Single locus sex determination and female heterogamety in the basket willow (Salix viminalis L.)

Most eukaryotes reproduce sexually and a wealth of different sex determination mechanisms have evolved in this lineage. Dioecy or separate sexes are rare among flowering plants but have repeatedly evolved from hermaphroditic ancestors possibly involving male or female sterility mutations. Willows (Salix spp.) and poplars (Populus spp.) are predominantly dioecious and are members of the Salicaceae family. All studied poplars have sex determination loci on chromosome XIX, however, the position differs among species and both male and female heterogametic system exists. In contrast to the situation in poplars, knowledge of sex determination mechanisms in willows is sparse. In the present study, we have for the first time positioned the sex determination locus on chromosome XV in S. viminalis using quantitative trait locus mapping. All female offspring carried a maternally inherited haplotype, suggesting a system of female heterogamety or ZW. We used a comparative mapping approach and compared the positions of the markers between the S. viminalis linkage map and the physical maps of S. purpurea, S. suchowensis and P. trichocarpa. As we found no evidence for chromosomal rearrangements between chromosome XV and XIX between S. viminalis and P. trichocarpa, it shows that the sex determination loci in the willow and the poplar most likely do not share a common origin and has thus evolved separately. This demonstrates that sex determination mechanisms in the Salicaceae family have a high turnover rate and as such it is excellent for studies of evolutionary processes involved in sex chromosome turnover.

绝大多数真核生物以有性生殖方式繁衍，该演化支中演化出了多种多样的性别决定机制。雌雄异株（即个体间分雌雄性）在显花植物中较为罕见，但该类群多次从雌雄同株的祖先类群演化而来，这一过程可能涉及雄性或雌性不育突变。柳树（柳属Salix spp.）与杨树（杨属Populus spp.）均隶属于杨柳科（Salicaceae），且绝大多数为雌雄异株物种。所有已被研究的杨树，其性别决定位点均位于第19号染色体，但不同物种的位点位置存在差异，且同时存在雄性异配（male heterogamety，XY型）与雌性异配（female heterogamety，ZW型）两套性别决定系统。与杨树的研究现状相反，学界对于柳树性别决定机制的认知仍较为匮乏。本研究首次借助数量性状位点（quantitative trait locus, QTL）定位技术，将S. viminalis的性别决定位点定位于第15号染色体。所有雌性后代均携带母系遗传的单倍型，这提示该物种的性别决定系统属于雌性异配（ZW型）。本研究采用比较定位策略，对比了S. viminalis连锁图谱与S. purpurea、S. suchowensis以及P. trichocarpa的物理图谱中分子标记的位置。本研究未在S. viminalis与P. trichocarpa的第15号与第19号染色体间发现染色体重排的证据，这表明柳树与杨树的性别决定位点很可能并非起源于共同的祖先，而是各自独立演化而来。该结果表明杨柳科的性别决定机制具有极高的演化更替速率，因此该类群是研究性染色体演化更替相关进化过程的绝佳模型。