Response to joint selection on germination and flowering phenology depends on the direction of selection

Abstract

Flowering and germination time are components of phenology, a complex phenotype that incorporates a number of traits. In natural populations, selection is likely to occur on multiple components of phenology at once. However, we have little knowledge of how joint selection on several phenological traits influences evolutionary response. We conducted one generation of artificial selection for all combinations of early and late germination and flowering on replicated lines within two independent base populations in the herb Campanula americana. We then measured response to selection and realized heritability for each trait. Response to selection and heritability were greater for flowering time than germination time, indicating greater evolutionary potential of this trait. Selection for earlier phenology, both flowering and germination, did not depend on the direction of selection on the other trait, whereas response to selection to delay germination and flowering was greater when selection on the other trait was in the opposite direction (e.g., early germination and late flowering), indicating a negative genetic correlation between the traits. Therefore, the extent to which correlations shaped response to selection depended on the direction of selection. Furthermore, the genetic correlation between timing of germination and flowering varies across the trait distributions. The negative correlation between germination and flowering time found when selecting for delayed phenology follows theoretical predictions of constraint for traits that jointly determine life history schedule. In contrast, the lack of constraint found when selecting for an accelerated phenology suggests a reduction of the covariance due to strong selection favoring earlier flowering and a shorter life cycle. This genetic architecture, in turn, will facilitate further evolution of the early phenology often favored in warm climates.

Phenotypic but not genotypic selection for earlier flowering in a perennial herb

“…This is in contrast to studies with other species that have found significant heritabilities for flowering time (e.g. Burgess, Etterson, & Galloway, ; Franks, Sim, & Weis, ; Galloway, Watson, & Prendeville, ; Sedlacek et al, ; Van Dijk, Boudry, McCombre, & Vernet, ; Yu, Harding, Byrne, & Famula, ), and to a study indicating that there is a genetic component to flowering time in our study species (Widén & Schiemann, ). Given the low additive genetic variance for FFD found in our study, it is not surprising that observed phenotypic selection for early flowering did not correspond to genotypic selection on FFD.…”

Simultaneous selection on vegetative and reproductive phenology in a perennial herb

“…Such correlations imply that selection might only rarely be exerted independently on the timing of a single event. Instead, selection on a focal phenological trait is often a combination of direct selection, and indirect selection acting via other phenological traits (Ehrlén, 2015; Galloway et al, 2018; Kelly, 1992; Rathcke & Lacey, 1985). Selection could also target the relative timing of phenological traits.…”

Spring and autumn phenology in an understory herb are uncorrelated and driven by different factors

“…Phenological events are often temporally and developmentally correlated, and the timing of one event can constrain or otherwise affect the timing of subsequent events (e.g., Diggle, 1999; Mentzel et al 2006; Donohue, 2014; Galloway et al, 2018; Zacchello et al, 2020; Bucher and Römermann, 2021). Within seasons, spring and autumn phenology have been found to be positively correlated among individuals in many woody species (e.g., Crawley and Akhteruzzaman, 1988; Keenan and Richardson, 2015), with individuals both advancing their growing season and senescing earlier when exposed to experimental warming in early spring (Fu et al, 2014; Zohner and Renner, 2019).…”