Department of Ecology and Evolutionary Biology, University of California, 130 McAllister Way, Santa Cruz, CA 95060, USA.
Proc Biol Sci. 2020 Dec 23;287(1941):20202593. doi: 10.1098/rspb.2020.2593.
Heteranthery, the presence of two or more anther types in the same flower, is taxonomically widespread among bee-pollinated angiosperms, yet has puzzled botanists since Darwin. We test two competing hypotheses for its evolution: the long-standing 'division of labour' hypothesis, which posits that some anthers are specialized as food rewards for bees whereas others are specialized for surreptitious pollination, and our new hypothesis that heteranthery is a way to gradually release pollen that maximizes pollen delivery. We examine the evolution of heteranthery and associated traits across the genus (Onagraceae) and study plant-pollinator interactions in two heterantherous species. Across species, heteranthery is associated with bee pollination, delayed dehiscence and colour crypsis of one anther whorl, and movement of that anther whorl upon dehiscence. Our mechanistic studies in heterantherous species show that bees notice, forage on and export pollen from each anther whorl when it is dehiscing, and that heteranthery promotes pollen export. We find no support for division of labour, but multifarious evidence that heteranthery is a mechanism for gradual pollen presentation that probably evolved through indirect male-male competition for siring success.
异型雄蕊，即在同一朵花中存在两种或多种花药类型，在传粉给蜜蜂的被子植物中具有广泛的分类学意义，但自达尔文以来一直困扰着植物学家。我们检验了关于其进化的两个相互竞争的假说：长期存在的“分工假说”，该假说认为某些花药专门作为蜜蜂的食物奖励，而其他花药则专门用于偷偷传粉；以及我们的新假说，即异型雄蕊是一种逐渐释放花粉的方式，可以最大限度地提高花粉传递。我们研究了异型雄蕊及其相关特征在（柳叶菜科）属中的进化，并在两种异型雄蕊的物种中研究了植物-传粉者的相互作用。在物种间，异型雄蕊与蜜蜂传粉、延迟开裂和一个花药轮的颜色伪装、以及该花药轮在开裂时的运动有关。我们在异型雄蕊物种中的机制研究表明，当每个花药轮开裂时，蜜蜂会注意到并在其上觅食和输出花粉，并且异型雄蕊促进了花粉输出。我们没有发现分工的证据，但有大量证据表明，异型雄蕊是一种逐渐呈现花粉的机制，可能是通过间接的雄性间竞争来提高繁殖成功而进化而来的。
