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Ribosomal DNA evolution and phylogeny in Aloe (Asphodelaceae)¹

² School of Biological Sciences, Queen Mary and Westfield College, University of London, E1 4NS, UK; and ³ Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK

All Aloe taxa (400 species) share a conserved bimodal karyotype with a basic genome of four large and three small submetacentric/acrocentric chromosomes. We investigated the physical organization of 18S–5.8S–26S and 5S ribosomal DNA (rDNA) using fluorescent in situ hybridization (FISH) to 13 Aloe species. The organization was compared with a phylogenetic tree of 28 species (including the 13 used for FISH) constructed by sequence analysis of the internal transcribed spacer (ITS) of 18S–5.8S–26S rDNA. The phylogeny showed little divergence within Aloe, although distinct, well-supported clades were found. FISH analysis of 5S rDNA distribution showed a similar interstitial location on a large chromosome in all species examined. In contrast, the distribution of 18S–5.8S–26S rDNA was variable, with differences in number, location, and size of loci found between species. Nevertheless, within well-supported clades, all species had the same organizational patterns. Thus, despite the striking stability of karyotype structure and location of 5S rDNA, the distribution of 18S–5.8S–26S rDNA is not so constrained and has clearly changed during Aloe speciation.

Key Words: 5S and 18S-5.8S-26S rDNA • Aloe • Asphodelaceae • bimodal karyotype • chromosomes • ITS phylogeny

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