Cytogenetic basis of sexual dimorphism in <i>Cycas rumphii</i>: Insights from mitotic and meiotic analysis

Cycas rumphii Miq. (queen sago) is a dioecious gymnosperm of considerable evolutionary and ecological significance. This study provides a comprehensive cytogenetic comparison of male and female C. rumphii through integrated mitotic and meiotic analyses. Mitotic investigation revealed a uniform diploid chromosome complement (2n=22) in both sexes. Detailed karyotype assessment revealed subtle morphological distinctions, with male plants having heteromorphic sex chromosomes and females having homomorphic karyotypes. Both sexes lack satellite-bearing chromosomes; however, males bear distinct heteromorphic sex chromosome pairs, suggesting a reliable sex-linked chromosomal marker. Meiotic evaluation of male sporocytes revealed normal progression from leptotene to tetrad formation. During diakinesis, 11 ring bivalents were observed with their subsequent equatorial alignment at metaphase, followed by regular segregation at anaphase I and balanced tetrad formation, indicating the absence of meiotic irregularities. According to Stebbins’ (1971) asymmetry index, the male C. rumphii karyotype was assigned to the 2A class, showing low asymmetry, while the female counterparts fell into the 2B class, revealing moderate asymmetry, thereby reflecting a subtle intra-karyotype heterogeneity. These findings provide deeper insights into the cytogenetics of C. rumphii, elucidating the chromosomal features underlying sex determination and meiotic stability. These insights offer valuable markers for early sex identification and contribute to conservation strategies aimed at safeguarding the diversity of cycads.