COMPARISON OF NUCLEAR RIBOSOMAL-RNA GENES AMONG SOLANUM SPECIES AND OTHER SOLANACEAE

The organization of the nuclear-encoded 18S, 5.8S, and 25S ribosomal R NA genes (ribosomal DNA; rDNA) of 21 New World species from different sections of the genus Solanum, of two Old World Solanum species, and o f representatives of other Solanaceae (Nicotiana, Atropa, Datura, Phys alis, and Capsicum) was analyzed by restriction enzyme mapping using d ifferent rDNA specific hybridization probes. All Solanum species inves tigated exhibited rDNA repeats between 8.7 and 9.3 kb in length; the o nly exception was S. neorossii with a repeat length of 10.3 kb. Sequen ce heterogeneity was observed mostly in the intergenic spacer (IGS) re gion. Restriction sites for EcoRI and DraI in the spacer sequences wer e found to be characteristic for the New World species of the genus So lanum and for Lycopersicon esculentum. An additional XbaI site was det ected in the spacer region of two nontuber-bearing species, S. brevide ns and S. etuberosum (subsection Estolonifera Hawkes; series Etuberosa ), as well as in the primitive tuber-bearing species of the series Pin natisecta and Polyadenia (subsection Potatoe G.Don), thus demonstratin g that these Mexican species are separated from the other tuber-bearin g species but are closely linked to the nontuber-bearing Estolonifera group. Two EcoRI sites mapped at the 3' end of the 25S rRNA coding reg ion seem to be characteristic for members of the Solanaceae; the first EcoRI site is apparently methylated in approximately 50% of the rDNA repeats. Southern hybridization with an IGS fragment of Solanum tubero sum as hybridization probe and nucleotide sequence analysis of the phy logenetically informative 3' end of the 25S rDNA support the assumptio n that the New World species of the genus Solanum are closely related to Lycopersicon (tomato) in contrast with other Solanaceae investigate d, Nicotiana, Atropa, Datura, Physalis, and Capsicum. Moreover, the Ne w World potatoes and tomato appear to be more closely related to each other than potatoes and the Old World species Solanum nigrum and Solan um dulcamara.