Plasticity, genetic diversity, and natural selection: A native polyploid under simulated climate change

Background/Question/Methods

Although plant response to climate change has been observed in many systems, it is unknown whether different ploidy levels within species respond similarly.  Solidago altissima (Asteraceae) occurs throughout Minnesota in populations polymorphic for ploidy level (diploid, tetraploid, and hexaploid).  Annual temperature is expected to increase, and average summer precipitation decrease, in Minnesota within 50 years, and this combination of altered climate variables may have a different effect than either factor alone.  Ploidy levels may differ in their ability to adapt immediately to altered conditions through phenotypic plasticity.  In the longer term, polyploids may evolve faster via natural selection than diploids if they harbor greater genetic diversity.  The goals of this research were to compare plasticity, genetic diversity, and patterns of selection between diploid and tetraploid genotypes of S. altissima grown in the field, but exposed to conditions simulating climate change (+1.9 ° C, -13 % soil moisture) (2 ploidy levels x 20 genotypes x 2 watering treatments x 2 temperature treatments x 4 blocks = 640 plants).  Physiological, morphological, life history traits, and fitness correlates were measured throughout the growing season. 

Results/Conclusions

The watering treatment had a more pronounced effect than temperature, and both ploidy levels exhibited plasticity in response to water availability; diploids grew taller, produced more rosettes, and thicker leaves in drought, and tetraploids flowered earlier, grew taller, and had greater stomatal density.  Under elevated temperature and decreased water availability, selection was significant for 60% of traits measured for polyploids and 40% of traits for diploids, yet diploids had significant heritable genetic variation for twice as many traits as polyploids.  Both ploidy levels had significant heritability and selection for earlier flowering in a warmer, drier environment, with similar predicted responses to selection.  Tetraploids may be at a disadvantage compared to diploids in their ability to respond to climate change depending on which traits have the greatest impact on fitness, the extent to which those traits can manifest adaptive plastic responses, and how quickly those traits can evolve.