Plant Ecological Genetics, Institute of Integrative Biology & Zurich-Basel Plant Science Center, ETH Zurich, Zurich, Switzerland.
Ann Bot. 2018 Jan 25;121(1):107-118. doi: 10.1093/aob/mcx133.
Defective hybrid seed development in angiosperms might mediate the rapid establishment of intrinsic post-zygotic isolation between closely related species. Extensive crosses within and among three lineages of wild tomatoes (Solanum section Lycopersicon) were performed to address the incidence, developmental timing and histological manifestations of hybrid seed failure. These lineages encompass different, yet fairly recent, divergence times and both allopatric and partially sympatric pairs.
Mature seeds were scored visually 2 months after hand pollinations, and viable-looking seeds were assessed for germination success. Using histological sections from early-developing seeds from a sub-set of crosses, the growth of three major seed compartments (endosperm, embryo and seed coat) was measured at critical developmental stages up to 21 d after pollination, with a focus on the timing and histological manifestations of endosperm misdevelopment in abortive hybrid seeds.
For two of three interspecific combinations including the most closely related pair that was also studied histologically, almost all mature seeds appeared 'flat' and proved inviable; histological analyses revealed impaired endosperm proliferation at early globular embryo stages, concomitant with embryo arrest and seed abortion in both cross directions. The third interspecific combination yielded a mixture of flat, inviable and plump, viable seeds; many of the latter germinated and exhibited near-normal juvenile phenotypes or, in some instances, hybrid necrosis and impaired growth.
The overall results suggest that near-complete hybrid seed failure can evolve fairly rapidly and without apparent divergence in reproductive phenology/biology. While the evidence accrued here is largely circumstantial, early-acting disruptions of normal endosperm development are most probably the common cause of seed failure regardless of the type of endosperm (nuclear or cellular).
被子植物杂种胚败育可能介导了近缘种间迅速建立的固有合子后隔离。为了研究杂种败育的发生、发育时间和组织学表现，对野生番茄（番茄属 Lycopersicon 组）的三个谱系内和谱系间的广泛杂交进行了研究。这些谱系包含不同的、但相当近期的分歧时间，以及异地和部分同域对。
授粉 2 个月后，通过肉眼对成熟种子进行评分，对外观正常的种子进行发芽成功率评估。通过对来自部分杂交的早期发育种子的组织切片，在授粉后 21 天内，对三个主要种子室（胚乳、胚和种皮）的生长进行了测量，重点关注败育杂种种子中胚乳发育异常的时间和组织学表现。
对于包括在组织学上也进行了研究的最密切亲缘对的两个种间组合，几乎所有成熟种子都显得“扁平”且无活力；组织学分析显示，在早期球形胚阶段，胚乳增殖受损，同时在两个杂交方向上胚胎停滞和种子败育。第三个种间组合产生了扁平、无活力和饱满、有活力的种子混合物；后者中的许多种子发芽并表现出近乎正常的幼体表型，或者在某些情况下，出现杂种坏死和生长受损。
总体结果表明，近完全的杂种种子败育可以相当迅速地进化，而生殖物候学/生物学没有明显的分歧。虽然这里获得的证据主要是间接的，但正常胚乳发育的早期中断很可能是导致种子败育的共同原因，而与胚乳类型（核型或细胞型）无关。
