Elsevier

Aquatic Toxicology

Volume 85, Issue 4, 30 December 2007, Pages 278-284
Aquatic Toxicology

Consequences of inbreeding and reduced genetic variation on tolerance to cadmium stress in the midge Chironomus riparius

https://doi.org/10.1016/j.aquatox.2007.04.015Get rights and content

Abstract

Inbreeding and loss of genetic variation are considered to be major threats to small and endangered populations. The reduction of fitness due to inbreeding is believed to be more severe under stressful environmental conditions. We generated nine strains of the ecotoxicological model organism Chironomus riparius of different inbreeding levels in order to test the hypothesis that the inbreeding level and thus the degree of genome-wide homozygosity influences the life-history under cadmium exposure. Therefore, midge populations were exposed to a gradient of sediment-bound cadmium. The level of genetic variation in the used strains was assessed using microsatellite markers.

In the life-cycle tests, inbreeding reduced fitness within C. riparius populations both under control and stressed conditions. However, differences between genetically diverse and impoverished strains were greatest at high cadmium exposure. Overall, inbreeding effects were not only dependent on cadmium concentrations in the sediment, but also on the life-history trait investigated. While some parameters where only affected by inbreeding, others were altered by both, inbreeding and cadmium. For the larval developmental time, a significant interaction was found between inbreeding and cadmium stress. While all strains showed a similar developmental time under control conditions, high rates of inbreeding led to a significantly delayed emergence time under high cadmium concentrations, resulting in longer generation periods and reduced population growth rates as population-relevant effects.

The results show, that bioassays with C. riparius are affected by the level of inbreeding within Chironomus test strains. Pollution stress is therefore likely to affect the survival of rare and endangered populations more severe than that of large and genetically diverse ones.

Introduction

When populations decrease in size, genetic drift can lead to reduced levels of genetic variation. This loss of variation has been shown to enhance extinction risk of populations in the long term due to loss of evolutionary potential which enables populations to adapt to changing environmental conditions (Frankham et al., 2004). Additionally, small populations may suffer from inbreeding due to increased levels of homozygosity of deleterious alleles, which frequently results in the loss of fitness (Charlesworth and Charlesworth, 1987). This process, termed inbreeding depression, has been documented to affect both natural and caged organisms subjected to severe population bottlenecks (Armbruster et al., 2000). In summary, loss of genetic variation and increased genome-wide homozygosity are likely to accelerate extinction processes in plants and animals (Charlesworth and Charlesworth, 1987, Saccheri et al., 1998, Reed et al., 2003).

Furthermore, empirical studies showed that inbreeding depression can be reinforced under stressful environmental conditions, like chemical exposure (summarized in Armbruster and Reed, 2005). For instance, Kristensen et al. (2003) showed that only under environmental stress highly inbred Drosophila buzzatii strains produce significantly less progeny than outbred individuals. These findings might have severe implications for both conservation strategies in polluted environments as well as for toxicological exposure assays, for populations of endangered species as well as caged laboratory test strains show reduced levels of genetic variability (Norris et al., 2001, Joron and Brakefield, 2003, Frankham et al., 2004). Furthermore, inbreeding is unavoidable in small populations (Bijlsma et al., 2000). Therefore, it is of high ecotoxicological importance to investigate interactions between inbreeding and chemical stress. These studies will allow us to assess the consequences of environmental stress on the fitness of natural populations.

To test if inbreeding and the level of genetic variation influence tolerance to chemical exposure, we generated nine laboratory strains of the non-biting midge Chironomus riparius (Diptera: Chironomidae) which differed in their level of inbreeding (inbreeding coefficient F = 0, 0.125 and 0.375). The level of genetic variation within the strains was verified using microsatellite analysis (Nowak et al., 2006). All strains were exposed to a gradient of sediment-associated Cd and various life-history traits, including reproduction parameters, were recorded.

The main questions of this study were:

  • (i)

    Does inbreeding affect life-history traits of C. riparius strains in the laboratory?

  • (ii)

    Does inbreeding and reduced genetic variation affect susceptibility of C. riparius to Cd stress?

  • (iii)

    Are ‘validity criteria’, like thresholds for control mortality, sufficient to prevent test results from bias due to inbreeding effects?

The results of this study are discussed with special emphasis for their consequences on both ecotoxicological test bioassays and species conservation strategies.

Section snippets

Inbreeding procedure and culture conditions

We established nine independent C. riparius strains from a laboratory culture which has been reared in our laboratory for 3 years. In order to obtain a high level of genetic variation, this culture had been established from a crossbred of 11 laboratory strains from seven different countries (Nowak et al., 2007). The nine strains were generated using different numbers of egg ropes for the founder generation. Three independent highly inbred strains (high In) were established performing two

Residual analysis of sediment and water

At the end of the life-cycle experiments, mean percentages of measured Cd concentrations in the sediment ranged from 60% (1.2 mg Cd/kg dw) to 74% (0.9 mg Cd/kg dw) of nominals. Cd concentrations in the water samples varied between 3.87 μg/L (0.6 mg Cd/kg dw) and 9.55 μg/L (0.3 mg Cd/kg dw, Table 1).

Genetic analyses

Measuring of allelic variation at five microsatellite loci led to observed heterozygosity levels ranging from 0.17 (high In III) to 0.41 (Out III). Generally, both observed and expected heterozygosity, the

Discussion

To our knowledge, we present the first study which investigates the combined effects of inbreeding, reduced heterozygosity and chemical stress with an ecotoxicological model species under standardized conditions. Recovery rates of Cd in the experiments were similar among replicates and treatments. Thus, observed variation in life-history response between C. riparius strains cannot be explained by variations in sediment contamination between the tests. Because earliest larval stages have been

Acknowledgements

We thank Agnes Sieratowicz and Simone Galluba for their kind help in test accomplishment and Maren Heß for expert technical assistance. Markus Pfenninger gave valuable hints concerning the experimental design. This project was financially supported by the ‘Baden-Württemberg Programm Lebensgrundlage Umwelt und ihre Sicherung’ (BWPLUS, BWR 22018).

References (28)

  • J.J. O’Grady et al.

    Realistic levels of inbreeding depression strongly affect extinction risk in wild populations

    Biol. Conserv.

    (2006)
  • C. Vogt et al.

    Multi-generation studies with Chironomus riparius – effects of low tributyltin concentrations on life history parameters and genetic diversity

    Chemosphere

    (2007)
  • P. Armbruster et al.

    Equivalent inbreeding depression under laboratory and field conditions in a tree-hole-breeding mosquito

    Proc. R. Soc. Lond. B Biol.

    (2000)
  • P. Armbruster et al.

    Inbreeding depression in benign and stressful environments

    Heredity

    (2005)
  • P. Armitage et al.

    The Chironomidae, the Biology and Ecology of Non-biting Midges

    (1995)
  • R. Bijlsma et al.

    Does inbreeding affect the extinction risk of small populations? Predictions from Drosophila

    J. Evol. Biol.

    (2000)
  • B.W. Brook et al.

    Contribution of inbreeding to extinction risk in threatened species

    Conserv. Ecol.

    (2002)
  • D. Charlesworth et al.

    Inbreeding depression and its evolutionary consequences

    Annu. Rev. Ecol. Syst.

    (1987)
  • P.O. Cheptou et al.

    Effects of competition on lifetime-estimates of inbreeding depression in the outcrossing plant Crepis sancta (Asteraceae)

    J. Evol. Biol.

    (2000)
  • M. Dudash

    Relative fitness of selved and outcrossed progeny in a self-compatible, protandrous species, Sabatia angularis L. (Gentianaceae): a comparison in three environments

    Evolution

    (1990)
  • V.E. Forbes et al.

    Effects of the pyrethroid esfenvalerate on life-cycle traits and population dynamics of Chironomus riparius – importance of exposure scenario

    Environ. Toxicol. Chem.

    (2005)
  • R. Frankham et al.

    A Primer of Conservation Genetics

    (2004)
  • M. Joron et al.

    Captivity masks inbreeding effects on male mating success in butterflies

    Nature

    (2003)
  • T.N. Kristensen et al.

    Effects of inbreeding and environmental stress on fitness – using Drosophila buzzatii as a model organism

    Conserv. Genet.

    (2003)
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