Resurrecting an Urban Sunflower Population: Phenotypic and Molecular Changes Over 36 Years

Abstract

Resurrection experiments, in which dormant propagules of antecedent populations are grown alongside modern populations, provide a unique opportunity to directly evaluate phenotypic and molecular evolution in response to environmental challenges. To understand evolution of an urban population of Helianthus annuus (Asteraceae) over 36 years, we resurrected samples obtained from a 1980 USDA National Plant Germplasm System accession alongside contemporary successors gathered in 2016. Molecular changes in transcript expression using RNA-seq data revealed 200 differentially expressed transcripts between antecedent and modern groups. Transcript expression was higher in modern samples as compared to antecedent samples, while expression patterns indicated evolution due to genetic drift, gene introgression, or adaptive evolution. After a refresher generation in greenhouse conditions, we grew the resulting family lines in an outdoor common garden under varied water availability (high-water and low-water) and temperature conditions (ambient and elevated > 3°C) corresponding to cooler 1980 and warmer 2016 conditions to observe phenotypic differences and plastic response. Seventy-seven percent of measured traits differed, with modern individuals displaying traits similar to cultivated varieties and antecedent individuals displaying more customary wild-type traits. For example, modern plants were larger and showed more apical dominance while antecedent plants produced more branches and inflorescences. Modern trait means were often selected for across varied environmental treatments, especially those resembling modern conditions. This indicates that modern plants are well-adapted to their current environment. However, the modern population displayed little genetic variation underlying important reproductive traits which may limit the potential for further evolution of this population in response to changing conditions. The resurrection method allowed us to understand molecular and phenotypic evolution as a response to environmental pressures, gene flow from cultivated H. annuus, or some combination of evolutionary mechanisms resulting in the observed differences between the 1980 and 2016 populations.