Pollen movement in a natural population of Arisaema serratum (Araceae), a plant with a pitfall-trap flower pollination system

Arisaema serratum possesses a pitfall-trap flower pollination system. However, little is known about the efficiency and pattern of pollen movement in A. serratum. Thus, the aims of this study are to (1) determine the paternal parents of the seeds and (2) elucidate pollen movement in a natural population. Paternity analysis using microsatellite markers was performed. Seeds were collected from a natural population of A. serratum in 2001 at Horigane, Japan. Small midges became trapped in female spathe tubes during the flowering period. We found that (1) seeds in a fruit were fertilized by multiple sires; (2) seeds sired by a paternal parent were either clumped, exclusively, or randomly distributed on the spadix, depending on the parent; (3) to a great extent, a few males contributed as sires; (4) distance from a female was not a factor in the inequality of reproductive success among males; (5) male reproductive success was not correlated with its size. We conclude that pollen carryover and the trap-flower pollination system are likely to result in multiple paternity and inequality in male success. The aroids (the family Araceae) can be easily recognized by their unique spathe and spadix inflorescence. The spadix is a long cylindrical inflorescence with minute, sessile, and sometimes unisexual flowers. The spathe is a large bract, usually conspicuous in color and shape, that subtends and partly surrounds the spadix. Various diversifications are found in aroid inflorescences, not only in the design and architecture of the spathe and spadix, but also in the arrangement of male and female flowers on an axis. These diversifications of inflorescence have been believed to be closely related to pollination systems (Grayum, 1990). One of the more conspicuous diversifications is a trap system for pollinators. Kugler (1955) defined a trap as the architecture of a flower or an inflorescence that prevents pollinators from escaping immediately by capturing them briefly until pollination is completed. The trap system in aroids is a modification of the whole inflorescence (Richards, 1997). A well-known trap system in aroids is the pitfall trap in the genus Arisaema (van der Pijl, 1953; Vogel and Martens, 2000). In this system, species always form a spathe tube, and 1