School of Natural Sciences and ARC Training Centre for Forest Value, University of Tasmania, Hobart, Australia.
Ann Bot. 2022 Jan 8;129(1):1-14. doi: 10.1093/aob/mcab103.
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.
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.
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.
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.
杂交被越来越多地认为是物种分布范围扩张和收缩动态的一个组成部分。因此，了解共存物种之间的生殖障碍很重要。在之前的研究中，我们认为稀有的红桉正在通过种子和花粉传播扩展到周围的杏仁桉的范围内，在此，我们研究了这两个物种及其杂种的长期适应性，以及扩展是否在继续。
我们评估了 29 年后自然杂种群中代表两个纯物种之间连续体的表型的存活率，以及幼苗的补充。还在 28 年的时间里，在一个共同花园试验中评估了纯物种以及人工和自然杂种的表现。
在杂种区，杏仁桉成虫的死亡率高于红桉，而当前的幼苗群体仍然以红桉表型为主。形态上的中间个体似乎适应性最差。在共同花园试验中，与纯物种家族一起种植人工第一代和自然杂种后，观察到了类似的结果。在这里，中间杂种的存活率、繁殖力、健康和生长明显低于纯物种，这与杂种劣势一致，尽管这种情况直到后期繁殖年龄才显现出来。在自然中间杂种的可变后代中，最像红桉的表型最适应。
本研究增加了关于杂种劣势在桉树中的报告数量，表明即使在亲缘关系密切的物种之间，合子后障碍也有助于维持物种的完整性。然而，由于与纯物种的回交，杂种的适应性迅速恢复，因此可以认为杂交仍然是一个重要的进化过程，在这种情况下，它似乎有助于稀有的红桉对气候变化的范围扩张。
