1. Brachionus calyciflorus is a recently described monogonont rotifer species complex that comprises four species. The observation of hybridisation between two of these species challenges this species delimitation. The mechanisms of reproductive isolation essential to the maintenance of species integrity remain unclear.
2. Here, we conducted upscaled hybridisation experiments to obtain large numbers of hybrid and non-hybrid dormant propagules. Through hatching assays, we compared the zygote viability of hybrid with non-hybrid dormant propagules. Furthermore, we investigated populations of F1 hybrid clones and assessed their clonal growth rate and the ability to reproduce sexually.
3. Our results demonstrated higher rates of morphological abnormality and associated mortality in dormant propagules of hybrids compared to non-hybrids yet hatching rates of healthy-looking propagules proved similar. F1 hybrids exhibited high clonal population growth rates, nevertheless, we also observed strong differences between clones and a strong influence of parental genotype identity. Two-thirds of the F1 hybrid clones showed a low incidence of sexual reproduction and almost never produced dormant propagules. Clones with high population growth rates seemed to invest less in sexual reproduction.
4. Our results clearly demonstrate the existence of intrinsic postzygotic barriers caused by relatively high mortality of dormant propagules. Furthermore, the low ability of most hybrid clones to engage in sexual reproduction may reduce the long-term fitness of hybrid clones. These postzygotic barriers probably impede genetic exchange between parental species and contribute to the maintenance of their integrity.

中文翻译：

---

内在的合子后障碍限制了 Brachionus calyciflorus 物种复合体的两个杂交同胞轮虫物种之间的交叉受精

1. Brachionus calyciflorus 是最近描述的单角轮虫物种复合体，包括四个物种。观察到其中两个物种之间的杂交对这种物种划界提出了挑战。对维持物种完整性至关重要的生殖隔离机制仍不清楚。
2. 在这里，我们进行了放大的杂交实验，以获得大量的杂交和非杂交休眠繁殖体。通过孵化试验，我们比较了杂种与非杂种休眠繁殖体的合子活力。此外，我们调查了 F1 杂交克隆的种群并评估了它们的克隆生长率和有性繁殖的能力。
3. 我们的结果表明，与非杂交种相比，杂交种的休眠繁殖体的形态异常率和相关死亡率更高，但看起来健康的繁殖体的孵化率证明相似。F1 杂种表现出高克隆种群增长率，然而，我们还观察到克隆之间的强烈差异和亲本基因型同一性的强烈影响。三分之二的 F1 杂种无性系表现出有性繁殖的低发生率，几乎从不产生休眠繁殖体。人口增长率高的克隆似乎对有性生殖的投资较少。
4. 我们的结果清楚地表明存在由休眠繁殖体相对较高的死亡率引起的内在合子后障碍。此外，大多数杂交无性系进行有性生殖的能力低下，可能会降低杂交无性系的长期适应度。这些合子后障碍可能会阻碍亲本物种之间的遗传交换，并有助于维持它们的完整性。