Crossover frequencies often differ substantially between sexes (i.e. heterochiasmy). Although this phenomenon is widespread throughout taxa, the mechanisms that lead to heterochiasmy remain unclear. One pattern that has emerged is that the overall length of the synaptonemal complex likely has a direct influence on the total number of crossovers in each sex. However, this has only been investigated in a handful of species. The threespine stickleback fish (Gasterosteus aculeatus) is an excellent species to explore whether synaptonemal complex length is associated with differences in the total number of crossovers, as females have much longer linkage maps than males. We used an immunocytological approach to quantify synaptonemal complex length in late pachytene female and male meiocytes in two different populations of threespine stickleback fish. Overall, the freshwater population had shorter synaptonemal complex lengths than the marine population. In both populations we observed sexual dimorphism, with females possessing longer axes. Our results support a model where chromosome axis length determines overall crossover frequency and establish the threespine stickleback as a useful species to explore the mechanistic basis of heterochiasmy as well as the genetic basis underlying variation in synaptonemal complex length.

中文翻译：

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三脊棘背鱼种群突触复合体长度的性二态

跨性别的频率通常在性别之间存在很大差异（即异位性交）。尽管此现象在整个分类单元中都很普遍，但导致异位嵌合的机制仍不清楚。已经出现的一种模式是，突触复合物的总长度可能直接影响每种性别的交叉总数。但是，仅在少数物种中对此进行了研究。三脊刺背鱼（Gasterosteus aculeatus）由于雌性比雄性具有更长的连锁图谱，因此它是探讨突触复合体长度是否与交叉总数差异相关的极好物种。我们使用了一种免疫细胞学方法来量化在两个不同种群的三棘棘背鱼中晚粗线期雌性和雄性肌细胞的突触复合体长度。总体而言，淡水种群的突触复合体长度比海洋种群短。在这两个人群中，我们观察到性二态性，其中雌性具有更长的轴。我们的结果支持一个模型，其中染色体轴长决定总体交叉频率，并将三棘棘背鱼建立为有用物种，以探索异位嵌合的机制基础以及突触复合体长度变异的遗传基础。