Croton is a mega-diverse genus of more than 1,200 species with great morphological diversity and highly dimorphic flowers. Staminate flowers generally possess petals and a variable number of stamens and lack of an ovary. Pistillate flowers generally lack petals or have filamentous structures instead; stamens are lacking, and the ovary is generally tricarpellate with divided styles. However, well-developed petals can be found in pistillate flowers of some African species and two New World sections, i.e., sect. Alabamenses and sect. Eluteria subsect. Eluteria. Our objectives are to compare ontogeny in dimorphic flowers of Croton which may elucidate the origin of petals, homology of the filamentous structures, nectaries, and diversity of the androecium. The development of staminate and pistillate flowers of C. alabamensis (sect. Alabamenses) and C. schiedeanus (sect. Eluteria) was studied under the scanning electron microscope (SEM) and compared with C. chilensis (sect. Adenophylli) which has filamentous structures in pistillate flowers. In staminate flowers, petals develop in alternation with sepals and later the outermost stamen whorl develops opposite to the petals. In a much later stage, nectar glands emerge in alternation with the petals. In pistillate flowers, filamentous structures and petals share the same early development in alternation with the sepals. However, growth of the filamentous structures of C. chilensis becomes arrested, while petals of C. alabamensis and C. schiedeanus develop similar to those of the staminate flowers. The main conclusions of the study are: (1) Petals and filamentous structures are homologous based on their location and shape in early development. (2) Nectaries with variable morphology develop in antesepalous position and probably represent receptacular outgrowths and are not staminodial. (3) Staminate flower development displays an unexpected diversity, including a floral cup in C. alabamensis. All investigated species have an unusual centrifugal initiation of the second stamen whorl. (4) The possible evolutionary loss and potential regain of petals among different species of Croton is discussed in relation to heterochrony. We suggest that the filamentous structures in pistillate flowers represent pedomorphic forms of petals of staminate flowers, and that the well-developed petals in C. alabamensis and C. schiedeanus are derived via a developmental reversion.

中文翻译：

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一些不寻常巴豆（大戟科）的雄花和雌花之间的不同发育途径

巴豆是一个拥有 1,200 多个物种的超级多样化属，具有巨大的形态多样性和高度二态性的花朵。雄花通常具有花瓣和可变数量的雄蕊，并且没有子房。雌花通常没有花瓣或具有丝状结构；雄蕊缺乏，子房一般为三心体，花柱分开。然而，在一些非洲物种的雌蕊花和两个新世界部分，即sect. 中，可以找到发育良好的花瓣。阿拉巴门斯和教派。Eluteria 亚科。Eluteria。我们的目标是比较巴豆二形花的个体发育，这可以阐明花瓣的起源、丝状结构的同源性、蜜腺和雄蕊的多样性。C. alabamensis (sect. Alabamenses) 和 C. 雄花和雌花的发育。在扫描电子显微镜 (SEM) 下研究了 schiedeanus (sect. Eluteria) 并与在雌蕊花中具有丝状结构的 C. chilensis (sect. Adenophylli) 进行了比较。在雄花中，花瓣与萼片交替发育，后来最外面的雄蕊轮发育在花瓣的对面。在更晚的阶段，花蜜腺与花瓣交替出现。在雌蕊花中，丝状结构和花瓣与萼片交替出现相同的早期发育。然而，C. chilensis 的丝状结构的生长被阻止，而 C. alabamensis 和 C. schiedeanus 的花瓣发育类似于雄花的花瓣。研究的主要结论是：（1）花瓣和丝状结构在早期发育的位置和形状上具有同源性。(2) 形态各异的蜜腺发育在对生位置，可能代表囊状生长物，而不是退化雄蕊。(3) 雄花发育表现出意想不到的多样性，包括阿拉巴马花中的花杯。所有被研究的物种都有一个不寻常的第二雄蕊轮离心起始。(4) 讨论了与异时性相关的不同巴豆属植物之间可能的进化损失和花瓣恢复的可能性。我们建议雌蕊花中的丝状结构代表雄花花瓣的足形形式，并且 C. alabamensis 和 C. schiedeanus 中发育良好的花瓣是通过发育逆转衍生的。