Species of Arisaema Mart. are perennial, tuberous geophytes that occur primarily as forest understory herbs, persisting across years via a subterranean corm but relying largely on sexual reproduction rather than clonal spread (Bierzychudek 1982; Murata 2011; Murata et al. 2018). Although typically associated with shaded forest interiors, many taxa, including A. serratum (Thunb.) Schott, readily establish in canopy gaps, forest edges, and managed plantation forests where light availability is spatially heterogeneous (Kinoshita 1987; Masaki et al. 2017; Matsumoto et al. 2018). Pollination is mediated mainly by small dipterans attracted and temporarily trapped within the spathe, a system that can facilitate heterospecific pollen transfer when congeners flower synchronously, while seeds are dispersed locally by gravity and small animals into newly opened microsites (Atkinson 1898; Bierzychudek 1982; Murata and Ohno 1989; Kudo et al. 2008).

The development of sexual expression in plants, here defined as the production of male or female flowers by an individual, is often constrained by intrinsic factors such as plant size and resource availability (Policansky 1981). In sexually diphasic species, these constraints can influence or trigger transitions between reproductive phases. In Arisaema (Araceae), sex expression has traditionally been considered size-dependent, with smaller individuals producing staminate (male) flowers and larger individuals producing “pistillate” or ovulate (female) flowers, due to the higher energetic demands associated with female function (Policansky 1981; Bierzychudek 1982). This pattern is supported by early morphological and demographic studies in North American congeners such as A. triphyllum (L.) Schott and A. dracontium (L.) Schott, where sex changes are linked to plant size (Atkinson 1898; Camp 1932; Doust and Cavers 1982; Ewing and Klein 1982; Clay 1993), nutrient status (Schaffner 1922), and age (Gow 1913; Camp 1932). Similar trends have been documented in several Japanese Arisaema species. For example, individuals of A. urashima H.Hara are reported to transition from male to female flowering with increasing plant size (Takasu 1987). The same size-dependent model has been applied to A. japonicum Blume (now considered a synonym of A. serratum), with early studies noting a general male-to-female transition in flowering accompanied by increases in plant age and plant size under controlled settings (Maekawa 1924, 1927). However, these studies were conducted under homogeneous greenhouse conditions, limiting their ability to assess the influence of natural environmental gradients.

More recent field-based work by Kinoshita (1987) provides explicit support for a size–sex relationship in A. serratum populations from Nagano Prefecture in north-central Japan. This study documented that larger plants were more likely to produce female flowers, consistent with the canonical size-dependent model. Yet, this pattern may not be universal. Preliminary field observations of A. serratum populations on Shikoku Island in western Japan suggested an absence or weakening of this correlation, with female-flowering individuals frequently observed across a broad range of plant sizes, including individuals below traditionally inferred size thresholds. These observations prompted a re-evaluation of the relative roles of intrinsic (size-based) and extrinsic (environmentally induced) cues in determining sex expression in this variable species.

The size-dependent model of sexual expression has been widely documented across plant families and is thought to optimize reproductive success under stable ecological conditions (Charnov and Bull 1977). However, in disturbed environments, where rapid colonization is favoured, strict size-dependent sex expression may be maladaptive (Dorken and Barrett 2004). In these situations, environmental parameters might select for, or result in the selection of, greater plasticity in reproductive strategies (Barot et al. 2005) including the timing of transition between the production of male vs female flowers.

Disturbance regimes, particularly those affecting canopy structure and light availability, can substantially alter reproductive dynamics in forest understory plants by modifying patterns of resource allocation and growth (Protchazka et al. 2024). In species capable of facultative or environmentally sensitive sex expression, external cues such as light intensity may override or interact with intrinsic, size-dependent factors to determine reproductive phase transitions (Obeso 2002). Light availability is especially critical in shaping fitness within early successional or plantation forests, where fluctuating microsite conditions favour rapid growth and reproductive responsiveness (Lichstein et al. 2021). In such contexts, sexually diphasic plants that relax their dependence on intrinsic size thresholds and respond directly to environmental conditions may gain a selective advantage, particularly in shaded or periodically disturbed landscapes common throughout western Japan.

Across many plant lineages, ecological expansion into novel or transitional habitats can also increase opportunities for interspecific hybridization. Species capable of colonizing light-variable, post-disturbance environments often come into secondary contact with congeners occupying adjacent native forests, creating conditions conducive to gene flow and introgression (Ellstrand and Schierenbeck 2000; Rieseberg et al. 2007). When such contact zones coincide with individuals exhibiting flexible or asynchronous sex expression, the likelihood of heterospecific pollen transfer may increase, facilitating admixture and the emergence of hybrid lineages.

The interaction between environmentally responsive sex expression and ecological disturbance may therefore have broader evolutionary implications. Morphological observations alone cannot determine whether apparent intermediacy among sympatric Arisaema taxa reflects phenotypic plasticity, shared ancestral variation, or genetic introgression, limiting inferences based solely on form. Because environmentally responsive sex expression may increase flowering synchrony and microsite overlap among congeners under variable light regimes, we evaluate introgression not as a predictor of sex expression itself, but as a potential evolutionary outcome of this reproductive flexibility in the plantation and secondary-forest landscapes of western Japan.

In this study, we test how environmental and morphological factors interact to shape reproductive phase transitions in Arisaema serratum, a widespread Japanese species commonly found in post-disturbance evergreen plantations and secondary forests (Matsumoto et al. 2018). Specifically, we evaluate whether light availability serves as a stronger predictor of sex expression than plant size—challenging the traditional size-dependent model—and whether this ecological flexibility may contribute to the species’ success in dynamic, human-modified landscapes. To explore potential evolutionary consequences of this flexibility, we also analyse genome-wide SNP data as an exploratory framework for detecting possible gene flow between A. serratum and sympatric congeners. Together, these approaches aim to clarify how environmentally responsive reproductive strategies may facilitate both local adaptation and the maintenance of porous species boundaries in forest understory taxa.

Our results demonstrate that Arisaema serratum exhibits a relaxed dependence on plant size for determining sex expression, with light availability emerging as a more consistent predictor of reproductive phase among western Japanese populations. This contrasts sharply with the canonical size–sex relationship documented in other Arisaema species, including the four congeners analysed here, and challenges the long-held assumption that sex expression in this genus is strictly allometric (Atkinson 1898; Camp 1932; Doust and Cavers 1982; Ewing and Klein 1982; Takasu 1987; Clay 1993). In A. serratum, the probability of producing pistillate inflorescences increases under brighter microsite conditions, regardless of individual size, implying that light availability may act as an environmental cue overriding the intrinsic thresholds governing sex expression (Obeso 2002).

While similar light–sex relationships have been observed in A. triphyllum populations in high-light habitats of Ontario (Doust and Cavers 1982), those systems showed a strong correlation between light and plant size, making the causal driver ambiguous. In contrast, our analyses revealed no significant correlation between light intensity and either plant height or pseudostem diameter in A. serratum, suggesting that growth traits are not the proximate mediators of the light effect. This decoupling may reflect a shift toward direct physiological sensitivity to light in determining reproductive investment, a response that could confer adaptive advantages in patchy or periodically illuminated forest floors. For an understory geophyte such as A. serratum, this plasticity allows individuals to opportunistically reproduce when canopy openings increase resource availability, rather than waiting to achieve a fixed vegetative size threshold.

Our findings contradict early foundational work by Maekawa (1924, 1927), who reported that individuals of A. japonicum (now synonymized under A. serratum (Scholten et al. 2025)) generally transitioned from male to female with increasing size and age. However, Maekawa’s studies were conducted under homogeneous greenhouse conditions and did not assess the influence of light on reproductive expression. In fact, Maekawa himself acknowledged that energy assimilation and nutrient status likely played a role in sex transitions, hinting at complexity beyond mere size thresholds.

The departure from strict size dependence in A. serratum also provides an explanation for regional variation in reproductive strategies across Japan. Kinoshita (1987) found a clear size–sex correlation in A. serratum populations from Nagano Prefecture, where deciduous broadleaf forests dominate. In contrast, the evergreen conifer plantations and secondary mixed forests of Shikoku Island—characterized by reduced understory light and irregular canopy gaps (Ovington 1983)—present a markedly different selective environment. In such low-light systems, a reproductive strategy responsive to transient illumination may increase both individual fitness and population persistence. These findings suggest that forest composition and management history can exert a strong influence over the evolution of reproductive plasticity in perennial understory plants.

The ecological consequences of such flexibility extend beyond individual fitness. Light-mediated sex expression likely facilitates colonization of plantation landscapes, where disturbance and canopy thinning create transient windows of high light. Increased reproductive output in these microsites can elevate local population density and the probability of contact among species. The co-occurrence of A. serratum with congeners such as A. ehimense, A. tosaense, and A. ringens within the same plantation mosaics provides numerous opportunities for heterospecific pollen transfer and hybridization. Our genomic analyses—although exploratory due to limited sampling—revealed extensive allele sharing among A. serratum and sympatric species, consistent with recurrent introgression. These results align with previous reports of hybrid individuals and morphologically intermediate populations throughout western Japan (Murata and Ohno 1989; Murata and Ohashi 2009) and indicate that anthropogenic habitats may serve as arenas for both ecological convergence and genetic exchange.

This pattern of admixture supports a causal sequence in which light-driven reproductive plasticity promotes colonization of plantation forests, thereby increasing opportunities for hybridization. Specifically, our demographic analyses show that sex expression in A. serratum is decoupled from plant size and instead strongly associated with light availability (Fig. 1A), a pattern not observed in sympatric congeners, indicating that ecological flexibility is a species-specific trait rather than a consequence of hybridization. In parallel, phylogenetic and f-branch analyses reveal widespread allele sharing between A. serratum and multiple sympatric taxa (Fig. 2B–C), consistent with recurrent gene flow occurring in the same disturbed, light-variable habitats where relaxed sex expression is observed. Together, these results favour an interpretation in which ecological flexibility enables range expansion and increased interspecific contact, after which gene flow among sympatric taxa reinforces variation and potentially facilitates adaptation, rather than hybridization acting as the primary driver of sex-expression plasticity. The frequent occurrence of morphologically intermediate individuals in plantation understories further supports this feedback, in which environmental heterogeneity, reproductive plasticity, and introgression operate jointly to maintain demographic and genetic diversity within the A. serratum complex (Fig. 3). While our data do not allow us to test whether introgression quantitatively modifies sex-expression strategies, the concurrence of relaxed size dependence and widespread allele sharing suggests that reproductive plasticity may facilitate opportunities for gene flow rather than result from it.

An intriguing question emerging from these findings is why sympatric species such as A. ehimense, A. tosaense, and A. ringens retain strict size-dependent sex determination despite occupying similar habitats. One possibility is that their divergent phenology or differing energy reserves limit responsiveness to transient light changes, constraining the expression of plasticity. Alternatively, underlying hormonal or gene-regulatory mechanisms may differ among taxa, with A. serratum having a greater physiological sensitivity to photic cues. Future work should experimentally test these hypotheses through controlled shading and light-manipulation experiments, coupled with hormone assays to determine whether endogenous regulators such as gibberellins or cytokinins mediate the observed responses. Comparative transcriptomic or methylation studies could also identify candidate pathways underlying light responsiveness and reveal whether these mechanisms are shared or species-specific.

The genomic results presented here, though limited in resolution, underscore the evolutionary fluidity within this species complex. The paraphyly of A. serratum relative to A. ehimense and A. tosaense, combined with widespread allele sharing, points to weak reproductive barriers and frequent historical gene flow. While the small number of individuals analysed prevents robust population-genetic inference, the congruence of morphological and molecular patterns strongly suggests that introgression is ongoing. Broader sampling across the geographic range of A. serratum—including both plantation and native-forest populations—will be essential to test whether hybridization rates are indeed higher in managed habitats and to disentangle incomplete lineage sorting from contemporary gene exchange. Such work could also clarify whether adaptive alleles associated with light sensitivity have experienced introgression across species boundaries, potentially linking hybridization to the evolution of such plastic sexual expression.

The unexpected polyphyly of A. ehimense in the phylogeny is particularly significant, as this study represents the first phylogenetic resolution of Arisaema species on Shikoku Island. The original description of A. ehimense by Murata and Ohno (1989) identifies two distinct morphotypes, one with a red appendix and another with a green appendix. Our analysis indicates that individuals with a green appendix do not form a monophyletic group with the more widely recognized red-appendix form. This suggests that the green-appendix morphotype, while superficially matching the morphological criteria of A. ehimense, may represent a hybrid between A. serratum and another species outside the A. serratum–A. tosaense hybrid complex. The f-branch analysis (Fig. 2C) supports this hypothesis, indicating one-way allele sharing with A. serratum and no other sampled species, suggesting that the second parental species was not included in our analysis. Further genetic sampling across Japan, targeting more individuals and taxa not included in this study, could help provide insight into the taxonomy of this species.

The increasing number of documented natural hybrids between A. serratum and its congeners (Murata and Ohno 1989; Murata and Ohashi 2009) further supports the idea that hybridization and introgression significantly influence the genetic structure of Arisaema populations on Shikoku Island. Collectively, our findings suggest that the demographic success of A. serratum across western Japan arises from the interaction of environmental heterogeneity, reproductive plasticity, and gene flow. By decoupling the reproductive phase from plant size and coupling it instead to light availability, individuals of A. serratum can exploit brief periods of high irradiance within plantation forests, ensuring reproductive output under otherwise limiting conditions. This ecological strategy promotes persistence and expansion in anthropogenically altered habitats, while also facilitating contact and hybridization with congeners. The resulting feedback between ecological opportunity and genetic exchange likely contributes to the morphological diversity and phylogenetic complexity characteristic of this species. Future studies integrating demographic monitoring, pollinator observations, and expanded genomic datasets will be crucial for quantifying the fitness consequences of this flexibility and for understanding how human-modified environments continue to shape the evolutionary trajectories of Arisaema and other clonal, sexually plastic forest herbs.

Taken together, our results demonstrate that light availability, rather than plant size, is the dominant predictor of sex expression in Arisaema serratum populations on Shikoku Island. This shift from a size- to a light-dependent reproductive strategy likely reflects adaptation to the shaded, spatially heterogeneous conditions characteristic of evergreen conifer plantations. The decoupling of size and light effects suggests that reproductive transitions in A. serratum are triggered directly by environmental cues rather than by accumulated biomass, allowing individuals to reproduce under the variable light regimes typical of managed forests. These ecological patterns coincide with genomic evidence of allele sharing between A. serratum and several sympatric congeners, consistent with ongoing or historical introgression within plantation and secondary-forest mosaics. We therefore interpret reproductive plasticity as the initial driver of colonization in these modified habitats, with hybridization emerging as a secondary outcome of increased spatial overlap and flowering synchrony among species. Future work combining broader population sampling, experimental manipulations of light intensity, and higher-resolution genomic data will be necessary to test whether the observed plasticity is heritable, to quantify the direction and extent of gene flow, and to clarify how anthropogenic forest structure continues to influence reproductive and evolutionary dynamics in Arisaema.

All sample sequence data, the multisample VCF, and the logistic regression matrix are openly available for download from Zenodo (https://doi.org/10.5281/zenodo.18163207). Additional supporting information may be found online in Suppl. material 1.