Larcombe Matthew J, Costa E Silva João, Tilyard Paul, Gore Peter, Potts Brad M
Department of Botany, University of Otago, PO Box 56, Dunedin 9011, New Zealand
Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal.
Ann Bot. 2016 Sep;118(3):431-44. doi: 10.1093/aob/mcw115. Epub 2016 Jul 10.
Many previous studies conclude that pre-zygotic barriers such as mechanical isolation account for most reproductive isolation between pairs of taxa. However, the inheritance and persistence of barriers such as these after the first generation of hybridization is rarely quantified, even though it is a vital consideration in understanding gene flow potential. There is an asymmetrical pre-zygotic mechanical barrier to hybridization between Eucalyptus nitens and Eucalyptus globulus, which completely prevents small-flowered E. nitens pollen from mating with large E. globulus flowers, while the reverse cross is possible. We aimed to determine the relative importance of pre- and post-zygotic barriers in preventing gene flow following secondary contact between E. nitens and E. globulus, including the inheritance of barriers in advanced-generation hybrids.
Experimental crossing was used to produce outcrossed E. nitens, E. globulus and their F1, F2, BCg and BCn hybrids. The strength and inheritance of a suite of pre- and post-zygotic barriers were assessed, including 20-year survival, growth and reproductive capacity.
The mechanical barrier to hybridization was lost or greatly reduced in the F1 hybrid. In contrast, intrinsic post-zygotic barriers were strong and persistent. Line-cross analysis indicated that the outbreeding depression in the hybrids was best explained by epistatic loss.
The removal of strong mechanical barriers between E. nitens and E. globulus allows F1 hybrids to act as a bridge for bi-directional gene flow between these species. However, strong and persistent post-zygotic barriers exist, meaning that wherever F1 hybridization does occur, intrinsic post-zygotic barriers will be responsible for most reproductive isolation in this system. This potential transient nature of mechanical barriers to zygote formation due to additive inheritance in hybrids appears under-appreciated, and highlights the often important role that intrinsic post-mating barriers play in maintaining species boundaries at zones of secondary contact.
许多先前的研究得出结论，诸如机械隔离等合子前屏障是大多数分类单元对之间生殖隔离的主要原因。然而，尽管这是理解基因流潜力的一个至关重要的考虑因素，但这些屏障在第一代杂交后的遗传和持续性很少被量化。在蓝桉和直干桉之间存在一种不对称的合子前机械杂交屏障，它完全阻止小花的蓝桉花粉与大花的直干桉交配，而反交则是可能的。我们旨在确定合子前和合子后屏障在阻止蓝桉和直干桉二次接触后基因流方面的相对重要性，包括高级代杂种中屏障的遗传。
采用实验杂交的方法产生蓝桉、直干桉及其F1、F2、BCg和BCn杂种的异交后代。评估了一系列合子前和合子后屏障的强度和遗传性，包括20年的存活率、生长和繁殖能力。
F1杂种中杂交的机械屏障丧失或大大降低。相比之下，内在的合子后屏障很强且持续存在。品系杂交分析表明，杂种中的远交衰退最好由上位性丧失来解释。
蓝桉和直干桉之间强大机械屏障的消除使F1杂种能够成为这两个物种之间双向基因流的桥梁。然而，存在强大且持久的合子后屏障，这意味着无论F1杂交发生在哪里，内在的合子后屏障将是该系统中大多数生殖隔离的原因。由于杂种中的加性遗传导致的合子形成机械屏障的这种潜在短暂性质似乎未得到充分认识，并突出了内在交配后屏障在维持二次接触区域物种边界方面经常发挥的重要作用。
