Systematic implications of isozyme number variation in Tribe Brassiceae (Brassicaceae).

Abstract

Tribe Brassiceae (haploid chromosome number, n=6$-$75) is generally recognized as one of the few natural (monophyletic) groups in the Brassicaceae. Recent molecular analyses, however, have questioned the validity of this subtribal classification and have proposed new generic and subtribal circumscriptions. The present study examined isozyme number, which is usually highly conserved in diploid plants, in order to assess the evolution of chromosome number and systematic relationships in the tribe. Ten enzyme systems were surveyed for 108 species in 35 genera of tribe Brassiceae and for eleven species from seven other tribes of the family. The observed variation in isozyme number among taxa was phylogenetically informative. Duplications for cytosolic phosphoglucomutase (Pgm-2) and plastid triosephosphate isomerase (Tpi-1) were evident in 33 of the 35 Brassiceae genera examined, supporting the monophyletic nature of the tribe, with inclusion of Orychophragmus and exclusion of Calepina and Conringia. Duplications for cytosolic isocitrate dehydrogenase (Idh-2) were observed in members of Brassica, Diplotaxis, Moricandia and Pseuderucaria, confirming that subtribes Moricandiinae and Brassicinae are artificially separated, as indicated by restriction site analysis of chloroplast DNA and hybridization data. The only subtribe clearly supported as monophyletic by the isozyme data was the Cakilinae which displayed duplications for cytosolic glucose-6-phosphate isomerase (Gpi-2) and Idh-2 in several taxa. The present study also compared allozyme patterns of select allopolyploids and their proposed parental species. These analyses verified Diplotaxis tenuifolia (n = 11) and D. viminea (n = 10), and D. erucoides (n = 7) and Erucastrum nasturtiifolium (n = 8) as respective parental taxa of D. muralis (n = 10 + 11) and E. gallicum (n = 7 + 8). In addition, inheritance studies conducted on Sinapis arvensis confirmed Mendelian inheritance at seven isozyme loci.