Background and Aims Hybridization is increasingly recognized as an integral part of the dynamics of species range expansion and contraction. Thus, it is important to understand the reproductive barriers between co-occurring species. Extending previous studies that argued that the rare Eucalyptus risdonii was expanding into the range of the surrounding E. amygdalina by both seed and pollen dispersal, we here investigate the long-term fitness of both species and their hybrids and whether expansion is continuing. Methods We assessed the survival of phenotypes representing a continuum between the two pure species in a natural hybrid swarm after 29 years, along with seedling recruitment. The performance of pure species as well as of artificial and natural hybrids was also assessed over 28 years in a common garden trial. Key Results In the hybrid zone, E. amygdalina adults showed greater mortality than E. risdonii, and the current seedling cohort is still dominated by E. risdonii phenotypes. Morphologically intermediate individuals appeared to be the least fit. Similar results were observed after growing artificial first-generation and natural hybrids alongside pure species families in a common garden trial. Here, the survival, reproduction, health and growth of the intermediate hybrids were significantly less than those of either pure species, consistent with hybrid inferiority, although this did not manifest until later reproductive ages. Among the variable progeny of natural intermediate hybrids, the most E. risdonii-like phenotypes were the most fit. Conclusions This study contributes to the increasing number of reports of hybrid inferiority in Eucalyptus, suggesting that post-zygotic barriers contribute to the maintenance of species integrity even between closely related species. However, with fitness rapidly recovered following backcrossing, it is argued that hybridization can still be an important evolutionary process, in the present case appearing to contribute to the range expansion of the rare E. risdonii in response to climate change.

中文翻译：

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尽管杂交劣势，但通过与密切相关的物种杂交，在气候变化下扩大稀有桉树

背景和目的杂交越来越被认为是物种范围扩张和收缩动态的一个组成部分。因此，了解共生物种之间的生殖障碍很重要。扩展先前的研究，即稀有桉树通过种子和花粉传播扩展到周围的 E. amygdalina 的范围，我们在这里调查了这两个物种及其杂交种的长期适应性以及扩张是否仍在继续。方法 我们评估了 29 年后自然杂交群中代表两个纯种之间连续体的表型的存活率，以及幼苗的补充。纯种以及人工和天然杂交种的性能也在 28 年的共同花园试验中进行了评估。主要结果 在混合区，E. amygdalina 成虫的死亡率高于 E. risdonii，目前的幼苗队列仍以 E. risdonii 表型为主。形态学中间个体似乎最不适合。在普通花园试验中，在与纯种科一起种植人工第一代和自然杂交种后，观察到了类似的结果。在这里，中间杂种的生存、繁殖、健康和生长明显低于任何一个纯种，这与杂种的劣势一致，尽管直到后来的生育年龄才表现出来。在天然中间杂种的可变后代中，最适合 E. risdonii 的表型是最合适的。结论 这项研究有助于增加桉树杂交劣势的报道，表明即使在密切相关的物种之间，合子后屏障也有助于维持物种完整性。然而，随着回交后适应度迅速恢复，有人认为杂交仍然是一个重要的进化过程，在目前的情况下，似乎有助于稀有 E. risdonii 响应气候变化的范围扩大。