花是如何起源的?

How did the flower originate?

花的起源是植物学和进化生物学中的重要问题,长期以来备受关注.近年来,分子系统学、进化发育生物学和基因组学的兴起和蓬勃发展,不仅为花起源的研究带来了新思路,而且更新了人们对该问题的认识.基于这些领域的新进展,人们不仅纠正了前人在理论和模型中的错误、提出了可验证的新假说,而且揭示了花发育MADS-box基因的进化对花起源的贡献.然而,已有研究只能笼统地解释花作为一个整体是如何起源的,并不足以全面揭示花和花器官起源的根本机制.本文在总结与花和花器官起源相关理论和假说的基础上,介绍人们对花和花起源问题的新见解,探讨花起源机制的复杂性,指出花的起源和基本结构的建立涉及诸多"小"的关键性状的改变,认为只有对这些性状进化的分子机制进行研究才能真正揭示花起源的内在原因.

The flower is a characteristic feature of angiosperms. The innovation and diversification of the flower significantly improve the reproductive efficiency and environmental adaptation of angiosperms, and thus contribute to the great success of angiosperms in terrestrial ecosystem. How the flower originated is a major aspect of Darwin＇s ＂abominable mystery＂ of the origin and rapid diversification of angiosperms, and has long been a basic question of evolutionary biology and botany with intense investigations. Over the past two centuries, many theories and hypotheses have been proposed to explain the origin of the flower based on a broad spectrum of multidisciplinary research involving comparative morphology, paleontology, anatomy, and developmental biology. However, the answer to how the flower originated has remained controversial because the phylogeny of the seed plants was poorly resolved and the molecular basis of flower development was not unraveled. In 2005, Science posed this question as one of the 125 most challenging scientific questions in a special issue to commemorate the 125 th anniversary of the journal, implying that the study of the process and mechanisms of the origin of the flower is still a hot topic of current evolutionary biology and botany. In recent years, thanks to the rise and rapid progress in molecular phylogenetics, evolutionary developmental biology（evo-devo）, and genomics, new ideas have been brought and our knowledge on the origin of the flower has been updated. Now, it is generally believed that（i） the flower is a determinate, compressed, and bisexual reproductive axis, which consists of perianth, androecium, and gynoecium from the outer to the inner;（ii） the flower did not originate abruptly but evolved gradually, with bisexuality being the first step, followed by the compression of reproductive axis and the evolution of the floral determinacy, and the floral structural elements originated at different evolutionary stages; and（iii） the floral MADS-box genes that specify the identities of floral organs have experienced duplication events prior to the origin of the angiosperms and the changes in their expression patterns, functions, and modes of protein-protein interaction have contributed to the origin of the flower. The new advances have not only corrected some mistakes in previous theories and models, stimulated the proposal of new and testable hypotheses, but also revealed the contribution of the evolution of floral MADS-box genes to the origin of the flower. However, previous studies can only provide general explanations for how the flower as a whole originated, but are insufficient for a complete understanding of the underlying mechanisms of the origins of different floral organs. In this review, we summarize the theories and hypotheses explaining the origin of the flower and floral organs. On the basis of this framework, we provide the current understanding of the flower itself and its origin. In our opinion, changes of many ＂small＂ key characters have been involved in the origin of the flower and the establishment of its basic structure. We propose that elucidation of the mechanisms underlying the evolution of these characters is the prerequisite for understanding the intrinsic basis for the origin of the flower.