Department of Human Genetics and Department of Biological Chemistry, University of California, Los Angeles, Los Angeles, CA, USA.
Howard Hughes Medical Institute, Chevy Chase, MD, USA.
Nature. 2022 Jun;606(7913):329-334. doi: 10.1038/s41586-022-04757-3. Epub 2022 Jun 1.
The sexual strain of the planarian Schmidtea mediterranea, indigenous to Tunisia and several Mediterranean islands, is a hermaphrodite. Here we isolate individual chromosomes and use sequencing, Hi-C 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 of the reproductive system are located on chromosome 1. We used RNA interference 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.
性系地中海扁形虫（Schmidtea mediterranea），原产于突尼斯和几个地中海岛屿，是雌雄同体。在这里，我们分离出个体染色体，并使用测序、Hi-C 和连锁作图来组装染色体级别的基因组参考。连锁图谱显示染色体 1 上的重组率极低。我们通过对单个精子和卵子进行基因分型，证实了染色体 1 上的重组抑制。我们表明，即使经过长时间的近亲繁殖，先前确定的维持杂合性的基因组区域构成了染色体 1 的大部分。对在野外分离的个体进行基因组测序表明，这种现象是在来自撒丁岛和科西嘉岛的种群中特异性进化而来的。我们发现，大多数已知的生殖系统主要调控因子位于染色体 1 上。我们使用 RNA 干扰敲低了一个具有单倍型偏向表达的基因，这导致了更明显的雌性交配器官的形成。基于这些观察结果，我们提出染色体 1 是一个为进化成性染色体而预先准备的性染色体激活的常染色体。
