Cytogenetic constraints on hybridization: A meta-analysis investigating the role of chromosome number in monocot hybrid evolution using a newly developed tool, the ploidy deviation index (PDI)

BACKGROUND AND AIMS: Hybridization is a major driver of plant diversity, yet the role of cytogenetic compatibility, particularly differences in chromosome number, remains poorly understood. Differences in parental chromosome number can present barriers to hybrid formation by disrupting meiotic stability, but the extent to which biological and ecological factors influence the chromosomal architecture of hybrids remains poorly quantified, especially in monocots. This study aims to investigate how chromosome number divergence interacts with biological and ecological factors to shape hybrid formation in monocots, using a novel quantitative metric, the Ploidy Deviation Index (PDI), to standardize comparisons of hybrid cytogenetic architecture. MATERIAL AND METHODS: We developed and applied the PDI, a continuous index quantifying chromosome-number deviation between a hybrid and its two parents, across approximately 200 hybrid cases with documented parental karyotypes. Hybrids were categorized as homoploid, uniparentally homoploid, intermediate, or polyploid based on their PDI values. We analyzed the distribution of PDI scores in relation to type of hybrid origin (natural vs. artificial), growth habit, size of the genus (a proxy for richness), and range of chromosome number within a genus (proxy for diversity). Comparisons across categories employed Anderson-Darling k-sample tests, multinomial logistic regression, and Mann-Whitney U tests to determine significance. KEY RESULTS: Homoploid hybrids were found to be the most frequent. We found no significant difference in PDI distributions between natural and artificial hybrids. Significant variation in PDI distributions was found among growth habits, with aquatic hybrids more likely to be homoploid and geophytic hybrids showing higher proportions of polyploidy. Intermediate hybrids were common in larger genera with broader chromosome-number ranges, whereas polyploid hybrids showed the highest PDI values in large and karyotypically diverse genera. CONCLUSION: These results challenge long-held assumptions that polyploidy dominates hybrid formation and reveal that homoploid and intermediate chromosomal configurations are common in monocots. The PDI framework offers a powerful, standardized approach for assessing cytogenetic constraints on hybridization, with implications for systematics, evolutionary biology, and conservation.