The evolution of sexual dimorphism and expansion of sex chromosomes are both driven through sexual conflict, arising from differing fitness optima between males and females. Here, we pair work in poplar (Populus) describing one of the smallest sex-determining regions known thus far in complex eukaryotes (~100 kbp) with comprehensive tests for sexual dimorphism using >1300 individuals from two Populus species and assessing 96 non-reproductive functional traits. Against expectation, we found sexual homomorphism (no non-reproductive trait differences between the sexes), suggesting that gender is functionally neutral with respect to non-reproductive features that affect plant survival and fitness. Combined with a small sex-determining region, we infer that sexual conflict may be effectively stymied or non-existent within these taxa. Both sexual homomorphism and the small sex-determining region occur against a background of strong environmental selection and local adaptation in Populus. This presents a powerful hypothesis for the evolution of dioecious species. Here, we suggest that environmental selection may be sufficient to suppress and stymy sexual conflict if it acts orthogonal to sexual selection, thereby placing limitations on the evolution of sexual dimorphism and genomic expansion of sex chromosomes.

Département des sciences du bois et de la forêt Faculté de foresterie de géographie et de géomatique Université Laval Québec QC G1V 0A6 Canada

Department of Botany University of British Columbia Vancouver BC V6T 1Z4 Canada

Department of Dendrology and Forest Tree Breeding Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague 165 21 Czech Republic

Department of Forest and Conservation Sciences Faculty of Forestry University of British Columbia Forest Sciences Centre Vancouver BC V6T 1Z4 Canada

Department of Wood Science Faculty of Forestry University of British Columbia Forest Sciences Centre Vancouver BC V6T 1Z4 Canada

Saskatoon Research and Development Centre Agriculture and Agri Food Canada Saskatoon SK S7N 0X2 Canada

Scion Whakarewarewa Rotorua 3046 New Zealand

United States Department of Agriculture Agricultural Research Service Corn and Soybean Research Wooster OH 44691 USA

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Sci Rep. 2018 Nov 22;8(1):17410. doi: 10.1038/s41598-018-35443-y. PubMed

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