Cytogenetic assessment of Haworthia using flow cytometry and fluorescence in situ hybridization

The purpose of this study was to investigate cytogenetic variations between the nine Haworthia species necessary for subsequent breeding. Flow cytometric analysis of H. badia 'Murasaki', H. splendens, H. truncata 'Seiko', H. cooperi var. obtusa, H. cymbiformis, H. mirabilis var. mundula, and H. retusa showed a wide range of DNA content from 10.17 to 12.17 pg/1C, whereas two tetraploids, H. limifolia and H. angustifolia var. baylisii, showed higher DNA content of 16.81 and 12.64 pg/1C, respectively. Seven diploid Haworthia expressed three karyotype formulas, whereas two tetraploid Haworthia each expressed a different karyotype formula. Secondary constriction was observed in H. truncata 'Seiko' and H. cooperi var. obtusa. Results of fluorescence in situ hybridization (FISH) showed a major difference between the diploid and tetraploid Haworthia in the distribution of 5S and 18S ribosomal DNA (rDNA) loci. There were eight loci in two out of seven diploids and both tetraploid Haworthia, while H. cymbiformis and H. mirabilis var. mundula each showed seven 18S rDNA loci. H. badia 'Murasaki', H. splendens, and H. retusa had four, three, and five 18S rDNA loci, respectively. In addition, a single pair of 5S rDNA loci was observed in all diploid Haworthia except H. cooperi var. obtusa, which had four 5S rDNA loci. Each tetraploid Haworthia had four 5S rDNA loci, with two strong and two weak signals. All 5S and 18S rDNA loci were observed on the long arm, in the subtelomeric or telomeric region, while a significant variation in chromosome length was observed among the Haworthia. This cytological analysis provides the necessary genetic information to elucidate the genetic makeup of newly introduced Haworthia hybrids.