Island-specific evolution of a sex-primed autosome in a sexual planarian
Longhua Guo, Joshua S. Bloom, Daniel Dols-Serrate, James Boocock, Eyal Ben‐David, Olga T. Schubert, Kaiya Kozuma, Katarina Ho, Emily Warda, Clarice Chui, Yubao Wei, Daniel Leighton, Tzitziki Lemus, Marta Riutort, Alejandro Sánchez Alvarado, Leonid Kruglyak
Abstract
Abstract The sexual strain of the planarian Schmidtea mediterranea , indigenous to Tunisia and several Mediterranean islands, is a hermaphrodite 1,2 . Here we isolate individual chromosomes and use sequencing, Hi-C 3,4 and linkage mapping to assemble a chromosome-scale genome reference. The linkage map reveals an extremely low rate of recombination on chromosome 1. We confirm suppression of recombination on chromosome 1 by genotyping individual sperm cells and oocytes. We show that previously identified genomic regions that maintain heterozygosity even after prolonged inbreeding make up essentially all of chromosome 1. Genome sequencing of individuals isolated in the wild indicates that this phenomenon has evolved specifically in populations from Sardinia and Corsica. We find that most known master regulators 5–13 of the reproductive system are located on chromosome 1. We used RNA interference 14,15 to knock down a gene with haplotype-biased expression, which led to the formation of a more pronounced female mating organ. On the basis of these observations, we propose that chromosome 1 is a sex-primed autosome primed for evolution into a sex chromosome.