Effective Mate Choice in Wild Radish: Evidence for Selective Seed Abortion and Its Mechanism

Abstract

Maternal plants that can affect the paternity of their seeds may influence the quality of their offspring. However, mate choice in plants has proved difficult to demonstrate because it is usually confounded by the effects of pollen competition and because its mechanisms are not well understood. We show here that stressed wild radish plants selectively abort seeds sired by particular donors. This selection occurs by position-dependent seed abortion and position-dependent resource allocation within fruits, coupled with position-dependent fertilization of ovules by pollen donors. The result is that stressed plants select for pollen donors that sire seeds in the basal and middle, rather than the stylar, thirds of fruits. Thus, in wild radish, the position of seeds fathered by pollen donors may act as a selective arena. Because of this position-dependent abortion, successful fathers sire the seeds most likely to mature and to be larger than average. However, siring these high-quality seeds may reduce the quantity of seeds sired by a donor after mixed pollination because pollen tubes must grow past fertilizable stylar ovules to reach middle and basal ovules, allowing pollen tubes from other donors to sire many seeds in the less favored positions. Since multiple paternity is common in wild radish fruits in the field, selective seed abortion probably occurs there as well. Finally, the position-dependent seed abortion necessary to this effective maternal choice occurs in other plants; thus, the mechanism suggested here may be a general mechanism of effective mate choice in plants that selects for position of fertilization, not just speed of pollen-tube growth.