Frequent origins and unidirectional gene-flow sustain cytotype coexistence in a mixed-ploidy species

Mixed-ploidy species provide key insights into polyploid plant evolution, but the role of inter-cytotype gene-flow in their establishment and maintenance remains unclear due to the complex polysomic and heterogeneous genetic background of polyploids. Here, we established a mathematical model to describe inter-cytotype gene-flow patterns and validated it with migration rate data from the mixed-ploidy species Chamerion angustifolium. We further assessed the mechanisms underlying cytotype establishment and maintenance by estimating genetic diversity, habitat suitability distribution and population structure. Both the model and genetic analyses revealed that intermediate-level cytotypes (e.g., tetraploids in C. angustifolium) received higher inter-cytotype gene-flow, maintained greater genetic diversity, and harbored more private alleles. Habitat suitability distribution modeling revealed a significantly smaller and overlapping suitable area for polyploids relative to diploids. Genetic structure analyses revealed weak differentiation among cytotypes and a geography-driven clustering pattern, indicating multiple origins of polyploids. Taken together, we hypothesize that incomplete reproductive isolation and weak genetic isolation in C. angustifolium enables inter-cytotype gene-flow. Moreover, cytotype coexistence in mixed-ploidy species is sustained by: (i) unidirectional gene-flow (from diploids to polyploids), (ii) climatic-niche differentiation of polyploid populations, and (iii) frequent origins of polyploids.