Why Sequence Campanulales Chloroplasts?

Something happened in the plant order Campanulales that fundamentally destabilized the chloroplast genome. Chloroplast gene order is highly conserved among virtually all land plants, and foreign DNA is not normally incorporated into the chloroplast genome. In the Campanulales, however, inversions and other genome rearrangements occur exceptionally often, and these are commonly associated with the insertion of foreign DNA with open reading frames and a suite of other unusual characteristics. What are the underlying molecular bases for this unusual genomic instability? By sequencing the chloroplast genomes of 50 selected species, we will obtain the comparative evidence needed to address this question for the Lobeliaceae and Cyphiaceae, two of the large families in the Campanulales.

In this group, the tempo of speciation has generally exceeded the tempo of genome rearrangement, such that almost every rearrangement step leading to a complex, highly rearranged extant genome occurred sequentially, has therefore been "preserved" in the phylogeny, and can be reconstructed by comparative analysis. This is the first group of exceptionally rearranged organelle genomes--certainly for any chloroplast lineage and probably for any mitochondrial lineage too--where it is feasible to phylogenetically disentangle a series of complex rearrangements as individual events. This chloroplast system thus provides the best opportunity yet to elucidate the underlying molecular bases for exceptional instability in organelle genome evolution. This project aims to do just that through targeted sequencing of chloroplast genomes that triangulate the nodes just below and just above each rearrangement event. In addition, as the first study to sequence a large number of chloroplast genomes throughout a large, phylogenetically well-resolved group of plants of moderate (5-50 million years) divergence time, this work will yield unparalleled information on the tempo and pattern of chloroplast genome evolution in general.

CSP project participants: Eric B. Knox (Indiana Univ.).