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(American Journal of Botany. 2004;91:2051-2059.)
© 2004 Botanical Society of America, Inc.

Reproductive Biology

Temporal changes in calyx tube length of Clematis stans (Ranunculaceae): a strategy for pollination by two bumble bee species with different proboscis lengths1

Ikumi Dohzono2,5, Kazuo Suzuki3 and Jin Murata4

2Makino Herbarium, Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan; 3Faculty of Integrated Arts and Sciences, The University of Tokushima, 1-1 Minami-josanjima, Tokushima 770-8502, Japan; 4Department of Biological Sciences, Botanical Gardens, Koishikawa, Graduate School of Science, University of Tokyo, 3-7-1 Hakusan, Bunkyo-ku, Tokyo 112-0001, Japan

We examined the adaptive significance of a temporal decrease in the calyx tube length of Clematis stans, a dioecious species pollinated by Bombus diversus (long proboscis) and B. honshuensis (short proboscis). We compared visitation frequency, pollen removal, pollen deposition, and fruit set after a single visit among three flower stages, differentiated by calyx tube length. Bombus diversus frequently visited and removed significantly more pollen from long flowers. Bombus honshuensis visited and tended to remove more pollen from short flowers. Both pollinators deposited more pollen in short flowers, resulting in higher fruit set. These results indicate that size correspondence between the proboscis and the calyx tube enhances visitation frequency and pollen removal, but not pollen deposition. Because a single visit does not fertilize all ovules of a flower, multiple visits by two bumble bee species may increase seed production and genetic diversity of offspring. By temporally changing calyx tube length, C. stans can use two bumble bee pollinators and maintain specialized relationships with each. This strategy may be adaptive when the pollinator fauna fluctuates, and is economical because it eliminates costs required to produce different types of flowers. This constitutes a novel pattern of temporal specialization in flower-pollinator relationships.

Key Words: Bombus • Clematis stans • floral morphology • fruit set • pollen removal • Ranunculaceae • specialized pollination system • visitation frequency






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