Does size matter? The integrated roles of light, adaptive sex expression, and hybridization in a widespread Arisaema (Araceae) species from western Japan

Background – For any given plant species, reproductive plasticity and hybridization are known to play key roles in local adaptation and range expansion. Arisaema serratum (Araceae), a widespread species in Japan, has been characterized as having size-dependent sex determination, a trait in which individuals produce inflorescences with only male flowers until they reach a certain size, at which point they shift to the production of female-flowered inflorescences.

Materials and methods – We conducted field-based demographic surveys across five populations of Arisaema on Shikoku Island, Japan, quantifying plant size, sex expression, and light intensity to evaluate environmental and morphological predictors of reproductive phase. To assess hybridization, we generated genome-wide SNP data through genotyping-by-sequencing, constructed a maximum likelihood phylogeny, and performed f-branch analyses to detect patterns of introgression among sympatric species.

Key Results – Our data indicate that size in fact may not be the key factor in sex determination, with light availability emerging as a stronger predictor of sex expression than size alone for populations on Shikoku Island. Preliminary genomic analyses support evidence of gene flow between A. serratum and sympatric species, indicating ongoing hybridization and introgression.

Conclusions – Our findings indicate that the shift away from a strict allometric model of sex determination on Shikoku Island to a photosensitive model allowing for diphasic sexual expression likely enhances reproductive flexibility and leads to an increase in seed production in open-canopy and disturbed environments. Together, these findings provide ecological and demographic insight contributing to the broad distribution, sex determination, and morphological diversity of A. serratum .