Department of Biology, Faculty of Science, Silpakorn University, Sanam Chandra Palace Campus, Nakhon Pathom, Thailand.
Real Jardín Botánico (RJB), CSIC, Madrid, Spain.
J Plant Res. 2024 Sep;137(5):721-743. doi: 10.1007/s10265-024-01572-x. Epub 2024 Aug 29.
Floral diversity of Croton, the second largest genus in Euphorbiaceae, is currently under-explored. Several clades demonstrate an unusual floral morphology, e.g., lower or higher stamen number, bilateral symmetry and reduced ovary, but have never been investigated in a comparative study with typical Croton. This study examined morphology and ontogeny of flowers in nine Croton species from different clades within the genus with light and scanning microscopy, resin sectioning and micro-computed tomography. In staminate flowers, great variations of stamen number and arrangement are observed. The ancestral androecium likely consisted of two or more whorls with the outermost antepetalous stamen whorl developing centrifugally. Modification by reduction of the antepetalous whorl resulted in an outer alternipetalous stamen whorl in Croton section Moacroton, subgenus Quadrilobi. Several species in the subgenus Geiseleria show an independent reduction of stamen numbers by absence of a centrifugal development with the antepetalous whorl the first whorl to develop. Petal losses are observed in the distantly related C. setiger and C. dioicus. Chaotic stamen arrangement is found in C. celtidifolius (subgenus Adenophylli) as a result of a secondary stamen increase. In pistillate flowers, reduction of carpel numbers happened three times in the subgenus Geiseleria. C. monanthogynus has a bicarpellate ovary, while in C. setiger and C. michauxii the ovary is monocarpellate. Reduction of carpel number is linked with merism change and perianth reduction. The ovary in C. michauxii has basal placentation which is unique among all Croton. Moreover, strong bilateral sepals and nectaries are observed in species from section Julocroton. Therefore, the floral diversity of some species in the genus Croton could be explained by developmental modification of an ancestral form via reduction, rearrangement of stamen whorls, and symmetry shifts.
大戟科（Euphorbiaceae）第二大属——巴豆属（Croton）的花卉多样性目前尚未得到充分探索。一些分支表现出不寻常的花形态，例如较低或较高的雄蕊数量、双边对称和缩小的子房，但从未与典型的巴豆属进行过比较研究。本研究使用光学显微镜、扫描电子显微镜、树脂切片和微计算机断层扫描，对来自该属不同分支的 9 种巴豆属植物的花形态和发育进行了研究。在雄花中，雄蕊数量和排列存在很大的变化。原始雄蕊可能由两个或更多轮组成，最外层的花瓣状雄蕊轮向心发育。花瓣状雄蕊轮的退化导致在 Croton 组 Moacroton 和亚属 Quadrilobi 中形成外部对生雄蕊轮。亚属 Geiseleria 的几个种表现出雄蕊数量的独立退化，因为没有离心发育而缺失花瓣状雄蕊轮，该轮是第一个发育的轮。在亲缘关系较远的 C. setiger 和 C. dioicus 中观察到花瓣丢失。在 C. celtidifolius（Adenophylli 亚属）中，由于次生雄蕊增加，导致雄蕊排列混乱。在雌花中，亚属 Geiseleria 中的心皮数量减少了 3 次。C. monanthogynus 具有双心皮子房，而在 C. setiger 和 C. michauxii 中，子房为单心皮。心皮数量的减少与轮数的变化和花被的减少有关。在 C. michauxii 中，子房具有基底胎座，这在所有巴豆属中是独一无二的。此外，在 Julocroton 组的一些物种中观察到强烈的双边萼片和蜜腺。因此，一些巴豆属物种的花卉多样性可以通过减少、雄蕊轮的重新排列和对称性变化来解释。
