Tuesday, August 3, 2010

PS 44-149: Sex-allocation plasticity and the stability of subdioecy in Fragaria virginiana (Rosaceae)

Rachel B. Spigler and Tia-Lynn Ashman. University of Pittsburgh


Resource-dependent plasticity in sex expression, often referred to as sex-allocation plasticity, has been suggested to play an important role in the evolution of separate sexes (dioecy) in plants by promoting both the invasion of females in hermaphrodite populations (gynodioecy) and the maintenance of hermaphrodites once females are abundant and males exist (subdioecy). In subdioecious Fragaria virginiana (Rosaceae), sex is determined by loci on a proto-sex chromosome, yet fruit set in hermaphrodites from at least one population is known to be plastic in response to resource availability, underscoring the importance of understanding both the genetic basis of and plasticity in sex expression in order to identify key parameters influencing the evolution of plant sexual systems and within-population sex ratios. Identifying and characterizing these parameters will require ascertaining the degree to which plasticity occurs across populations and whether plasticity varies among sexes.  In this study, we characterized sex-allocation plasticity in F. virginiana by growing clones of female and pollen-bearing (hermaphrodite and male)  F. virginiana individuals collected from 17 natural populations and subjecting them to high and low resource treatments in the greenhouse  We asked whether components of sex expression, including proportion fruit set, proportion seed set, ovule number, anther number, and flower number, and vegetative traits were plastic in response to resource availability.  Moreover, we asked whether sexes differed significantly in plasticity and whether the degree of plasticity in pollen-bearing individuals was related to the sex ratio (female frequency) of their originating populations.


Preliminary analyses reveal significant plasticity for most traits and that for several traits, including flower number and proportion fruit set, populations vary in their mean plastic response. These analyses further reveal that sexes are differentially plastic for proportion fruit set. Interestingly, while mean fruit-setting ability of pollen-bearing individuals estimated in the field varied negatively with population female frequency, as expected from theoretical predictions, this relationship did not hold when fruit set of those individuals was measured in the greenhouse.  Together these results suggest that plasticity in sex expression across populations of F. virginiana may impede the evolution of dioecy in this species by retarding female-mediated among-population genetic differentiation in fruit-setting ability of pollen-bearing individuals. We further interpret these results in the context of current theories on the evolution of dioecy that incorporate plasticity, including whether subdioecy may represent an evolutionary stable system rather than a transitional phase en route to dioecy.