Pucholt Pascal, Hallingbäck Henrik R, Berlin Sofia
Department of Plant Biology, Uppsala BioCenter, Linnean Centre for Plant Biology, Swedish University of Agricultural Sciences, P.O. Box 7080, SE - 75007, Uppsala, Sweden.
Department of Forest Genetics and Plant Physiology, Umeå Plant Science Center, Swedish University of Agricultural Sciences, SE - 90183, Umeå, Sweden.
BMC Genomics. 2017 Mar 23;18(1):251. doi: 10.1186/s12864-017-3634-5.
Biased sex ratios are common among dioecious plant species despite the theoretical prediction of selective advantage of even sex ratios. Albeit the high prevalence of deviations from even sex ratios, the genetic causes to sex biases are rarely known outside of a few model species. Here we present a mechanism underlying the female biased sex ratio in the dioecious willow species Salix viminalis.
We compared the segregation pattern of genome-wide single nucleotide polymorphism markers in two contrasting bi-parental pedigree populations, the S3 with even sex ratio and the S5 with a female biased sex ratio. With the segregation analysis and comparison between the two populations, we were able to demonstrate that sex determination and sex ratio distortion are controlled by different genetic mechanisms. We furthermore located the sex ratio distorter locus to a Z/W-gametologous region on chromosome 15, which was in close linkage with the sex determination locus. Interestingly, all males in the population with biased sex ratio have in this sex ratio distorter locus the same genotype, meaning that males with the Z/Z-genotype were missing from the population, thereby creating the 2:1 female biased sex ratio.
We attribute the absence of Z/Z males to an allelic incompatibility between maternally and paternally inherited alleles in this sex ratio distorter locus. Due to the tight linkage with the sex determination locus only male individuals are purged from the population at an early age, presumably before or during seed development. We showed that such allelic incompatibility could be stably maintained over evolutionary times through a system of overdominant or pseudooverdominant alleles. Thus, it is possible that the same mechanism generates the female biased sex ratio in natural willow populations.
尽管理论上预测雌雄比例均衡具有选择优势，但在雌雄异株植物物种中，偏倚的性别比例很常见。尽管偏离均衡性别比例的情况普遍存在，但除了少数模式物种外，性别偏倚的遗传原因鲜为人知。在此，我们提出了雌雄异株柳树物种垂柳中雌性偏倚性别比例的潜在机制。
我们比较了两个具有对比性的双亲谱系群体中全基因组单核苷酸多态性标记的分离模式，即性别比例均衡的S3群体和具有雌性偏倚性别比例的S5群体。通过两个群体之间的分离分析和比较，我们能够证明性别决定和性别比例扭曲受不同的遗传机制控制。此外，我们将性别比例扭曲位点定位到第15号染色体上的一个Z/W配子同源区域，该区域与性别决定位点紧密连锁。有趣的是，性别比例偏倚群体中的所有雄性在这个性别比例扭曲位点都具有相同的基因型，这意味着具有Z/Z基因型的雄性在群体中缺失，从而产生了2:1的雌性偏倚性别比例。
我们将Z/Z雄性的缺失归因于这个性别比例扭曲位点中母本和父本遗传等位基因之间的等位基因不相容性。由于与性别决定位点紧密连锁，只有雄性个体在幼年时被从群体中清除，大概是在种子发育之前或期间。我们表明，这种等位基因不相容性可以通过超显性或假超显性等位基因系统在进化时间内稳定维持。因此，有可能相同的机制在天然柳树种群中产生了雌性偏倚的性别比例。
