Karyotype analysis and physical mapping of the 5S and 45S rDNA genes in Hymenocallis howardii (Amaryllidaceae) by FISH

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Abstract Hymenocallis howardii Bauml is a bulbous species belonging to the monocotyledonous family Amaryllidaceae and is distributed in southwestern Mexico. Chromosome numbers reported for the genus Hymenocallis vary widely, ranging from 2n = 24 and 2n = 110. The aim of this study was to determine the chromosome number and to conduct karyotype analysis and physical mapping of 45S and 5S ribosomal DNA (rDNA) in H. howardii. These analyses were performed in root meristematic cells using DAPI staining and fluorescence in situ hybridization (FISH). The number of chromosomes found for this species was 96. The 45S rDNA signals were detected at two loci located in the telomeric regions of the short chromosome arms, while six loci of 5S rDNA were observed in telomeric and subtelomeric positions, also on the short arms. The karyotype formula was 64m + 30sm + 2st, and the karyotype symmetry/asymmetry indices (TF% = 40.43, AsK% = 59.56, Syi% = 67.88) indicated a slightly asymmetric karyotype, characterized by a gradual decrease in chromosome length from 10.46 µm to 3.36 µm, with a predominance of metacentric and submetacentric chromosomes. The presence of only two 45S rDNA sites in H. howardii suggests that it is a paleopolyploid species.

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