The effects of becoming taller: direct and pleiotropic effects of artificial selection on plant height in Brassica rapa

Abstract

Plant height is an important trait for plant reproductive success. Plant height is often under pollinator-mediated selection, and has been shown to be correlated with various other traits. However, few studies have examined the evolutionary trajectory of plant height under selection and the pleiotropic effects of plant height evolution. We conducted a bi-directional artificial selection experiment on plant height with fast cycling Brassica rapa plants to estimate its heritability and genetic correlations, and to reveal evolutionary responses to artificial selection on height and various correlated traits. With the divergent lines obtained through artificial selection, we subsequently conducted pollinator-choice assays and investigated resource limitation of fruit production. We found that plant height variation is strongly genetically controlled (with a realized heritability of 41-59%). Thus, plant height can evolve rapidly under phenotypic selection. In addition, we found remarkable pleiotropic effects in phenology, morphology, floral scent, color, nectar and leaf glucosinolates. Most traits were increased in tall-line plants, but flower size, UV reflection and glucosinolates were decreased, indicating potential trade-offs. Pollinators preferred plants of the tall selection lines over the short selection lines in both greenhouse experiments with bumblebees and field experiment with natural pollinators. We did not detect any differences in resource limitation between plants of the different selection lines. Overall, our study predicts that increased height should evolve under positive pollinator-mediated directional selection with potential trade-offs in floral signals and herbivore defense.

Floral signals evolve in a predictable way under artificial and pollinator selection inBrassica rapausing a G-matrix

“…The line used needs only ca. 35 to 40 days to complete a life cycle and maintains sufficient genetic variability for selection experiments (Miller & Schemske, 1990; Ågren & Schemske, 1992; Zu et al , 2016; Zu & Schiestl, 2017).…”

“…Our analysis includes a total of 16 traits with 12 floral volatile organic compounds (VOCs), and 4 morphological traits (plant height, petal width, petal length, and flower diameter). The measurement methods were described in detail in Zu & Schiestl (2017). Floral VOCs were collected from at least four freshly opened flowers per plant at a flow rate of 100 mL per min for 3 hours.…”

“…Mathematically, genetic (co)variance matrix (G-matrix) and phenotypic selection (β) are the two parameters for predicting the evolutionary changes (Δz) of a suite of traits by using multivariate breeder’s equation Δz = G* β (Lande, 1979; Lande & Arnold, 1983). A great number of empirical studies have documented significant heritability and genetic (co)variance of diverse floral traits (Ashman & Majetic, 2006; Kaczorowski et al , 2008; Zu et al , 2016; Zu & Schiestl, 2017), as well as phenotypic selection acting on them (Gómez, 2003; Irwin & Strauss, 2005; Sandring & Ågren, 2009; Sletvold & Ågren, 2010; Hopkins & Rausher, 2012; Parachnowitsch et al , 2012; Ågren et al , 2013; Gross et al , 2016; Gervasi & Schiestl, 2017). Among those traits, floral scents have rarely been considered.…”

“…We used data from two forward-in-time experimental evolution experiments that documented genetic co-variation and evolutionary responses in floral traits of fast cycling Brassica rapa plants. The first parameter G of the plant population was estimated from a three-generation bi-directional artificial selection experiment on plant height (Zu & Schiestl, 2017). In that study, tall- and short-plants were selected artificially for building the two directional lines, in addition to a control line built with randomly selected plants.…”

“…Control lines in this experiment were used to estimate G-matrix. The other parameter β was calculated from four evolutionary scenarios: two from the tall- and short-selection lines in the artificial selection experiment mentioned above (Zu & Schiestl, 2017); the other two from a 9-generation pollinator selection experiment (Gervasi & Schiestl, 2017). The pollinator selection experiment was carried out with bumblebees and hoverflies as the selection agents separately.…”