drift and inbreeding, resulting in loss of genetic variation, an increase in homozygosity, and possibly a decrease in viability (inbreeding depression). This process, here referred to as genetic erosion, may significantly increase the extinction probability of populations or even species, and is therefore currently an important issue in conservation biology. The research presented here aimed to determine the occurrence, extent, and significance of genetic erosion in natural populations of Scabiosa columbaria and Salvia pratensis, two species that are considered endangered in The Netherlands.

Family Distances and Human Lymphocyte Antigens

We compared family distances of homozygotes and heterozygotes for HLA-A and -B. When matched on number of inhabitants per birthplace, no significant differences were found. However, when homozygotes were compared with heterozygotes from larger birthplaces, homozygotes showed significantly smaller family distances in the grandparental generation. We suggest that matching for population size of birthplace and the choice of the geographic study area are important factors in studies of family distances.

Genetic and Phenotypic Variation in Relation to Population Size in Two Plant Species: Salvia pratensis and Scabiosa columbaria

Due to human activities many populations have become small, fragmented and isolated and consequently more sensitive to genetic drift and inbreeding, resulting in loss of genetic variation and fixation of deleterious alleles. This process often involves a decrease in viability (inbreeding depression) and therefore may significantly affect the probability of extinction of populations. To assess the relevance of these predictions, the amount of genetic variation was measured in relation to population size for two plant species, Salvia pratensis and Scabiosa columbaria. Both species are endangered in The Netherlands and show a considerable decline in number of populations during the last 30 years. For allozymes significant correlations were observed between population size and both the proportion of polymorphic loci and the mean observed number of alleles, the large populations being more variable than the small populations. In addition, substantial genetic differentiation was observed between populations, the differentiation being more extensive among small populations than among large populations. Concerning morphological characters also a positive relationship between the total amount of phenotypic variation and population size was observed. The results suggest that, predominantly due to genetic drift, small populations are depauperate in genetic variation. Although small populations, at least for Salvia, showed some indications of decreased viability, much more research is needed to decide whether these low levels of genetic variation influence the probability of extinction significantly.