Introduction
The timing of seed germination affects fitness by determining conditions
during seedling establishment and growth (Donohue et al. 2010;
Postma and Ågren 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
and Ågren 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). The degree to which among-population variation in
seed dormancy is related to climatic differences across different
spatial scales thus becomes of considerable interest for assessing the
potential of adaptive evolution in response to climate change (Dawsonet al. 2011).
In many species, timing of germination is regulated by the strength of
seed dormancy. Primary seed dormancy, i.e., dormancy at the time of seed
maturation, and the rate at which it is lost will determine when seed
germination will be triggered in response to environmental cues
(Vleeshouwers et al. 1995; Li and Foley 1997; Finch-Savage and
Leubner-Metzger 2006; Bewley et al. 2013; Baskin and Baskin
2014). Optimal seed dormancy should be positively correlated with the
length of the period following seed release that is unfavourable for
seedling establishment (Meyer and Monsen 1991; Allen and Meyer 1998;
Llorens et al. 2008; Wagmann et al. 2012), and may thus
vary among populations. Genetic variation in seed dormancy has been
documented among populations of several species and across various
spatial scales, for example in Arabidopsis thaliana within Europe
(Kronholm et al. 2012; Debieu et al. 2013), inBromus tectorum in North America (Allen and Meyer 2002), and inDigitaria melanjiana in central and eastern Africa (Hacker 1984).
Several environmental factors may drive divergence of seed dormancy, and
one approach to identify potential agents of selection is to examine the
correlation between phenotype and environment at the site of origin
(Wadgymar et al. 2017). Characterizing spatial variation in seed
dormancy and its association with differences in environmental factors
can thus provide an insight to the possible drivers of such variation.
Among-population variation in seed dormancy can be the result not only
of genetic differentiation, but also of environmental effects and the
interaction between these two factors (Young et al. 1991; Benderet al. 2003; Schütz and Rave 2003; Donohue et al. 2005;
Donohue 2009; Postma and Ågren 2015; Fenner 2018). Most studies that
document within-species variation in seed dormancy have grown
populations in the greenhouse (e.g., Allen & Meyer, 2002; Debieu et
al., 2013; Kronholm et al., 2012; Vidigal et al., 2016; Wagmann et al.,
2012). However, plants raised in the greenhouse are typically exposed to
temperature, light and watering regimes that are very different from
those experienced in natural populations. Studies comparing dormancy of
seeds produced in the greenhouse and in the field have also detected
strong genotype × maternal environment interactions (Schütz and Rave
2003; Fernández-Pascual et al. 2013; Postma and Ågren 2015). To
determine the importance of genetic differentiation, environmental
effects and their interaction for variation in seed dormancy, seeds of
different source populations should thus ideally be produced in multiple
relevant field environments (Young et al. 1991; Schütz and
Milberg 1997).
In this study, we quantify variation in primary seed dormancy among
populations of the annual model organism A. thaliana in two
regions in Europe and examine the association between within-region
variation in seed dormancy and climate. A. thaliana is native to
Africa and Eurasia (Durvasula et al. 2017), and occurs in
habitats that vary widely in the length of the dry summer period. In
winter-annual populations of A. thaliana , seeds are matured in
late spring and early summer and germinate in autumn (Ågren and Schemske
2012). As in other species with physiological dormancy, germination inA. thaliana cannot occur until seed dormancy has been released by
a process called after-ripening (Finch-Savage and Leubner-Metzger 2006;
Montesinos-Navarro et al. 2012). Considerable within-species
variation in the required length of after-ripening has been observed at
different spatial scales, but its association with climatic factors is
still unclear. Kronholm et al. (2012) found a negative
association between seed dormancy and summer precipitation among
European populations, and Vidigal et al. (2016) similarly found
that high dormancy was associated with high temperature and low summer
precipitation within the Iberic peninsula. By contrast, Debieu et
al. (2013) found no climatic association but only a latitudinal cline
across Europe. Moreover, in a study of regional variation in
north-eastern Spain, high primary dormancy was associated with dry, but
cold environments of origin (Montesinos-Navarro et al. 2012),
indicating that large-scale climatic associations are not necessarily
reflected at smaller spatial scales. Additional studies of regional
variation are therefore needed to characterize spatial patterns of seed
dormancy differentiation in A. thaliana . Furthermore, seed
dormancy in A. thaliana is strongly affected by the maternal
environment and by the interaction between genotype and maternal
environment (Postma & Ågren, 2015), suggesting that the possibility of
such interactions should be considered when examining correlations
between trait expression and environment at site of origin.
Here, we planted individuals from 28 Fennoscandian populations and 17
Italian populations (4-5 maternal lines per population for a total of
224 maternal lines) in three common gardens, one at the site of a
natural population in north-central Sweden, one at the site of a natural
population in central Italy, and one in a greenhouse in Uppsala, Sweden
(Fig. 1 ). Fennoscandia and Italy represent the northern and
southern range margins of A. thaliana in Europe. The summer
period during which intermittent droughts is likely to kill any young
emerging seedlings is longer in Italy compared to Fennoscandia
(Fig. 2A and B ). Consequently, we expect Italian
populations to show stronger seed dormancy than Fennoscandian
populations, as previously observed in a comparison between the
populations native to the Swedish and Italian field sites used in the
present study (Postma and Ågren 2015). Similarly, within regions, seed
dormancy can be expected to be positively correlated with the length of
the period with summer conditions and with temperature during this
period, and negatively correlated with precipitation during summer.
We tested the hypotheses that (i) dormancy of seeds produced by Italian
populations is stronger than that of seeds produced by Fennoscandian
populations, (ii) seed dormancy is affected by the maternal environment
and its interaction with region and population of origin, and (iii)
within regions, seed dormancy is related to climatic conditions at the
site of origin in summer.