Comparison of chloroplast and nuclear phylogeny in the autogamous annual Microseris douglasii (Asteraceae: Lactuceae)

Morphology suggests that the Californian annual Microseris douglasii is a monophyletic sister group to the other three diploid annuals of Microseris. Phylogenetic analysis of 44 inbred strains of M. douglasii derived from 23 populations with 72 RAPD markers in the nuclear DNA strongly supports this phylogeny. However, 13 chloroplast RFLPs divide M. douglasii into four distinct groups. Two of these each share one or more cpRFLPs with M. bigelovii and M. pygmaea. Several hypotheses can explain the incongruence between nuclear and chloroplast phylogeny: (1) random sorting out of chloroplasts during phylogeny from a polymorphic pool, (2) cytoplasmic introgression from the related annual M. bigelovii into M. douglasii after hybridization followed by elimination of the M. bigelovii nuclear genome. We suggest cytoplasmic introgression as the most likely origin. Possible remnants of nuclear introgression have been found in two populations of M. douglasii that are polymorphic for chloroplast types. In these populations M. bigelovii type chloroplast DNA seems to be accompanied by nuclear genes for flower color and leaf shape.