Polygenic Sex Determination System in Zebrafish

BackgroundDespite the popularity of zebrafish as a research model, its sex determination (SD) mechanism is still unknown. Most cytogenetic studies failed to find dimorphic sex chromosomes and no primary sex determining switch has been identified even though the assembly of zebrafish genome sequence is near to completion and a high resolution genetic map is available. Recent publications suggest that environmental factors within the natural range have minimal impact on sex ratios of zebrafish populations. The primary aim of this study is to find out more about how sex is determined in zebrafish. Methodology/Principal FindingsUsing classical breeding experiments, we found that sex ratios across families were wide ranging (4.8% to 97.3% males). On the other hand, repeated single pair crossings produced broods of very similar sex ratios, indicating that parental genotypes have a role in the sex ratio of the offspring. Variation among family sex ratios was reduced after selection for breeding pairs with predominantly male or female offspring, another indication that zebrafish sex is regulated genetically. Further examinations by a PCR-based “blind assay" and array comparative genomic hybridization both failed to find universal sex-linked differences between the male and female genomes. Together with the ability to increase the sex bias of lines by selective breeding, these data suggest that zebrafish is unlikely to utilize a chromosomal sex determination (CSD) system. Conclusions/SignificanceTaken together, our study suggests that zebrafish sex is genetically determined with limited, secondary influences from the environment. As we have not found any sign for CSD in the species, we propose that the zebrafish has a polygenic sex determination system.

背景 尽管斑马鱼作为研究模型已被广泛应用，但其性别决定（sex determination, SD）机制至今仍未明确。多数细胞遗传学研究未能检测到具有二态性的性染色体；即便斑马鱼基因组组装已接近完成、高分辨率遗传图谱已可获取，学界仍未鉴定出核心的性别决定开关。近期研究显示，自然环境范围内的环境因子对斑马鱼种群的性别比例影响极微。本研究的核心目标为进一步解析斑马鱼的性别决定方式。

研究方法与主要结果 借助经典育种实验，我们发现不同家系的性别比例跨度极大（雄性占比4.8%至97.3%）。而重复开展的单对交配实验，所得子代的性别比例均极为相近，这表明亲本基因型对子代性别比例具有调控作用。在筛选以雄性或雌性子代占比为主的育种配对进行繁育后，家系间的性别比例差异显著降低，进一步佐证了斑马鱼性别受遗传调控。后续通过基于PCR的“盲法检测”与阵列比较基因组杂交技术开展的分析，均未发现雌雄基因组间存在普遍的性别连锁差异。结合通过选择性育种可提升品系性别偏向性的实验结果，上述数据表明斑马鱼不太可能采用染色体性别决定（chromosomal sex determination, CSD）系统。

研究结论与意义 综合来看，本研究证实斑马鱼的性别由遗传决定，仅受环境因素的微弱、间接影响。由于未在该物种中发现染色体性别决定的相关迹象，我们推测斑马鱼采用多基因性别决定系统。