Data from: Postzygotic isolation involves strong mitochondrial and sex-specific effects in Tigriopus californicus, a species lacking heteromorphic sex chromosomes

Detailed studies of the genetics of speciation have focused on a few model systems, particularly Drosophila. The copepod Tigriopus californicus offers an alternative that differs from standard animal models in that it lacks heteromorphic chromosomes (instead, sex determination is polygenic) and has reduced opportunities for sexual conflict, because females mate only once. Quantitative trait loci (QTL) mapping was conducted on reciprocal F2 hybrids between two strongly differentiated populations, using a saturated linkage map spanning all 12 autosomes and the mitochondrion. By comparing sexes, a possible sex ratio distorter was found but no sex chromosomes. Although studies of standard models often find an excess of hybrid male sterility factors, we found no QTL for sterility and multiple QTL for hybrid viability (indicated by non-Mendelian adult ratios) and other characters. Viability problems were found to be stronger in males, but the usual explanations for weaker hybrid males (sex chromosomes, sensitivity of spermatogenesis, sexual selection) cannot fully account for these male viability problems. Instead, higher metabolic rates may amplify deleterious effects in males. Although many studies of standard speciation models find the strongest genetic incompatibilities to be nuclear–nuclear (specifically X chromosome–autosome), we found the strongest deleterious interaction in this system was mito–nuclear. Consistent with the snowball theory of incompatibility accumulation, we found that trigenic interactions in this highly divergent cross were substantially more frequent (>6 × ) than digenic interactions. This alternative system thus allows important comparisons to studies of the genetics of reproductive isolation in more standard model systems.物种形成遗传学的精细研究多聚焦于少数模式系统，尤以果蝇（Drosophila）为甚。桡足类物种加州虎斑猛水蚤（Tigriopus californicus）则提供了一类另类研究体系，其与经典动物模式生物的差异在于：不具备异形性染色体（性别决定由多基因调控），且因雌性仅交配一次，性冲突发生的概率大幅降低。本研究针对两个高度分化种群间的正反交F2杂种开展数量性状位点（Quantitative trait loci, QTL）定位分析，使用覆盖全部12条常染色体与线粒体的饱和连锁图谱。通过对不同性别进行比较，研究人员发现了一处潜在的性别比例畸变位点，但未检测到性染色体相关的遗传座位。尽管经典模式生物的相关研究常发现大量杂种雄性不育相关因子，但本研究未检测到不育相关的QTL，却鉴定出多个与杂种存活力（以非孟德尔式成虫性别比例为表征）及其他性状相关的QTL。研究发现雄性的存活力缺陷更为显著，但学界用于解释杂种雄性弱势的常规机制——包括性染色体效应、精子发生敏感性缺陷及性选择压力——均无法完全解释该现象。取而代之的假说为：雄性较高的代谢速率可能放大了遗传不相容带来的有害效应。尽管诸多经典物种形成模式研究显示，最强的遗传不相容性多发生于核-核互作（尤其是X染色体-常染色体间），但本体系中最强的有害互作却为线粒体-核互作。与不相容性累积的雪球理论（snowball theory）相符，本研究发现该高度分化杂交组合中的三基因互作频率显著高于双基因互作（高出6倍以上）。因此，该另类研究体系可为更经典模式系统中的生殖隔离遗传学研究提供重要的对比参照。