壳斗演化的假说及分子系统学和化石证据

A hypothesis on cupule evolution and the evidence from molecular phylogenies and fossils

壳斗作为壳斗科Fagaceae的重要特征,其起源、演化和多样性形成长期以来为系统学家们所关注,有众多的观点和假说。本文在综合这些假说的基础上,结合分子系统学和大化石证据,讨论了壳斗演化的规律。研究结果支持壳斗具枝性性质,来源于二歧聚伞花序最外侧小枝的假说。在比较3个壳斗演化模型的基础上,根据4种分子系统关系的一致性重建了壳斗的演化历史:壳斗是单系起源;水青冈属的2果4裂瓣壳斗是最早分化的壳斗类型,三棱栎型壳斗是较晚分化的类型;并支持具有内裂瓣的金鳞果类型壳斗可能与其他横切面为圆形的壳斗有较近的共同祖先。这一结果也得到了多个大化石证据的支持。壳斗科多个分子系统关系的一致性和壳斗化石证据共同证实了壳斗演化的规律是从开裂向裂瓣融合、逐步简化、多向发展;支持开裂壳斗、坚果三角形、较小、具棱或狭翅是祖征,融合壳斗、坚果圆形、较大、不具棱是衍征。这些结果为进一步探索壳斗科的演化历史和系统发育提供了新的证据。

The cupule is the most distinguishable character of the Fagaceae and has been the focus of much research. Many hypotheses have been proposed for the origin, evolution and morphological features of the cupule. These were based on different types of evidence, but here we used molecular phylogenetic and fossil evidence. Our results supported the view that cupule valves originated from the outermost branches of the dichasium, and that cupules of the Fagaceae were distinct from other cupule-like organs with perianth origins in Fagales. There were three models of cupule evolution, each characterized by distinct evolutionary tendencies, single, dual and multi-directional respectively. Based on these models, we reconstructed the cupule’s evolution using the consensus of four recent molecular phylogenies from different DNA fragments. Our results indicated that cupules had a monophyletic origin and that the Fagus cupule, with four cupule valves enclosing two triangular acorns, represented the ancestral type, the Trigonobalanus cupule was the inferior-ancestral type, and the Chrysolepis cupule which possesses inner valves, shared a common ancestor with other roundish cupules. Our conclusions are also unequivocally supported by fossil evidence, with affinity of extant homologous cupule. The consensus from four molecular phylogenetic hypotheses along with the age of fossil cupules that are similar to extant Fagaceae taxa indicate that the cupule evolved in multi-directions, and that the small triangular, narrow winged fruits surrounded by a dehiscent cupule was plesiomorphy, and that a bigger roundish fruit surrounded by an indehiscent cupule was apomorphy. All these results provide the new evidence to explore evolution and phylogeny of Fagaceae.