Sexual Dimorphism and the Evolution of Sex-Biased and Sex-Limited Genes in the Dioecious Tree Populus balsamifera

Animals have been shown to exhibit sexual dimorphism in gene expression in as many as 3/4 of their genes, and male-biased genes generally exhibit elevated rates of evolution as measured by dN/dS. Although ~5-6% angiosperm species are dioecious, little is known sexual dimorphism in gene expression or whether patterns of evolution differ for male- and female-biased genes. Populus exhibits clear sexual dimorphism in floral color, but few dimorphisms in non-floral traits. We characterized transcriptomes of two tissues (leaf and flower) of five male and five female individuals of Populus balsamifera from Fairbanks, Alaska. Five times as many genes exhibited sex limited expression in male floral tissues (476) than female floral tissues (76), and no genes exhibited sex limited expression in leaves. 11,068 genes were differentially expressed (DE) between male and female flowers, and only one gene exhibited significant differential expression between male and female leaves. Functional analyses of these genes revealed 15 enriched gene pathways. Notably, male and female flowers differed in energy-related pathways. Transcripts in photosynthesis and tetrapyrrole synthesis pathways were over-expressed in female flowers, whereas mitochondrial electron transport and TCA/org transformation pathways were over-expressed in male flowers. dN/dS ratio was calculated for sex-biased and sex-limited genes to explore whether different selective pressure acted on the sexually dimorphic genes. Taken together, these results demonstrate that sexual dimorphism in this species is limited to reproductive tissues, and provide insight into the evolutionary history that has shaped these differences.