O que define um cariótipo bimodal? revisitando a bimodalidade

Abstract

Bimodal karyotypes present two classes of chromosomes, one formed by large chromosomes and the other formed by small chromosomes, so that the difference between the classes is sharply distinct, representing the extreme of karyotypic asymmetry. This definition was originally proposed by Avdulov in 1931 and later discussed by Stebbins in 1971. Species such as Eleutherine bulbosa have a markedly bimodal karyotype, however this difference is not always obvious. In view of the absence of an objective criterion to define the level of discontinuity between two classes of chromosomes in a putative bimodal karyotype, the classification becomes subjective. The objective of this work was to revisit the concept of bimodal karyotype through a literature review, and to propose a quantitative criterion in agreement with Avdulov and Stebbins. Chromosome morphology and asymmetry were evaluated in 37 species mentioned as having bimodal karyotypes. These species were compared with eight reference species used by Stebbins, separated into asymmetric or bimodal. Cluster analysis by Euclidean distance was used to assess the discontinuity in chromosome size in each karyotype. The bimodality concepts presented in the papers were compared with the one originally proposed by Avdulov. The chromosome numbers ranged from 2n = 8 in Drosophila melanogaster, Hypochaeris brasiliensis, H. catharinensis and H. chillensis to 2n = 140 in Agave fourcroydes. Based on the A2 interchromosomal asymmetry index, Scaphura nigra had the most asymmetric karyotype with A2 = 1.0, while Puya mirabilis was the most symmetric with A2 = 0.25. For Stebbins' interchromosomal asymmetry categories, 20 species presented B karyotypes, while 16 species presented C karyotypes. Considering a Euclidean distance ≥ 40%, 19 species presented karyotypes with two chromosomal classes. On the other hand, some species presented three classes, while others showed continuous variation. Stebbins demonstrates through reference species that the discontinuity between the smallest chromosome of the largest subset and the largest chromosome of the smallest subset in species with a bimodal karyotype is ≥ 2: 1. On the other hand, all species with a strongly asymmetric karyotype had a ratio ≤ 1.5: 1 between the chromosomes involved in the discontinuity. One of the main criteria involved in the concept of bimodality is sharply distinct difference between two classes of chromosomes. For this reason, we propose the ratio ≥ 2: 1 between the smallest chromosome of the largest subset, and the largest chromosome of the smallest subset, as a quantitative criterion for the definition of a bimodal karyotype.