Opuntia (Cactaceae) is known for high rates of hybridization and ploidisation, resulting in the formation of new species. The occurrence of two sympatric and closely related species of Opuntia, O. elata and O. retrorsa, in Brazilian Chaco enabled us to test the hypothesis that pre-zygotic reproductive isolation mechanisms operate in both species. We monitored the flowering period, as well as floral biology, and compared the morphological variation of floral structures through measurements, performed intra- and interspecific cross-pollination tests, and recorded the guild of floral visitors and pollinators. Flowering was seasonal and highly synchronous. Floral biology exhibits similar strategies, and although floral morphology differs significantly in many of the compared structures, such morphological variation does not result in the selection of exclusive pollinators. Floral visitors and pollinators are oligolectic bees shared by both species. Opuntia elata and O. retrorsa are self-compatible. While interspecific cross-pollination (bidirectional) resulted in germination, the pollen tube did not penetrate the stigma. Opuntia elata and O. retrorsa are closely related; however, they are isolated and do not hybridise in Brazilian Chaco. We found that both have weak pre-pollination barriers, but that they are strongly isolated by pollen-pistil incompatibility, i.e. post-pollination barrier.

中文翻译：

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巴西查科地区两个同步无序仙人掌（仙人掌科）之间的合子前生殖隔离。

仙人掌（仙人掌科）以高杂交率和倍性化而闻名，导致形成新物种。在巴西Chaco中出现了两个同属的且密切相关的仙人掌物种O. elata和O. retrorsa，这使我们能够检验假说前合子生殖隔离机制在这两个物种中均起作用的假说。我们监测了开花期以及花卉生物学，并通过测量比较了花卉结构的形态变化，进行了种内和种间交叉授粉试验，并记录了花卉访客和传粉媒介的行会。开花是季节性的并且高度同步。花卉生物学表现出相似的策略，尽管在许多比较的结构中花卉形态均存在显着差异，这种形态上的变化不会导致独家授粉媒介的选择。花的访客和传粉者是两个物种共享的寡聚蜂。Eluntia elata和O. retrorsa是自相容的。种间异花授粉（双向）导致发芽，但花粉管未穿透柱头。仙人掌和O. retrorsa密切相关。但是，它们是分离的，在巴西Chaco中不杂交。我们发现两者均具有较弱的授粉前授粉障碍，但被花粉雌蕊不相容性（即授粉后授粉障碍）强烈隔离。花粉管没有穿透柱头。仙人掌和O. retrorsa密切相关。但是，它们是分离的，在巴西Chaco中不杂交。我们发现二者均具有较弱的授粉前授粉障碍，但它们之间的相互隔离是由花粉雌蕊不相容性所致，即授粉后授粉障碍。花粉管没有穿透柱头。仙人掌和O. retrorsa密切相关。但是，它们是分离的，在巴西Chaco中不杂交。我们发现二者均具有较弱的授粉前授粉障碍，但它们之间的相互隔离是由花粉雌蕊不相容性所致，即授粉后授粉障碍。