Nat Ecol Evol. 2021 Apr 5. doi: 10.1038/s41559-021-01435-x. Online ahead of print.
Daehan Lee # 1 2, Stefan Zdraljevic # 1 3 4 5, Lewis Stevens # 1, Ye Wang 1 6, Robyn E Tanny 1, Timothy A Crombie 1, Daniel E Cook 1, Amy K Webster 7 8, Rojin Chirakar 7, L Ryan Baugh 7 9, Mark G Sterken 10, Christian Braendle 11, Marie-Anne Félix 12, Matthew V Rockman 13, Erik C Andersen 14
1Department of Molecular Biosciences, Northwestern University, Evanston, IL, USA.
2Center for Integrative Genomics, University of Lausanne, Lausanne, Switzerland.
3Interdisciplinary Biological Sciences Program, Northwestern University, Evanston, IL, USA.
4Department of Human Genetics, University of California, Los Angeles, CA, USA.
5Howard Hughes Medical Institute, University of California, Los Angeles, CA, USA.
6Sichuan Key Laboratory of Conservation Biology on Endangered Wildlife, Chengdu Research Base of Giant Panda Breeding, Chengdu, People’s Republic of China.
7Department of Biology, Duke University, Durham, NC, USA.
8University Program in Genetics and Genomics, Duke University, Durham, NC, USA.
9Center for Genomic and Computational Biology, Duke University, Durham, NC, USA.
10Laboratory of Nematology, Wageningen University and Research, Wageningen, the Netherlands.
11Université Côte d’Azur, CNRS, Inserm, IBV, France, Nice, France.
12Institut de Biologie de l’Ecole Normale Supérieure, Centre National de la Recherche Scientifique, INSERM, École Normale Supérieure, Paris Sciences et Lettres, Paris, France.
13Center for Genomics and Systems Biology, Department of Biology, New York University, New York, NY, USA.
14Department of Molecular Biosciences, Northwestern University, Evanston, IL, USA. email@example.com.
Across diverse taxa, selfing species have evolved independently from outcrossing species thousands of times. The transition from outcrossing to selfing decreases the effective population size, effective recombination rate and heterozygosity within a species. These changes lead to a reduction in genetic diversity, and therefore adaptive potential, by intensifying the effects of random genetic drift and linked selection. Within the nematode genus Caenorhabditis, selfing has evolved at least three times, and all three species, including the model organism Caenorhabditis elegans, show substantially reduced genetic diversity relative to outcrossing species. Selfing and outcrossing Caenorhabditis species are often found in the same niches, but we still do not know how selfing species with limited genetic diversity can adapt to these environments. Here, we examine the whole-genome sequences from 609 wild C. elegans strains isolated worldwide and show that genetic variation is concentrated in punctuated hyper-divergent regions that cover 20% of the C. elegans reference genome. These regions are enriched in environmental response genes that mediate sensory perception, pathogen response and xenobiotic stress response. Population genomic evidence suggests that genetic diversity in these regions has been maintained by long-term balancing selection. Using long-read genome assemblies for 15 wild strains, we show that hyper-divergent haplotypes contain unique sets of genes and show levels of divergence comparable to levels found between Caenorhabditis species that diverged millions of years ago. These results provide an example of how species can avoid the evolutionary dead end associated with selfing.