PIGLIUCCI, MASSIMO*, HEIDI POLLARD, AND MITCHELL CRUZAN. Department of Botany, University of Tennessee, Knoxville, Tn 37996-1100. - Comparative studies of reaction norms in Arabidopsis: evolution of response to coarse and fine grained environmental variation in Scandinavian haplotypes.
There is general agreement that we need a better understanding of how
historical processes contribute to patterns of variation in phenotypic
plasticity within and among species; however, the evolution of
reaction norms has rarely been addressed from within an explicitly
phylogenetic comparative framework. Furthermore, the simultaneous
evolution of traits in response to environmental variation
characterized by different grains (coarse and fine from the standpoint
of the organism being considered) is also amenable to phylogenetic
comparative studies which have not been attempted so far. In this
paper we compare the reaction norms to foliage shade (changes in light
quality, spatially fine-grained environmental variation) and
photoperiod (daylength, spatially coarse-grained environmental
variation) in several haplotypes of Arabidopsis thaliana and of two
closely related species, A. arenosa and A. lyrata subsp. petraea, from
Scandinavia. We found that across-environment means evolved
continuously and very rapidly within this group, while plasticity
changed only rarely and especially between the outgroups and A.
thaliana. Character means evolved largely independently of each other,
while trait plasticities were highly integrated, as predicted by the
adaptive plasticity hypothesis for response to foliage shade (the
so-called “shade avoidance” syndrome). We found evidence of strong
constraints across environmental factors (daylength and foliar shade)
for some traits directly related to life history, but otherwise
largely independent evolution of the reaction norms of many traits in
response to either daylength or light quality. Some of the observed
patterns can be explained by a combination of shared ecological
circumstances and an underlying genetic constraint due to the fact
that an overlapping battery of photoreceptors perceive both aspects of
light availability.
Key words: Arabidopsis - phenotypic plasticity - light availability - ecotypes - comparative method