Regulation of the mutualism between yuccas and yucca moths: intrinsic and extrinsic patterns of fruit set

Abstract

In plants that produce many more flowers than fruit, nonrandom patterns of fruit set arise from (1) factors intrinsic to inflorescence architecture, such as flower position or timing in the blooming sequence, and (2) factors extrinsic to the plant, such as pollinator visitation patterns. Here, we address how intrinsic and extrinsic factors drive fruit set in the interaction between Yucca kanabensis McKelvey and its pollinating moths. On inflorescences from which moths were excluded and all flowers were hand pollinated, the flowers most likely to produce fruit were (1) flowers from the first of three waves of flowering, (2) earlier opening flowers but not the earliest, and (3) flowers lower on the inflorescence but not the lowest. However, inflorescences experiencing natural levels of pollination and oviposition showed no effect of flowering wave, and flowers later in the flowering sequence and higher on the inflorescence were the ones most likely to produce fruit. The intrinsic patterns of fruit set were associated with differences among flowers, with the largest flowers being wave 1 flowers and flowers lower on the inflorescence. However, these differences among flowers did not translate into commensurate differences among fruit.Key words: mutualism, abscission, fruit, yucca, yucca moth.

Consequences of variation in flowering time within and among individuals of Mertensia fusiformis (Boraginaceae), an early spring wildflower

“…No reductions in seed quantity or quality were observed in 2007, when early flowers set few seed, or in bagged plants on which few flowers were pollinated. Declining fecundity in late flowers has been observed in many plants (Nicholls, 1987; Medrano et al, 2000; Humphries and Addicott, 2004; Kliber and Eckert, 2004), although in some cases it may be due to structural constraints (e.g., distal positioning of late flowers when flowering is acropetal [Diggle, 1997], or declining ovule number in later flowers [Thomson, 1989]). In Cryptantha flava (also Boraginaceae), for example, seed set is higher in early flowers, but does not increase in later flowers if early flowers are prevented from setting seed (Casper, 1984).…”

“…Flowering distributions at the level of individual plants have received less attention than population‐level patterns, but these can be positively skewed, too (Thomson, 1985; Blionis et al, 2001). Finally, allocation of resources within plants also favors early flowers, these almost invariably having higher levels of fruit set if pollination is adequate (Stephenson, 1981; Thomson, 1989; Diggle, 1997; Ladio and Aizen, 1999; Medrano et al, 2000; Humphries and Addicott, 2004; Kliber and Eckert, 2004; Brown and McNeil, 2006; Buide, 2008).…”

“…For example, declining seed set is often observed in later or distal flowers within inflorescences (Stephenson, 1981; Thomson, 1989). So far, most evidence supports two alternative (but not mutually exclusive) hypotheses to explain this observation: The earliest‐fertilized ovules may take precedence in within‐plant competition for resources (Stephenson, 1981; Medrano et al, 2000; Humphries and Addicott, 2004), or the earliest flowers may have structural advantages such as proximity to nutrient sources or possession of a greater numbers of ovules (Thomson, 1989; Diggle, 1997; Buide, 2008). However, temporal variation in pollinator availability could also generate within‐plant variation in seed set.…”

Florivores Limit Cost of Mutualism in the Yucca–yucca Moth Association

“…Until recently, the obligate pollination mutualism between yuccas and yucca moths has been examined nearly exclusively with regard to the pairwise mutualism between the plants and pollinators. Furthermore, considerable attention has been devoted to understanding the intrinsic mechanisms that plants use to reduce costs in this mutualism (e.g., Pellmyr and Huth 1994, Addicott 1998, Addicott and Bao 1999, Shapiro and Addicott 2003, Humphries and Addicott 2004, but these studies have primarily focused on mechanisms pertaining to the pairwise interaction. Furthermore, considerable attention has been devoted to understanding the intrinsic mechanisms that plants use to reduce costs in this mutualism (e.g., Pellmyr and Huth 1994, Addicott 1998, Addicott and Bao 1999, Shapiro and Addicott 2003, Humphries and Addicott 2004, but these studies have primarily focused on mechanisms pertaining to the pairwise interaction.…”

“…Some notable exceptions include experiments that show that the presence of ants on yucca infructescences decreases seed predation by parasitic yucca moths, but does not deter pollinator moths (Perry et al 2004) and that parasitoids that attack yucca moth larvae can reduce the number of seeds consumed in a fruit (Crabb and Pellmyr 2006). Furthermore, considerable attention has been devoted to understanding the intrinsic mechanisms that plants use to reduce costs in this mutualism (e.g., Pellmyr and Huth 1994, Addicott 1998, Addicott and Bao 1999, Shapiro and Addicott 2003, Humphries and Addicott 2004, but these studies have primarily focused on mechanisms pertaining to the pairwise interaction. For example, yuccas can selectively abscise yucca fruit with many moth eggs within them and limit the number of eggs within retained fruits Huth 1994, Marr andPellmyr 2003; but see Addicott 2004, Holland andDeAngelis 2006).…”