Genetic incompatibilities are widespread within species

ResearchBlogging.org

The role and importance of epistasis (non-additive interactions between alleles) in shaping genetic variation, and its effects on fitness have been the focus of much debate. Russell et al. recently investigated the genomic footprint of epistatic fitness effects. Starting from two sets of 8 highly inbred Drosophila melanogaster strains, they created 4 panels of 400 recombinant inbred lines. Then, they investigated non independent allelic segregation of pairs of unlinked loci (i.e. on different chromosomes; figure 1 a and c). Indeed, it is expected that unfavourable allelic combinations will be under-represented in a the population. This deviation from Mendelian proportions was refereed as genotype ratio distortion (GDP).

Figure 1: a and c) P-value of the chi2 test computed for each pairs of loci between two chromosomes. It presents two examples of  significant genotype ratio distortion.  B and d) Average productivity of each genotypic class (progeny counts). As predicted from the GRD test (a and c), haplotypes presents strong negative epistasis for the aa;bb genotypes.
Figure 1: a and c) P-value of the chi2 test computed for each pairs of loci between two chromosomes. It presents two examples of significant genotype ratio distortion. B and d) Average productivity of each genotypic class (progeny counts). As predicted from the GRD test (a and c), haplotypes presents strong negative epistasis for the aa;bb genotypes.

22 pairs of epistatically interacting alleles could be identified. Many of the incompatible alleles are present in more than one founder strain, suggesting that they were segregating in natural populations. They estimated that any pairwise combination of founder strains has, on average, 1.15 pairs of epistatically interacting alleles .

They performed experimental crosses in order to investigate two incompatibilities in more details. They discovered that the negative interaction is caused by the minor alleles at each locus in both cases (Figure 1 b, d). Interestingly, both involved a reduction in male fertility (with respectively 74% and 22% fewer offsprings).
Finally, in order to confirm that these observations are not specific to D. melanogaster , they used a similar approach to screen for GRD in two additional species: Arabidopsis and maize. 7 cases of GRD were identified in Arabidopsis, 5 in maize.
This work shows that epistasis with fitness effects has a detectable genomic footrint, and supports the idea that incompatibilities are widespread within populations. The authors finally discussed the possibility that reproductive isolation and speciation could be achieved through divergence in frequencies of numerous incompatible variants, and would not necessarily require the emergence of incompatible mutations independently derived and fixed in allopatric lineages.

Reference:

Corbett-Detig, R., Zhou, J., Clark, A., Hartl, D., & Ayroles, J. (2013). Genetic incompatibilities are widespread within species Nature, 504 (7478), 135-137

Tutorial program autumn 2014: Genomics, ecology, evolution, etc

For this new session of our tutorial, these are the papers that we will discuss:

Boyle et al 2014 Comparative analysis of regulatory information and circuits across distant species. Nature 512, 453–456

Liu et al 2014 Population Genomics Reveal Recent Speciation and Rapid Evolutionary Adaptation in Polar Bears. Cell 157, 785–794

Lazaridis 2014 Ancient human genomes suggest three ancestral populations for present-day Europeans. Nature 513, 409–413

Wang et al 2014 The genome sequence of African rice (Oryza glaberrima) and evidence for independent domestication. Nature Genetics 46, 982–988

Gallant et al 2014 Genomic basis for the convergent evolution of electric organs. Science 344, 1522-1525

Carbone et al. 2014 Gibbon genome and the fast karyotype evolution of small apes.     Nature 513, 195–201

Moreno-Estrada et al. 2014 The genetics of Mexico recapitulates Native American substructure and affects biomedical traits. Science 344, 1280-1285

Soria-Carrasco et al. 2014 Stick Insect Genomes Reveal Natural Selection’s Role in Parallel Speciation. Science 344, 738-742

Dates: 31.10, 7.11, 14.11, 21.11, 28.11, 5.12, 12.12, 19.12.