Sperm Competition and Sexual Selection (1998) p 162
edited by Tim R. Birkhead, Anders Pape Møller

Flowering Competition in Flowering Plants
Lynda F. Delph and Kayri Havens


When stigmatic pollen loads derive from multiple donors, multiple paternity can result. Mixed-donor pollen loads are possible if pollen carryover occurs (pollen carryover refers to pollen from a particular flower being deposited on more than just the next flower visited because not all of the pollen is deposited in one visit — some of it is carried over to subsequent flowers). High rates of pollen carryover are common (see reviews by Robertson 1992 and Morris et al. 1994) and are likely to enhance the opportunity for female choice (Galen and Rotenberry 1988).

Multiply sired fruits have been documented in a number of species (Brown et al. 1986: Ellstrand and Marshall 1986: Broyles and Wyatt 1990; Dudash and Ritland 1991; Muona et al. 1991; D. Campbell, unpublished data; L. Delph, unpublished data) and this phenomenon is likely to be widespread in plants with multiple-seeded fruit. Ibarra-Perez et al. (1996) pointed out that they knew of 'no study in which within-fruit multiple paternity was sought and not found'. Even species in which pollen is packaged in pollinia or polyads (units with several pollen grains), such as Asclepias exaltata and Acacia melanoxylon, have been found to have some multiple paternity within fruits, although the percentages are relatively low (Broyles and Wyatt 1990; Muona et al. 1991).

Some studies have been able to show that multiple paternity is not merely the result of random paternal success following mixed-donor pollen load deposition, but that both seed abortion and fruit abortion by the maternal plant can alter the percentage of multiply-sired fruits matured by a plant. In other words, the maternal plant appears to be involved in the postfertilization sorting of mates. For example, in Raphanus sativus (wild radish) maternal plants preferentially abort stylar ovules (those closest to the style), thereby selecting against those pollen donors that preferentially fertilize stylar ovules (Marshall and Ellstrand 1988). Instead, they favour maturation of ovules towards the basal end of the fruit, especially when stressed. In addition, multiply-sired fruits of wild radish are selectively matured over singly-sired fruits (Marshall and Ellstrand 1986; Marshall 1988, 1991). Additional evidence of maternal influence on mate choice comes from work with O. organensis: when pollen from two donors was applied to stigmas, nonrandom seed abortion occurred and resulted in a more equal paternity of the seed crop than would have been predicted from fertilization success (Havens and Delph 1996). In this species, the pattern of seed was not related to ovule position in the ovary.

Multiple paternity may enhance the fitness of the maternal parent by increasing the number of offspring, the quality of offspring, or both. In some crosses in Costus allenii (Schemske and Pautler 1984) and in R. sativus (Marshall and Ellstrand 1986, 1988), mixed pollen loads increased the fitness of the female parent, measured as total seed weight. However, several other studies have not found a fitness benefit associated with mixed pollen loads (Lee and Bazzaz 1982; Bawa and Webb 1984; Sork and Schemske 1992). Karron and Marshall (1990, 1993) have tested two hypotheses pertaining to the fitness advantages of multiplysired progeny in R. sativus. The 'elbow room' hypothesis predicts that resource partitioning will increase with genetic diversity; therefore, half sibs in competition with each other will be more fit than full sibs. The 'lottery' hypothesis predicts that with genetically diverse offspring, a maternal plant will be more likely to produce successful phenotypes for each microsite in a patchy environment. Their studies did not provide empirical support for either hypothesis and they suggest further work is needed to determine why R .sativus preferentially matures multiply-sired fruit.