DOI: 10.5061/dryad.k3j9kd55k
title: Evolutionary divergence of potential drought adaptations between two subspecies of an annual plant: Are trait combinations facilitated, independent, or constrained?

description: Premise: Whether drought-adaptation mechanisms tend to evolve together,
 evolve independently, and/or evolve constrained by genetic architecture is
 incompletely resolved, particularly for water relations traits besides gas
 exchange. We addressed this issue in two subspecies of Clarkia
 xantiana (Onagraceae), California winter annuals that separated
 approximately 65,000 years ago and are adapted, partly by differences in
 flowering time, to native ranges differing in precipitation. Methods: In
 these subspecies and in recombinant inbred lines (RILs) from a cross
 between them we scored traits related to drought adaptation (timing of
 seed germination and of flowering; succulence; pressure-volume curve
 parameters) in common environments. Results: The subspecies native to more
 arid environments (parviflora) exhibited slower seed germination in
 saturated conditions, earlier flowering, and greater succulence, likely
 indicating superior drought avoidance, drought escape, and dehydration
 resistance via water storage, respectively. The other subspecies
 (xantiana) had lower osmotic potential at full turgor and lower water
 potential at turgor loss, implying superior dehydration tolerance. Genetic
 correlations among RILs suggest facilitated evolution of some trait
 combinations and independence of others. Where genetic correlations exist,
 subspecies differences fell along them, with the exception of differences
 in succulence and turgor loss point. In that case, subspecies difference
 overcame genetic correlations, possibly reflecting strong selection and/or
 antagonistic genetic correlations with other traits. Conclusions: Clarkia
 xantiana subspecies’ differ in multiple mechanisms of drought adaptation.
 Genetic architecture generally does not seem to have constrained the
 evolution of these mechanisms, and it may have facilitated the evolution
 of some of trait combinations.
description: These life-history and water-relations data come from plants in
 common environments in cultivation, in two temporal blocks. Taking
 wild-collected seeds from two parent populations, one of each named
 subspecies of <em>Clarkia xantiana</em> (Onagraceae), and
 greenhouse-generated seed from 14 F6 recombinant inbred lines of a cross
 between those populations, we sowed seeds on saturated germination
 blotting paper in a growth chamber, noting each seed's time to
 germination. Upon germination, we transplanted seedlings into artificial
 medium in containers and followed survivors to their date of first
 flowering, recording days elapsed. Hydrating and then harvesting plants on
 their date of first flower, we measured weight-based tissue succulence
 (saturated water contents of stems and leaves) and used pressure-volume
 analysis with a pressure-chamber to estimate several additional water
 relations traits. We used the spreadsheet tool of Sack and colleagues
 (Sack, L., J. Pasquet-Kok, and <i>PrometheusWiki</i>
 Contributors. 2011. Leaf pressure-volume curve parameters.
 <i>PrometheusWiki</i>. Website http://prometheuswiki.org/tiki-index.php?page=Leaf+pressure-volume+curve+parameters.) to estimate pressure-volume parameters from raw curves. Some individual plants' data we removed from the data set, to meet accepted standards of pressure-volume analysis and eliminate obvious errors (e.g., plants estimated to increase in weight as they dehydrated).
description: Missing values appear in the data table where it was not possible
 to estimate particular variables. The file
 "ReadMe_Burnette_and_Eckhart_AJB" explains the meanings of each
 variable.