Tas I C, Van Dijk P J
Department of Plant Population Biology, Netherlands Institute of Ecology, Center for Terrestrial Ecology (NIOO-CTO), PO Box 40, 6666 ZG Heteren, The Netherlands.
Heredity (Edinb). 1999 Dec;83 ( Pt 6):707-14. doi: 10.1046/j.1365-2540.1999.00619.x.
Some dandelions, Taraxacum, are diplosporous gametophytic apomicts. Crosses between closely related diploid sexuals and triploid apomicts were made to study the inheritance of apomixis. Seed-set was less than one-third of that in diploid x diploid crosses, probably because of the inviability of aneuploid pollen or zygotes. Almost 90% of the viable offspring were diploid and the result of selfing, as was shown by a discriminating allozyme marker. Aneuploid outcross pollen had a mentor effect on self-pollen, causing a breakdown of the sporophytic self-incompatibility system. A similar phenomenon has been reported before in wide crosses. Of the 26 allozyme-confirmed hybrids, four were diploids, 15 were triploids and seven were tetraploids. Diploid hybrids were significantly less frequent than triploid hybrids, suggesting either low fitness of haploid pollen or more numerous formation of diploid pollen. Emasculation and bagging of flowers indicated apomictic seed-set in none of the diploid, in one-third of the triploid and in all of the tetraploid hybrids. All apomictic hybrids showed partial seed-set, but additional cross-pollination did not increase seed-set. Cytological analysis of the F2 progeny confirmed that partial apomixis was caused by semisterility and not by residual sexuality (facultative apomixis). The difference in segregation for apomixis between triploid and tetraploid hybrids may be because the triploids originated from partially reduced diploid pollen grains, whereas the tetraploids originated from unreduced triploid pollen grains.
一些蒲公英属植物是二倍体孢子体无融合生殖体。将亲缘关系较近的二倍体有性生殖植物与三倍体无融合生殖植物进行杂交，以研究无融合生殖的遗传。结实率不到二倍体×二倍体杂交的三分之一，可能是由于非整倍体花粉或合子的不育性。几乎90%的可育后代是二倍体且是自交的结果，这通过一种有鉴别力的等位酶标记得以证明。非整倍体异交花粉对自花花粉有引导效应，导致孢子体自交不亲和系统的瓦解。之前在远缘杂交中也报道过类似现象。在26个经等位酶确认的杂种中，4个是二倍体，15个是三倍体，7个是四倍体。二倍体杂种的出现频率明显低于三倍体杂种，这表明要么单倍体花粉的适合度低，要么二倍体花粉的形成数量更多。对花朵进行去雄和套袋处理表明，二倍体杂种中无一表现出无融合生殖结实，三分之一的三倍体杂种和所有四倍体杂种表现出无融合生殖结实。所有无融合生殖杂种都表现出部分结实，但额外的异花授粉并未增加结实率。对F2后代的细胞学分析证实，部分无融合生殖是由半不育引起的，而非残留有性生殖（兼性无融合生殖）。三倍体和四倍体杂种在无融合生殖分离上的差异可能是因为三倍体源自部分减数的二倍体花粉粒，而四倍体源自未减数的三倍体花粉粒。
