What defines a bimodal karyotype? Bimodality revisited

Bimodal karyotypes, initially defined by Avdulov, are characterized by one large and one small set of chromosomes, reflecting a particular type of karyotype asymmetry. Despite later discussions by Stebbins, the absence of a quantitative criterion has led to subjective classifications. This study revisits the concept of bimodality through a literature review and proposes an objective criterion based on the ratio between the smallest chromosome of the larger set and the largest of the smaller set. Chromosome morphology and asymmetry were analyzed in 32 species previously classified as bimodal. Statistical tests were applied to detect size discontinuities and assess bimodality. We propose two forms of bimodality, interchromosomal and intrachromosomal, considering differences in size and morphology. Our results show that Drosophila melanogaster and Scaphura nigra exhibit trimodal karyotypes. A ratio of ≥1.5:1 between chromosomal subsets provides a clear and objective criterion for defining bimodality, aligning with the original concepts of Avdulov and Stebbins.