Purging of the genetic load in the Skyros wall lizard?

Posted by Anna Runemark

In our new Molecular Ecology-paper Has the inbreeding load for a condition-dependent sexual signalling trait been purged in insular lizard populations? we present a pattern which indicates that purging of the genetic load may have taken place in islet populations of the Skyros wall lizard Podarcis gaigeae. We find a pattern of heterozygosity-dependence for a sexually selected ornament, blue side patches, among the populations on the main island (see Figure). In spite of small population sizes and lower heterozygosity we do, however, not find any such pattern among islet populations. In previous studies we have found that the effective population sizes of these populations are small (Runemark et al. 2012) and that genetic drift may affect throat color morph frequency (Runemark et al. 2010) and pheromone composition (Runemark et al. 2011), indicating that stochastic fixation of deleterious alleles could occur at these islets. We find the pattern of heterozygosity dependence for this sexually selected signalling character intriguing and compare it to the patterns of characters mainly subjected to natural selection. We discuss purging and other non-exclusive explanations to the pattern, see abstract below.

This paper happened to be written because I started teasing Marcus Ljungqvist, telling him that the system of islets with small populations of the Skyros wall lizard was a lot better for studying inbreeding than his study system, a not too inbred meta-population of nest-box blue tits. I challenged him to do something with my data, and on Research School in Genomic Ecology meeting we decided to try to study inbreeding together. 

We found out that the blue side patches seemed to be condition dependent in Podarcis hispanica, so we asked Mikkel Brydegaard to do some Matlab-magic to quantify their size from photographs, and used my microsatellite data to estimate between population variation in heterozygosity, and if heterozygosity affected mean patch size. The results were not quite what we expected: we did not find any effects of heterozygosity among the islet populations, which had significantly lower heterozygosity than had the mainland populations. We started pondering about whether the data could be interesting in spite of this. After some thorough statistical scrutiny together with my supervisor Erik Svensson and my coadvisor Bengt Hansson and some comparisons with other data from the study system the project resulted in this paper. I do still – maybe even more now – think that the Skyros wall lizard is a suitable system for addressing topics regarding inbreeding. 

Abstract:

Sexually selected traits are often condition-dependent and are expected to be affected by genome-wide distributed deleterious mutations and inbreeding. However, sexual selection is a powerful selective force that can counteract inbreeding through purging of deleterious mutations. Inbreeding and purging of the inbreeding load for sexually selected traits has rarely been studied across natural populations with different degrees of inbreeding. Here we investigate inbreeding effects (measured as marker-based heterozygosity) on condition-dependent sexually selected signalling trait and other morphological traits across islet- and mainland populations (n = 15) of an endemic lizard species (Podarcis gaigeae). Our data suggest inbreeding depression on a condition-dependent sexually selected signalling character among mainland subpopulations with low or intermediate levels of inbreeding, but no sign of inbreeding depression among small and isolated islet populations despite their higher overall inbreeding levels. In contrast, there was no such pattern among ten other morphological traits which are primarily naturally selected and presumably not involved in sexual signalling. These results are in line with purging of recessive deleterious alleles, or purging in combination with stochastic fixation of alleles by genetic drift, for a sexual signalling character in the islet environment, which is characterized by low population sizes and strong sexual selection. Higher clutch sizes in islet populations also raise interesting questions regarding the possibility of antagonistic pleiotropy. Purging and other non-exclusive explanations of our results are discussed.