Trait specific consequences of fast and slow inbreeding: lessons from captive populations of Drosophila melanogaster

Abstract

The increased homozygosity due to inbreeding leads to expression of deleterious recessive alleles, which may cause inbreeding depression in small populations. The severity of inbreeding depression has been suggested to depend on the rate of inbreeding, with slower inbreeding being more effective in purging deleterious alleles of smaller effect. The effectiveness of purging is however dependent on various factors such as the effect of the deleterious, recessive alleles, the genetic background of inbreeding depression and the environment in which purging occurs. Investigations have shown inconclusive results as to whether purging efficiently diminish inbreeding depression. Here we used an ecologically relevant inbreeding coefficient (f & 0.25) and generated ten slow and ten fast inbred lines of Drosophila melanogaster by keeping the effective population size constant at respectively 32 and 2 for 19 or 2 generations. These inbred lines were contrasted to non-inbred control lines. We investigated the effect of inbreeding and inbreeding rate in traits associated with fitness including heat, cold and desiccation stress resistance, egg-to-adult viability, development time, productivity, metabolic rate and wet weight under laboratory conditions. The results showed highly trait specific consequences of inbreeding and generally no support for the hypothesis that slow inbreeding is less deleterious than fast inbreeding. Egg-to-adult viability and development time were investigated under both benign and heat stress conditions. Reduced viability and increased developmental time were observed at stressful temperatures and inbreeding depression was on average more severe at stressful compared to benign temperatures.

High genetic load in an old isolated butterfly population

“…Males in the PT population had an especially short lifespan (Table 3), which can be related to their high RMR (73,74), although we realize that association between RMR and lifespan is still under debate (75). Studies examining the effect of increased homozygosity (inbreeding) on metabolic rate are scarce (76)(77)(78). A study on the cricket Gryllodes sigillatus (only males) found an 80% increase in RMR of inbred crickets over that in outcrossed individuals, along with a slight decrease in metabolic rate during forced exercise (79).…”

Analysis of the effects of inbreeding on lifespan and starvation resistance in Drosophila melanogaster

“…Swindell and Bouzat 2006a;Vermeulen et al 2008;Bilde et al 2009). Lifespan is assumed to be extremely sensitive to a large number of environmental factors and it is therefore considered a suitable trait for studies investigating the effects that environmental factors can exert on the level of inbreeding depression (Vermeulen and Bijlsma 2004 and generally low levels of inbreeding depression on resistance to heat, cold and desiccation stresses have also been reported in D. melanogaster (Mikkelsen et al 2010). Studies investigating the effects of inbreeding on survival under starvation appear less numerous.…”

“…Whether good quality individuals have higher or lower rate of metabolism is however still an unresolved question (Hawkins and Day 1999;Konarzewski and Diamond 1995;Reinhold 1999;Nilsson 2002;Rantala and Roff 2006;Johnston et al 2007;Ketola and Kotiaho 2009a, b;Mikkelsen et al 2010). The environment in which a population has evolved to live may have fundamental effects on its metabolic strategy in the sense that a low metabolic rate is favored in areas where food supply is scarce, while high metabolic rate is favored in areas where food supplies are abundant (Mueller and Diamond 2001).…”

“…However, the authors emphasized the importance of noting the large number of instances in which inbreeding depression was not found to increase and in fact, in 24% of the data sets, inbreeding decreased under stress. Moreover, in a recently published paper Mikkelsen et al (2010) report strong trait specific consequences of inbreeding and generally low levels of inbreeding depression on resistance to such stress factors as heat, cold and desiccation in D. melanogaster.…”

Inbreeding interferes with the heat-shock response

“…In modern human-transformed landscapes, the populations of many species are under various pressures including habitat loss and degradation, resulting in increasingly small and isolated populations (Mikkelsen et al, 2010). Small population size, in turn, increases the risk of genetic erosion through drift and inbreeding (Hoffmann and Hercus, 2000).…”