Not just females and males: Unravelling the complex sex determinism of the hemp palm, Trachycarpus fortunei

The ornamental Asian palm Trachycarpus fortunei (Arecaceae: Coryphoideae) is widely planted in temperate regions. In Europe, it has spread outside of gardens, particularly on the southern side of the Alps. Sexual expression in the species is complex, varying from dioecy to polygamy. This study investigated (1) sexual floral development and (2) genetic markers implicated in sex determinism.

Methods

The morphology and anatomy of floral organs at different developmental stages were studied using SEM observations and anatomical section. Sex determinism was explored using a genome‐wide association study approach, searching for correlations between 31,000 single‐nucleotide polymorphisms and sex affiliation of 122 palms from 21 wild populations.

Results

We observed that sexual differentiation appears late in floral development of T. fortunei . Morpho‐anatomical characters of flowers conducive to panmixia were observed, such as well‐differentiated septal nectaries that are thought to promote cross‐pollination. At the molecular level, homozygous and heterozygous allelic systems with closely linked regions were found for sex determinism in individuals with female and “dominant‐male” phenotypes, respectively. Through our wide sampling in the southern Alps, the closely linked genetic regions in males suggest that at least fifteen percent of wild palms are the direct offspring of “males” that can also produce fertile pistillate flowers.

Conclusions

Trachycarpus fortunei is a further example of unstable sexual expression found in the family Arecaceae and represents an evolutionary path towards an XY genetic system. Our structural and genetic results may explain the high species dispersal ability in the southern Alps.