Khanal Ashmita, Hu Nan, Guo Minghao, Gambhir Diksha, Sanderson Brian, Olson Matthew S
Department of Biological Sciences, Texas Tech University, Lubbock, 79409, TX, USA.
Department of Biology, West Virginia University, Morgantown, 26506, WV, USA.
Am J Bot. 2025 Jun;112(6):e70059. doi: 10.1002/ajb2.70059. Epub 2025 Jun 17.
Most dioecious angiosperms have homomorphic sex chromosomes, and non-recombining sex-linked regions (SLRs) likely comprise a small portion of the sex chromosome. The fountain of youth hypothesis posits that rare recombination within heterogametic SLRs may act to maintain homomorphy. Alternatively, turnovers of the sex-determining genes could also contribute to sex chromosome homomorphy.
We used reduced representation sequencing and mapping to the Salix purpurea reference genome to determine the size of the sex-linked region in three dwarf willows (Salix phlebophylla, S. reticulata, and S. nivalis). To test the fountain of youth hypotheses, we performed allelic state and linkage disequilibrium (LD) analyses.
For all three species, sex-linked single nucleotide polymorphisms (SNPs) were identified on chromosome 15. Mean heterozygosity was higher in females, confirming that all three species were female heterogametic (ZW). A recombinant haplotype between the Z and W chromosomes was identified in S. nivalis, contributing to higher-than-expected mean heterozygosity in SLR and slow LD decay in males.
We propose that the uncommon ZW haplotype resulting from a recombination event identified here initiates the homogenization of Z and W chromosomes, leading to the maintenance of their homomorphy. Based on comparisons across all three SLRs, we further speculate that ongoing rare recombination events have influenced the shared patterns of sex-linked SNPs across S. phlebophylla, S. reticulata, and S. nivalis and is a process that influences sex chromosome evolution throughout the Salicaceae and flowering plants with homomorphic sex chromosomes.
大多数雌雄异株的被子植物具有同形性染色体，且非重组性连锁区域（SLR）可能仅占性染色体的一小部分。“青春之泉”假说认为，异配性SLR内罕见的重组可能有助于维持同形性。另外，性别决定基因的更替也可能导致性染色体同形性。
我们使用简化基因组测序并将其映射到紫柳参考基因组上，以确定三种矮柳（脉叶柳、网脉柳和雪柳）的性连锁区域大小。为了验证“青春之泉”假说，我们进行了等位基因状态和连锁不平衡（LD）分析。
在所有三个物种中，均在15号染色体上鉴定到了性连锁单核苷酸多态性（SNP）。雌性的平均杂合度更高，证实这三个物种均为雌性异配（ZW）。在雪柳中鉴定到了Z和W染色体之间的一个重组单倍型，这导致SLR中平均杂合度高于预期，且雄性的LD衰退缓慢。
我们提出，此处鉴定到的由一次重组事件产生的罕见ZW单倍型启动了Z和W染色体的同质化，从而维持了它们的同形性。基于对所有三个SLR的比较，我们进一步推测，正在发生的罕见重组事件影响了脉叶柳、网脉柳和雪柳之间性连锁SNP的共享模式，并且这一过程影响了整个杨柳科以及具有同形性染色体的开花植物的性染色体进化。
