Allogamy-Autogamy Switch Enhance Assortative Mating in the Allotetraploid Centaurea seridis L. Coexisting with the Diploid Centaurea aspera L. and Triggers the Asymmetrical Formation of Triploid Hybrids

Hybridization between tetraploids and their related diploids is generally unsuccessful in Centaurea, hence natural formation of triploid hybrids is rare. In contrast, the diploid Centaurea aspera and the allotetraploid C. seridis coexist in several contact zones where a high frequency of triploid hybrids is found. We analyzed the floral biology of the three taxa to identify reproductive isolation mechanisms that allow their coexistence. Flowering phenology was recorded, and controlled pollinations within and between the three taxa were performed in the field. Ploidy level and germination of progeny were also assessed. There was a 50% flowering overlap which indicated a phenological shift. Diploids were strictly allogamous and did not display mentor effects, while tetraploids were found to be highly autogamous. This breakdown of self-incompatibility by polyploids is first described in Centaurea. The asymmetrical formation of the hybrid was also found: all the triploid intact cypselae came from the diploid mothers pollinated by the pollen of tetraploids. Pollen and eggs from triploids were totally sterile, acting as a strong triploid block. These prezygotic isolation mechanisms ensured higher assortative mating in tetraploids than in diploids, improving their persistence in the contact zones. However these mechanisms can also be the cause of the low genetic diversity and high genetic structure observed in C. seridis.在矢车菊属（Centaurea）中，四倍体与其近缘二倍体之间的杂交通常难以成功，因此天然三倍体杂种的形成极为罕见。与之形成对比的是，二倍体糙矢车菊（Centaurea aspera）与异源四倍体C. seridis在多个接触带共存，这些区域中存在高频率的三倍体杂种。本研究对三个类群的花部生物学展开分析，旨在阐明其共存所依赖的生殖隔离机制。研究记录了开花物候，并在田间开展了三类群内部及类群间的控制性授粉实验；同时评估了子代的倍性水平与萌发率。三者的开花重叠率达50%，表明存在物候偏移。二倍体类群严格为异交且未表现出导师效应，而四倍体类群则表现出高度的自交亲和性。这种多倍体引发的自交不亲和性突破现象，在矢车菊属中尚属首次报道。研究还发现了杂种形成的不对称性：所有完整的三倍体瘦果均来自以四倍体花粉授粉的二倍体母本。三倍体产生的花粉与卵细胞均完全不育，构成了强烈的三倍体阻断效应。这些合子前生殖隔离机制使得四倍体类群相较于二倍体类群拥有更高的选型交配率，从而提升了其在接触带中的存续能力。然而，此类机制同样可能是C. seridis遗传多样性较低、遗传结构较高的成因。