MULTIPLE INDEPENDENT LOSSES OF THE PLASTID RPOC1 INTRON IN MEDICAGO (FABACEAE) AS INFERRED FROM PHYLOGENETIC ANALYSES OF NUCLEAR RIBOSOMAL DNA INTERNAL TRANSCRIBED SPACER SEQUENCES

A previous polymerase chain reaction based survey for the occurrence o f the intron in chloroplast gene rpoC1 revealed its absence in one of the eight species of Medicago (Fabaceae; Trifolieae) examined. We exte nd the survey of Medicago to include 65 of the 86 species, representin g all 12 sections and seven of the eight subsections recognized in the most recent comprehensive treatment of the genus. Our results indicat e that 17 species from five sections lack the intron and that three of these sections are heterogeneous with regard to intron content. DNA s equencing across the rpoC1 intron-exon boundary in three of these spec ies reveals the precise excision of the intron from the gene. Phylogen ies derived from nuclear ribosomal DNA internal transcribed spacer seq uences, estimated using maximum parsimony and maximum likelihood metho ds, suggest that most of the previously recognized sections in Medicag o are not monophyletic as currently circumscribed. Furthermore, these results suggest that the ipoC1 intron has been lost independently a mi nimum of three times during the evolution of the group. The occurrence of multiple independent intron losses severely reduces the utility of this character as a phylogenetic marker in Medicago.