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SIX INDEPENDENT LOSSES OF THE CHLOROPLAST DNA rpl 2 INTRON IN DICOTYLEDONS: MOLECULAR AND PHYLOGENETIC IMPLICATIONS
Author(s) -
Downie Stephen R.,
Olmstead Richard G.,
Zurawski Gerard,
Soltis Douglas E.,
Soltis Pamela S.,
Watson John C.,
Palmer Jeffrey D.
Publication year - 1991
Publication title -
evolution
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.84
H-Index - 199
eISSN - 1558-5646
pISSN - 0014-3820
DOI - 10.1111/j.1558-5646.1991.tb04390.x
Subject(s) - biology , intron , chloroplast dna , marchantia polymorpha , genetics , phylogenetic tree , gene
Previous studies have shown that in several angiosperms and the liverwort Marchantia the chloroplast gene rpl 2, encoding ribosomal protein L2, is interrupted by an intron, but that in spinach ( Spinacia oleracea , Caryophyllales) this intron has been lost. We have determined the distribution of the rpl 2 intron for 390 species representing 116 angiosperm families. Filter hybridizations reveal that the intron is absent from the chloroplast genomes of all examined families of the Caryophyllales, suggesting that the intron was lost in the common ancestor of the order. Sequencing of the rpl 2 gene in five genera of the Caryophyllales and in Rumex (Polygonales) not only confirms the filter hybridization results, but also shows that for all taxa lacking the intron, the rpl 2 gene has undergone a precise deletion of the intron. In all cases, it is the original rpl 2 gene that has sustained loss of its intron. This implies that in chloroplast DNA, integration of exogenous genes (e.g., a reverse transcript of a spliced mRNA) occurs mainly by homologous, replacement recombination, rather than by illegitimate recombination elsewhere in the genome. Filter hybridizations also reveal that the rpl 2 intron was lost independently in the common ancestors of at least five other lineages of dicotyledons: Saxifragaceae ( s.s .), Convolvulaceae (including Cuscuta ), Menyanthaceae, two genera of Geraniaceae, and one genus of Droseraceae. The molecular and phylogenetic implications of these independent intron losses are discussed.