Nuclear genome size variation in Phragmites at multiple spatial scales
Nuclear genome size (GS) can influence plant traits including growth rate, photosynthetic rate, minimum generation time, and seed size. GS is also inversely correlated with invasion success. According to the large genome constraint hypothesis, small-genome plants thrive in a range of diverse conditions, but large-genome plants are excluded from extreme environments because they are more strongly influenced by abiotic factors. Extreme conditions including drought, changes in temperature and salinity levels, and increased atmospheric CO2 concentrations are predicted to become more prevalent in some areas under global environmental change (GEC) and may favor small-genome plants. Since selection under GEC is unlikely to be uniform across spatial scales, effects on plant GS may vary at the patch, local, regional, and continental scales. A continent-wide invasion by the common reed, Phragmites australis, represents a unique opportunity to study GS variation across geographic ranges with diverse environmental conditions. This species is an excellent model organism because it has multiple phylogeographic groups that inhabit wetland environments across North America and exhibits high genetic variation and GS variability within and between ploidy levels. We quantified GS from Phragmites populations across North America using propodium iodide and DAPI flow cytometry and confirmed ploidy with chromosome counts.
Absolute GS in North America ranges from 1.683 picograms to 4.391 picograms. Relative GS varies at the continental scale between Gulf, Native, and Introduced lineages. At the regional level on the West Coast, relative GS (against Bellis perennis as an internal standard) ranges from 0.425 to 0.472 in tetraploids. GS also varied significantly locally between sites in Rhode Island (Jamestown, Warwick, and East Greenwich) and Massachusetts (Mashpee) with average site relative GS ranging from 0.570 to 0.610 for tetraploid plants. Preliminary data at these local sites show a positive relationship between GS and salinity, a result not expected under the large genome constraint hypothesis. Patch scale variation in relative GS also existed at a site in Crescent City, California.
Future research will focus on how abiotic factors influence GS variation in plant populations and how GEC may impact this relationship. Understanding the relationships between abiotic conditions and GS may be useful in conservation efforts by assisting in modeling plant invasions, range shifts, and species distributions under predicted future conditions due to GEC.