Phylogenetic and Evolutionary Implications of Interspecific Chloroplast DNA Variation in Krigia (Asteraceae-Lactuceae)

Abstract

Chloroplast DNA restriction site and length variation was examined among 12 taxa of Krigia, using 34 restriction endonucleases, filter hybridization experiments, and comparative mapping procedures. A total of 252 restriction site mutations was detected, 159 of which were phylogenetically informative. The resulting phylogenetic trees correlated well with cytological and morphological data. The two sects. Krigia and Cymbia differed by at least 73 restriction site mutations. Most species within each section were characterized by many restriction site changes. Interspecific sequence divergence in Krigia was much higher than in any other previously studied genus of the Asteraceae and was comparable to the high levels of divergence found in genera of other angiosperm families. Relative rates of chloroplast DNA evolution in Krigia were significantly different among the various chromosome number groups, resulting in the rejection of a molecular clock. However, a molecular clock could not be rejected within each chromosome number group. Different regions of the chloroplast genome also showed extremely different frequencies of mutations and four hotspot areas were identified. The areas occurred in long spacer regions between gene coding sequences. Furthermore, most of the homoplasy and length variation also occurred within these hotspots. Chloroplast DNA trees were consistent with the hypothesis that an ancient allopolyploid event between species with low chromosome base numbers, followed by subsequent aneuploid reduction, resulted in K. wrightii with n = 9. The results also suggest that x = 5 is the probable ancestral base number for the genus and that aneuploid reduction is the major mode of chromosome number change in Krigia and related genera.