School of Ecology and Environment Sciences, Yunnan University, Cuihubeilu No. 2, Kunming, 650091, Yunnan, China.
Faculty of Environmental Earth Science, Hokkaido University, Kita-ku, Sapporo, 060-0810, Japan.
J Plant Res. 2020 Mar;133(2):217-229. doi: 10.1007/s10265-020-01168-1. Epub 2020 Feb 3.
Stone oaks, or Lithocarpus species of Fagaceae are ecologically important canopy trees in the tropical and subtropical forests over East Asia, and the fruits of which are important food sources for insects and vertebrates there. The great fruit morphological variation of this genus represents two fruit types, acorn and enclosed receptacle fruit types. However, the evolutionary mechanisms of differentiation into these two fruit types with contrasting morphology remain a puzzle. To reveal the morphogenetic properties of two fruit types, we observed tissue differentiation and development among 20 Lithocarpus species from fruit set to maturity. Unlike in fruits of Quercus, the endocarp differentiation in Lithocarpus fruits occurred later than exocarp and mesocarp. Cupules provided further protection of developing seeds, particularly of acorn-type fruits. Fruits of Lithocarpus and Quercus acorns share similar insect predators. At fruit set, both acorn and enclosed receptacle types were largely identical, with similar tissue morphology and the sequence of differentiation. The distinct difference between two fruit types at maturity came from varied rates and degrees of development between the pericarp and receptacle tissues. We found that heterochrony between two tissues could create substantially divergent ecological strategies for protection and dispersal of their seeds, which is essential for the evolution of two fruit types.
石栎属（Lithocarpus）壳斗科植物是东亚热带和亚热带森林中具有重要生态意义的乔木层树种，其果实是该地区昆虫和脊椎动物的重要食物来源。该属果实具有显著的形态多样性，可分为两类，即橡果和封闭的聚合果。然而，这两种具有相反形态的果实类型是如何分化的，其进化机制仍不清楚。为了揭示两种果实类型的形态发生特性，我们观察了 20 种石栎属植物从果实开始形成到成熟的组织分化和发育过程。与栎属不同，石栎属果实的内果皮分化晚于外果皮和中果皮。壳斗进一步保护了发育中的种子，特别是橡果类型的果实。石栎属和栎属橡果的果实都有相似的昆虫捕食者。在果实开始形成时，橡果和封闭的聚合果类型在很大程度上是相同的，具有相似的组织形态和分化顺序。两种果实类型在成熟时的明显区别来自于果皮和容器组织之间的发育速度和程度的不同。我们发现两种组织之间的时变关系可以为它们的种子的保护和传播创造出截然不同的生态策略，这对于两种果实类型的进化至关重要。
