Sex-Allocation Plasticity in Hermaphrodites of Sexually Dimorphic Fragaria Virginiana (Rosaceae)
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231 Sex-allocation plasticity in hermaphrodites of sexually dimorphic Fragaria virginiana (Rosaceae) Eric J. Bishop, Rachel B. Spigler, and Tia-Lynn Ashman Abstract: Sex-allocation plasticity is thought to play an important role in the evolution of separate sexes in plants. Ac- cordingly, much attention has been paid to environmentally induced variation in fruit and seed production in sexually di- morphic species, but we know little about whether this variation arises as a direct response to environmental variation or is instead an indirect consequence of changes in plant size. In this study, we characterize sex-allocation plasticity across a resource gradient for several reproductive traits in hermaphrodites of gyno(sub)dioecious Fragaria virginiana Duch. We find significant plasticity, on average, for flower number, proportion fruit set, ovule number, proportion seed set, and run- ner number in response to resource variation. Plasticity of most traits examined tended to be at least partially independent of variation in plant size, suggesting that it is not simply an indirect consequence of plant allometry. Moreover, we find genetic variation for plasticity of key reproductive traits. Comparisons of relative plasticities among traits reveal that F. virginiana hermaphrodites are more likely to adjust female investment via changes in fruit and seed set than ovule num- ber, and most likely to adjust male investment via flower number rather than anther number or pollen per anther, although there is genotypic variation for plasticity in pollen per anther. Evidence of within-population variation can logically be ex- tended to suggest that variation in hermaphrodite sex-expression seen among natural populations of F. virginiana may be due, at least in part, to sex-allocation plasticity. Key words: dioecy, gynodioecy, phenotypic plasticity, sex allocation, strawberry, subdioecy. Re´sume´ : On croit que la plasticite´ sexuelle joue un roˆle important dans l’e´volution de la se´paration des sexes chez les plantes. Conse´quemment, on a accorde´ beaucoup d’attention aux variations induites par le milieu dans la production de fruits et de graines chez les espe`ces sexuellement dimorphiques, mais nous connaissons peu de choses a` savoir si cette va- riation constitue une re´action directe au milieu ou plutoˆt une conse´quence indirecte de la modification des dimensions de la plante. Les auteurs ont caracte´rise´ la plasticite´ de l’allocation sexuelle le long d’un gradient de ressources portant sur des caracte`res reproductifs chez les plants hermaphrodites du Fragaria virginiana Duch. gyno(sub)dioı¨que. Ils ont observe´ une plasticite´ significative, en moyenne, pour le nombre de fleurs, la proportion des mises a` fruit, le nombre d’ovules, la proportion des mises a` graine, et le nombre de stolons, en re´action a` une variation des ressources. La plasticite´ des caracte`- res observe´s a tendance a` eˆtre au moins partiellement inde´pendante en grosseur des plants, ce qui sugge`re qu’il ne s’agit pas simplement d’une conse´quence indirecte de l’allome´trie. De plus, ils ont observe´ une variation ge´ne´tique pour la plas- ticite´ de caracte`res reproductifs cle´s. Des comparaisons de plasticite´s relatives entre les caracte`res re´ve`lent que les herma- phrodites du F. virginiana augmenteront plus vraisemblablement l’investissement femelle via des modifications dans la mise a` fruit et a` graine, que dans le nombre d’ovules, et ajusteront l’investissement maˆle via le nombre de fleurs plutoˆt que le nombre d’anthe`res ou la quantite´ de pollen par anthe`re, bien qu’il n’y ait pas de variation ge´notypique pour la va- riation de pollen par anthe`re. On peut logiquement e´tendre la preuve de variation au sein de la population et sugge´rer que la variation dans l’expression sexuelle hermaphrodite observe´e entre les populations naturelles du F. virginiana pourrait eˆtre due, au moins en partie, a` la plasticite´ de l’allocation sexuelle. Mots-cle´s:dioı¨que, gynodioı¨que, plasticite´ phe´notypique, allocation sexuelle, fraise, subdioe´cie. [Traduit par la Re´daction] Introduction important phenomenon influencing the evolution of dimor- Sex-allocation plasticity, the ability of a hermaphrodite to phic sexual systems in plants (reviewed in Delph 2003; change allocation to male and female sex functions in re- Delph and Wolf 2005). Sex-allocation plasticity is thought sponse to changes in the environment, is thought to be an to influence both the likelihood that female individuals es- tablish in hermaphroditic populations (i.e., the evolution of gynodioecy), as well as whether hermaphrodites are main- Received 29 July 2009. Published on the NRC Research Press tained once females are abundant and males exist (i.e., sub- Web site at botany.nrc.ca on 26 February 2010. dioecy). For example, in low resource environments, if E.J. Bishop, R.B. Spigler,1 and T.-L. Ashman.2 Department of hermaphrodites preferentially reduce investment in female Biological Sciences, University of Pittsburgh, Pittsburgh, PA function to maintain or increase male function, females 15260-3929, USA; Pymatuning Laboratory of Ecology, could more easily achieve the seed fertility advantage re- Linesville, PA 16424, USA. quired to invade and establish (Delph 1990, 2003; Dorken 1Corresponding author (e-mail: [email protected]). and Mitchard 2008). In resource rich environments, how- 2Corresponding author (e-mail: [email protected]). ever, sexually plastic hermaphrodites that take advantage of Botany 88: 231–240 (2010) doi:10.1139/B10-005 Published by NRC Research Press 232 Botany Vol. 88, 2010 reduced trade-offs between male and female function and in- 1985; Sutherland 1987; Ashman and Penet 2007). Because crease investment in female function will reduce females’ genetic, functional, and (or) developmental constraints vary seed-fertility advantage (Dorken and Mitchard 2008), and among traits (reviewed in DeWitt et al. 1998; van Kleunen ultimately, this will be reflected in the equilibrium fre- and Fischer 2005) and trait plasticities as well (Waitt and quency of females. Observations of higher female frequen- Levin 1993; Pigliucci et al. 2003), some traits may be less cies and greater sex specialization in gynodioecious canalized than others. While previous work has revealed populations under resource-poor environments (Delph 1990; phenotypic and genetic correlations among reproductive Costich 1995; Wolfe and Shmida 1997; Ashman 1999a; traits in hermaphrodites (reviewed in Ashman 2003), we do Vaughton and Ramsey 2002; Delph 2003; Barr 2004; Case not know whether plasticities of these traits are integrated. and Barrett 2004; also see citations in Ashman 2006) are Given the predicted differences in fitness gain curves in re- consistent with these hypotheses. lation to resource availability and (or) plant size for male Yet in spite of the importance of the role that plasticity and female function, we might expect female-function traits, may play in sexual system evolution, we still know rela- in general, to be more plastic than male-function traits, as tively little about the mechanisms underlying variation in has been shown in other studies (Vogler et al. 1999; Sarkis- hermaphrodite sex allocation across natural resource gra- sian et al. 2001; Wolfe and Mazer 2005), but we do not dients. It is often assumed that this variation is due to plasti- know whether specific components of sex expression are city, but it may simply reflect past divergent selection on more important than others as conduits to variable sex ex- sex allocation across environments (Ashman 2006; Case pression. and Ashman 2007). Even if the variation in sex expression In this study, we characterize plasticity of multiple com- seen across environments is due to plasticity, we do not ponents of sex-expression in hermaphrodites of gyno(sub)- know whether such variation represents a direct response to dioecious Fragaria virginiana Duch. in response to resource variation or whether it is merely an indirect conse- variation in resource availability. Fragaria virginiana quence of changes in plant size with resource availability. presents an ideal system in which to study sex-allocation Such a distinction is important because it can inform on plasticity for several reasons. First, it is clonal, thus a given whether sex-allocation plasticity in response to resource genotype is prone to encounter different environments in a availability is a separate phenomenon from well-described heterogeneous habitat. Second, this clonality lends itself to allometric plant responses or size-dependent sex allocation experimental replication of genotypes in a controlled design. (de Jong and Klinkhamer 1989; Klinkhamer et al. 1997; Sar- Third, previous research suggests that plasticity of at least kissian et al. 2001; Zhang and Jiang 2002; Paquin and Aars- one trait, proportion fruit set, occurs in hermaphrodites and sen 2004; Ashman 2006). Theories based on both is associated with among-population variation in sex ratio mechanisms (direct vs. size-mediated) have similar predic- (i.e., the frequency of females) (Ashman 2006). Specifically, tions: individuals at low resources or small size should in- we asked whether reproductive traits are phenotypically vest more heavily in male than female function because of plastic in hermaphroditic F. virginiana, and whether there is the high cost of fruits relative to pollen, whereas individuals genetic variation for plasticity of these traits. In addition, we at high resources or large plant sizes should