Populus tremula (European aspen) shows no evidence of sexual dimorphism in gene expression, phenotype, secondary defence or herbivory.

Background: Although the majority of plant species are co-sexual, being either monoecious or hermaphroditic, a significant number are dioecious, having separate male and female individuals. Evolutionary theory suggests that males and females may develop sexually dimorphic phenotypic and biochemical traits concordant with each sex having different optimal strategies of resource investment to maximise reproductive success and fitness. The establishment of such sexual dimorphism would result in changes in gene expression patterns in non-floral organs. Results: We examined phenotypic, biochemical and leaf herbivory traits to reveal evidence of sexually dimorphic resource allocation strategies within a collection of sexually mature and immature Populus tremula (European aspen) trees. We additionally generated gene expression data from mature leaves of sexually mature wild trees using whole-genome oligo microarrays and RNA-Sequencing. Statistical analysis of the phenotypic traits revealed no evidence of sexual dimorphism or differential resource investment strategies between males and females. Similarly, single-gene differential expression and machine learning on combinations of genes identified no statistically robust evidence of expression level sexual dimorphism. Analysis of the RNA-Sequencing data did not identify any sex-specific Single Nucleotide Polymorphisms or genomic regions with contrasting SNP rates. Conclusions: We found no evidence of sexual dimorphism at the phenotypic, biochemical or genomic scales in P. tremula either before or after development phase transition (i.e. sexual maturity). There was no evidence that P. tremula males and females have evolved differential resource investment strategies or that they exhibit contrasting resistance or tolerance to leaf herbivores.