Claßen-Bockhoff Regine, Baczyński Jakub, Hanke Veronika, Henkes Svenja Sibylla, Ferdinand Nadine
Institute of Organismic and Molecular Evolution (iomE), Johannes-Gutenberg-University Mainz, Germany.
Institute of Evolutionary Biology, Faculty of Biology, Biological and Chemical Research Centre, University of Warsaw, Warsaw, Poland.
Ann Bot. 2025 Jul 9. doi: 10.1093/aob/mcaf144.
The capitulum of Asteraceae has traditionally been interpreted as a condensed raceme. However, morphological studies challenge this view, indicating that the capitulum does not arise from an inflorescence meristem (IM), but from a determinate floral unit meristem (FUM). Terminology, combined with conflicting evidence from developmental genetics, has hindered the formulation of a coherent evolutionary scenario for the origin of the capitulum. In this paper, we aim to refine the floral unit concept and critically re-examine the hypothesis that ray flowers in Asteraceae represent remnants of ancestral thyrsoid branching.
Approaching from evolutionary-developmental morphology, we performed a detailed analysis of capitulum development in 20 highly diverse Asteraceae species, encompassing three subfamilies and 11 tribes using SEM. We focused on early capitulum development, ray flower formation and changes in meristem geometry.
We show that the capitulum meristem is determinate and exhibits developmental features characteristic of flowers, thereby fulfilling the definition of a FUM. Continuous meristem expansion changes geometrical conditions and triggers spontaneous fractionation of flower meristems. The early developmental delay of ray flowers and the rare occurrence of bidirectional fractionation in the capitulum meristem can be explained morphologically by local expansion dynamics and mechanical pressure.
Our findings support the hypothesis that the capitulum meristem is distinct from an IM and instead recapitulates developmental properties of a flower meristem at a higher level of organization. This challenges the phylogenetic view that the capitulum evolved through gradual transformation of an ancestral thyrse. Instead, we propose that a single developmental shift - from an indeterminate reproductive meristem to a determinate FUM - was sufficient to give rise to the capitulum. The early determinacy of the meristem and the insertion of an additional step of fractionation are best explained by heterochronic changes, such as ontogenetic abbreviation and prolongation.
菊科的头状花序传统上被解释为密集的总状花序。然而，形态学研究对这一观点提出了挑战，表明头状花序并非起源于花序分生组织（IM），而是来自一个有限的花单元分生组织（FUM）。术语问题，再加上发育遗传学中相互矛盾的证据，阻碍了对头状花序起源形成一个连贯的进化场景。在本文中，我们旨在完善花单元概念，并批判性地重新审视菊科中舌状花代表祖先聚伞圆锥花序分支残余的假说。
从进化发育形态学角度出发，我们使用扫描电子显微镜（SEM）对20种高度多样化的菊科物种的头状花序发育进行了详细分析，这些物种涵盖三个亚科和11个族。我们重点关注头状花序的早期发育、舌状花的形成以及分生组织几何形状的变化。
我们表明头状花序分生组织是有限的，并表现出花的发育特征，从而符合FUM的定义。分生组织的持续扩展改变了几何条件，并触发了花分生组织的自发分离。舌状花的早期发育延迟以及头状花序分生组织中双向分离的罕见发生，可以通过局部扩展动态和机械压力从形态学上进行解释。
我们的研究结果支持这样的假说，即头状花序分生组织不同于IM，而是在更高的组织水平上概括了花分生组织的发育特性。这对头状花序是通过祖先聚伞圆锥花序的逐渐转变而进化的系统发育观点提出了挑战。相反，我们提出，从无限的生殖分生组织到有限的FUM的单一发育转变足以产生头状花序。分生组织的早期确定性和额外分离步骤的插入，最好用异时性变化来解释，如个体发育的缩短和延长。
