Fruit development of Lithocarpus (Fagaceae) and the role of heterochrony in their evolution

The fertilization process in Lithocarpus dealbatus (Fagaceae) and its implication on the sexual reproduction evolution of Fagales

MAIN CONCLUSION

The pistillate flowers of Lithocarpus dealbatus show two pollen tube (PT) arresting sites (the style-joining and micropyle) within the pistil during the postpollination-prezygotic stage. The PT, arrested at the pre-ovule stage, enhanced PT competition allowing the most compatible PTs to enter the ovary to ensure the highest fertilization success. During the shift from animal pollination to wind pollination, plants require a series of changes in reproductive traits. The mode of pollination is striking labile in Fagaceae. Lithocarpus is insect pollinated and is closely related to wind-pollinated Quercus. Little is known about the sexual reproduction of Lithocarpus. This study aimed to reveal the sexual reproduction of Lithocarpus dealbatus and to explore the evolutionary pattern of the key sexual reproduction traits to better understand their possible role in labile pollination. We found that after pollination, L. dealbatus PTs grew slowly in the style reaching style-joining in mid-January of the second year; then PT growth was arrested at style-joining for four months. Only two to three PTs resumed growth in mid-May to reach the micropyle, where PT growth ceased for one month before one PT resumed growth and passed through the micropyle to the embryo sac. Fagaceae showed a generalized mating system. Vast pollen production, small-sized pollen grains, long stigmatic receptive time, and reduced perianth were compatible with beetle pollination syndrome, representing the plesiomorphic status in Fagaceae. A large stigmatic surface and dry pollen grains linked to wind pollination might be independently derived several times in fagaceous lineages. Beetle pollination syndrome can cope with the uncertainty of pollinators to ensure conspecific pollen capture, which represents pre-adaptation status and has a selective advantage when conditions change, favouring wind pollination. The arrest of the PT at style-joining is a unique mechanism in later derived fagaceous lineages to enhance PT competition and promote outcrossing.

The fruit morphometric variation and fruit type evolution of the stone oaks (Fagaceae, Lithocarpus)

BACKGROUND

The great species diversity of Lithocarpus is associated with interspecific fruit morphological variation, represented by acorn (AC) and enclosed receptacle (ER) fruit types. Species representing both fruit types co-occur in the same forests and share two distribution centers in southern China and southeastern Asia. The predation selection hypothesis suggests that the fruit morphological mechanical trade-off between two fruit types could represent divergent dispersal strategies under varied predation pressures. By integrating phylogenetic construction with fruit morphometric study, we tried to verify the predation selection hypothesis and elucidate the fruit type evolution of Lithocarpus, which is critical in interpreting the distribution and diversification of the genus.

RESULTS

We identified the functional trade-off between the two fruit types: ER species have bigger seeds which are enclosed mainly by receptacle representing stronger physical defense; whereas the seeds of AC species are smaller and enclosed mainly by thin pericarp representing lower mechanical protection. Despite some reversals from ER back to AC, the ancestral state reconstruction in combination with thermal analysis supports the hypothesis that ER is the derived fruit type from AC-like ancestors independently across all clades.

CONCLUSION

Our results support the predation selection hypothesis by verifying the mechanical trade-off between the two fruit types. We propose a divergent selection theory for the two fruit types: the seed size and mechanical defense of AC species become smaller, whereas those of ER species become larger and require more morphological modifications in the receptacle. This signified the importance of the receptacle in differentiating the two fruit types and in the fruit morphological modification through evolutionary time. We found that the ER-type species evolved independently in all clades and across varied climates from tropical to warm temperate regions. As ER fruits are products of convergent evolution, we propose to examine the predation and dispersal variation between two fruit types in the future to verify whether predation selection is the reason behind fruit type evolution of the stone oaks.