Revisiting pollination mode in chestnut ( Castanea spp .): an integrated approach
Résumé
Wind, insects, or both? The pollination mode of chestnuts, an important genus of nut-producing forest trees of the Fagaceae family, is still unclear. We revisit this old question using an integrated approach, focusing on cultivated Castanea sativa trees and hybrids in South-western France. We first conducted a large-scale insect isolation experiment. We then monitored 16 trees, focusing on flowering phenology, flower abundance and insect visits. Half of these trees are male-sterile, helping explore the role of pollen in insect attraction. Finally, we characterized the pollination syndrome of chestnuts and contrasted it with that of wind-pollinated oaks using original and published data. Chestnut female flowers have erect styles resembling stamens from male flowers, a probable case of intersexual mimicry. The tree's unusual phenology includes two peaks of pollen production. Pollinator exclusion experiments demonstrated a predominant role of insects in chestnut pollination. Flowering trees attract large numbers of beetles, bees and flies. In contrast, the few insects seen on female flowers (66 in 32 h of observation, <2% of the total) were mostly beetles. Compared to male-fertile trees, male-sterile trees attract fewer insects overall but their female flowers are more frequently visited and they have higher fruit set. All chestnut flower traits examined, such as the tiny pollen grains and the huge rate of pollen production, resulting in the highest pollen/ovule ratio ever reported in plants, are compatible with a beetle pollination syndrome. The high uncertainty of this pollination mode and its convergence with wind pollination explain the pervading confusion regarding chestnut pollination.