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

Background and aims – Reproductive plasticity and hybridization are known to play key roles in local adaptation and range expansion in plants, yet the environmental conditions under which canonical size-dependent models of sex expression break down remain poorly understood. Arisaema serratum (Araceae), a widespread species in Japan, has long been characterized as exhibiting size-dependent sex determination, whereby individuals transition from producing staminate to pistillate inflorescences upon reaching a size threshold. Field observations from western Japan further suggest that sex expression in A. serratum may be decoupled from plant size, raising the possibility that environmental cues such as light availability override intrinsic size thresholds in some populations. Because such ecological flexibility can increase spatial and temporal overlap among sympatric congeners, we additionally evaluate whether relaxed sex determination may coincide with increased introgression within this species complex. 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 .