Strong stabilizing selection on timing of germination in a Mediterranean population of Arabidopsis thaliana

Abstract

Timing of germination can strongly influence plant fitness by affecting seedling survival and by having cascading effects on later life-history traits. In seasonal environments, the period favorable for seedling establishment and growth is limited, and timing of germination is likely to be under stabilizing selection because of conflicting selection through survival and fecundity. Moreover, optimal germination time may vary among genotypes because of inherent differences in later life-history traits. METHODS: To examine how germination time affects survival, fecundity, and the relative fitness of two genotypes differing in time to first flower, we conducted a field experiment in an Italian population of the winter annual Arabidopsis thaliana, in which seedling establishment occurs mainly in November. We transplanted seedlings of the local genotype and of a Swedish genotype monthly from August to December and monitored survival and fecundity. RESULTS: Only seedlings transplanted in November and December survived until reproduction, and fitness of the November cohort was 35 times higher than that of the December cohort, indicating strong stabilizing selection on timing of germination. There was no evidence of conflicting selection: seedling survival, adult survival, and fecundity were all highest in the November cohort. Moreover, the relative fitness of the two genotypes did not differ significantly between cohorts. CONCLUSIONS: The very narrow window of opportunity for seedling establishment was related to rapid seasonal changes in soil moisture and temperature, suggesting that rate of seasonal change is an important aspect to consider for understanding spatiotemporal variation in selection on phenological traits.

Life‐history trade‐offs and the genetic basis of fitness in Arabidopsis thaliana

“…QTL for phenological variation may also play a key role in stress tolerance and resource acquisition that would contribute to variation in condition. Previous work on the same populations has demonstrated strong selection to tune the timing of germination (Akiyama & Ågren, 2014; Postma & Ågren, 2016; Zacchello et al, 2020) and flowering time to match the local climate (Ågren et al, 2017). The timing of life‐history traits can have cascading effects on later life‐history stages (Beckerman et al, 2002; Donohue, 2014; Hepworth et al, 2020; Lindström, 1999; Martinez‐Berdeja et al, 2020; Postma & Ågren, 2016), meaning that direct effects on early traits can cause pleiotropic effects on multiple fitness components.…”

Seed dormancy varies widely among Arabidopsis thaliana populations both between and within Fennoscandia and Italy

“…Postma & Ågren, 2016 ). This expectation is supported by experiments conducted at the two field sites, in which August was identified as the optimal time of germination at the Swedish site (Akiyama & Ågren, 2014 ), and November at the Italian site (Zacchello et al, 2020 ). The difference in seed dormancy between Italian and Fennoscandian populations documented in the present study is consistent with previous observations, indicating a decrease in seed dormancy with increasing latitude of origin among A .…”

Seed dormancy varies widely among Arabidopsis thaliana populations both between and within Fennoscandia and Italy

“…Postma &Ågren, 2016). In experiments conducted at the two field sites, August was identified as the optimal time of germination at the Swedish site (Akiyama andÅgren 2014), and November at the Italian site (Zacchello et al 2020). The difference in seed dormancy between Italian and Fennoscandian populations documented in the present study is consistent with previous observations indicating a decrease in seed dormancy with increasing latitude of origin among A. thalianaaccessions sampled across Europe (Kronholm et al 2012;Debieuet al 2013), and is likely to be representative for differences between north European populations and southern populations at low altitude in general.…”

“…Fifth, seed dormancy of present-day populations may not mirror optimal seed dormancy at the sites of origin, but rather reflect founder events or genetic correlations with traits more strongly related to fitness. This may seem less likely considering the strong effects of germination date for likelihood of seedling establishment, survival and fecundity in A. thaliana (e.g., Donohue et al, 2005;Akiyama &Ågren, 2014;Postma &Ågren, 2016;Zacchello et al, 2020). However, germination date is determined not only by dormancy at the time of seed maturation, but also by processes affected by the post-dispersal environment, such as rate at which dormancy is released, and possible acquirement of secondary dormancy (Montesinos et al, 2012;Martínez-Berdeja et al, 2020).…”

“…The timing of seed germination affects fitness by determining conditions during seedling establishment and growth (Donohue et al 2010;Postma andÅgren 2016), but may also influence fitness by having cascading effects on later life-history traits such as flowering start (Evans and Cabin 1995;Donohue 2002;Wilczek et al 2009;Akiyama andÅgren 2014;Burghardt et al 2015;Martínez-Berdeja et al 2020;Zacchello et al 2020). In seasonal environments, there should be strong selection for timing of germination matching periods favourable for seedling establishment and growth (Donohueet al 2010;Wadgymar et al 2015), and adjustment of germination timing is expected to critically affect the ability of species to adapt to climate change (Fenner and Thompson 2005;Cochraneet al 2015).…”