Because sexual dimorphism in plants is often less morphologically conspicuous than in animals, studies of sex-biased gene expression may provide a quantitative metric to better address their commonality, molecular pathways, consistency across tissues and taxa, and evolution. The presence of sex-biased gene expression in tissues other than the androecium or gynoecium, termed secondary sexual characters, suggests that these traits arose after the initial evolution of dioecy. Patterns of sequence evolution may provide evidence of positive selection that drove sexual specialization. We compared gene expression in male and female flowers and leaves of Populus balsamifera to assess the extent of sex-biased expression, and tested whether sex-biased genes exhibit elevated rates of protein evolution. Sex-biased expression was pervasive in floral tissue, but nearly absent in leaf tissue. Female-biased genes in flowers were associated with photosynthesis, whereas male-biased genes were associated with mitochondrial function. Sex-biased genes did not exhibit elevated rates of protein evolution, contrary to results from other studies in animals and plants. Our results suggest that the ecological and physiological constraints associated with the energetics of flowering, rather than sexual conflict, have probably shaped the differences in male and female gene expression in P. balsamifera.
Bibliographical noteFunding Information:
We thank Jianquan Liu and Steve Difazio for help in accessing poplar genomes, Diana Wolf, Naoki Takebayashi, Gery Allen and Guanqiao Feng for help with photographs of poplar flowers, and Lynda Delph and four anonymous reviewers for helpful comments on the manuscript. This research was supported by Texas Tech University and National Science Foundation grants IOS-1137001 to MSO and PT, and DEB-1542599 to MSO.
© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust
- protein evolution
- sex determination
- sex-biased expression