Loss of heterozygosity through inbreeding or mitotic errors leads to reductions in progeny survival and fertility. Loss of heterozygosity is particularly exacerbated in geographically isolated populations, which are prone to inbreeding depression and faster rates of extinction. The regenerative capacities of the hermaphroditic biotype of the planarian Schmidtea mediterranea allowed us to perform a systematic genetic test of Mendelian segregation and study the loss of heterozygosity in the Spiralian superclade in general and planarians in particular. We discovered that ~300 Mb (~37.5%) of the genome retains heterozygosity even after ten generations of inbreeding, and show that these chromosomal regions have low diversity and recombination rates in wild populations. Our genetic and genomic analyses establish S. mediterranea as a genetically tractable system. The research also opens the door to study the evolutionary basis of non-Mendelian mechanisms, the adaptive advantages of chromosome structural heterozygotes and their potential relationship to the robust regenerative capacities of planarians.
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The authors thank D. Chao, R. Krumlauf, K. Golic, S. Hawley, T. Piotrowski, E. Jorgensen and D. Grunwald for comments, discussions and suggestions during the preparation of this manuscript. We thank H. Li, J. Vallandingham and M. Gogol for help with data analysis and visualization. We are grateful to M. Pala for the original gift in 1999 of the sexual specimens of S. mediterranea and the Stowers Institute Planarian Core facility for skilful maintenance of our planarian colony. We acknowledge S.M.C. Robb and P. Reddien for the initial establishment of the S2 line, A. Rossi for the discussions and S. Sánchez-Piotrowski for his help in specimen collection. This work was funded in part by the National Institutes of Health (NIH R37GM057260) to A.S.A.
The authors declare no competing financial interests.
Supplementary Note, Supplementary References, Supplementary Figures 1–9 (PDF 992 kb)
List of MI and non-MI SNPs identified (XLSX 1027 kb)
Probes used in the SNPtype dynamic array (XLSX 41 kb)
Genotyping Primers used in RFLP or PCR/Sanger sequencing (XLSX 39 kb)
Genotyping raw data (XLSX 63 kb)
The mating behavior in Schmidtea mediterranea (MP4 4553 kb)
The arrested single cells in D5D/D5I unhatched egg capsules were haploid oocytes (MP4 8211 kb)
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Guo, L., Zhang, S., Rubinstein, B. et al. Widespread maintenance of genome heterozygosity in Schmidtea mediterranea. Nat Ecol Evol 1, 0019 (2017). https://doi.org/10.1038/s41559-016-0019