AOBPreview originally published online on July 9, 2009
Annals of Botany 2009 104(4):681-688; doi:10.1093/aob/mcp161
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A bicontinental origin of polyploid Australian/New Zealand Lepidium species (Brassicaceae)? Evidence from genomic in situ hybridization
1 Universität Osnabrück, Abteilung Botanik, Barbarastrasse 11, 49076 Osnabrück, Germany
2 Masaryk University, Department of Functional Genomics & Proteomics, Institute of Experimental Biology, Kamenice 5, Building A2, CZ-62500 Brno, Czech Republic
* For correspondence. E-mail Tomiworld{at}gmx.de
Received: 23 March 2009 Returned for revision: 5 May 2009 Accepted: 1 June 2009 Published electronically: 9 July 2009
Background and Aims: Incongruence between chloroplast and nuclear DNA phylogenies, and single additive nucleotide positions in internal transcribed spacer (ITS) sequences of polyploid Australian/New Zealand (NZ) Lepidium species have been used to suggest a bicontinental hybrid origin. This pattern was explained by two trans-oceanic dispersals of Lepidium species from California and Africa and subsequent hybridization followed by homogenization of the ribosomal DNA sequence either to the Californian (C-clade) or to the African ITS-type (A-clade) in two different ITS-lineages of Australian/NZ Lepidium polyploids.
Methods: Genomic in situ hybridization (GISH) was used to unravel the genomic origin of polyploid Australian/NZ Lepidium species. Fluorescence in situ hybridization (FISH) with ribosomal DNA (rDNA) probes was applied to test the purported ITS evolution, and to facilitate chromosome counting in high-numbered polyploids.
Key Results: In Australian/NZ A-clade Lepidium polyploids, GISH identified African and Australian/NZ C-clade species as putative ancestral genomes. Neither the African nor the Californian genome were detected in Australian/NZ C-clade species and the Californian genome was not detected in Australian/NZ A-clade species. Five of the eight polyploid species (from 7x to 11x) displayed a diploid-like set of rDNA loci. Even the undecaploid species Lepidium muelleriferdinandi (2n = 11x = 88) showed only one pair of each rDNA repeat. In A-clade allopolyploids, in situ rDNA localization combined with GISH corroborated the presence of the African ITS-type.
Conclusions: The nuclear genomes of African and Australian/NZ C-clade species were detected by GISH in allopolyploid Australian/NZ Lepidium species of the A-clade, supporting their hybrid origin. The presumed hybrid origin of Australian/NZ C-clade taxa could not be confirmed. Hence, it is assumed that Californian ancestral taxa experienced rapid radiation in Australia/NZ into extant C-clade polyploid taxa followed by hybridization with African species. As a result, A-clade allopolyploid Lepidium species share the Californian chloroplast type and the African ITS-type with the C-clade Australian/NZ polyploid and African diploid species, respectively.
Key words: Lepidium, Brassicaceae, FISH, GISH, hybridization, polyploidy, long-distance dispersal, ITS, rDNA, Australia, New Zealand
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