Paraphyly of organelle DNAs in Cycas Sect. Asiorientales due to ancient ancestral polymorphisms.
BACKGROUND
This study addresses the apportionment of genetic diversity between Cycas revoluta and C. taitungensis, species that constitute the section Asiorientales and represent a unique, basal lineage of the Laurasian genus Cycas. Fossil evidence indicates divergence of the section from the rest of Cycas at least 30 million years ago. Geographically, C. taitungensis is limited to Taiwan whereas C. revoluta is found in the Ryukyu Archipelago and on mainland China.


RESULTS
The phylogenies of ribosomal ITS region of mtDNA and the intergenic spacer between atpB and rbcL genes of cpDNA were reconstructed. Phylogenetic analyses revealed paraphyly of both loci in the two species and also in the section Asiorientales. The lack of reciprocal monophyly between these long isolated sections is likely due to persistent shared ancestral polymorphisms. Molecular dating estimated that mt- and cp DNA lineages coalesced to the most recent common ancestors (TMRCA) about 327 (mt) and 204 MYA (cp), corresponding with the divergence of cycad sections in the Mesozoic.


CONCLUSIONS
Fates of newly derived mutations of cycads follow Klopfstein et al.'s surfing model where the majority of new mutations do not spread geographically and remain at low frequencies or are eventually lost by genetic drift. Only successful 'surfing mutations' reach very high frequencies and occupy a large portion of a species range. These mutations exist as dominant cytotypes across populations and species. Geographical subdivision is lacking in both species, even though recurrent gene flow by both pollen and seed is severely limited. In total, the contrasting levels between historical and ongoing gene flow, large population sizes, a long lifespan, and slow mutation rates in both organelle DNAs have all likely contributed to the unusually long duration of paraphyly in cycads.
Chiang
Yu-Chung
YC
1Department of Life Sciences, Pingtung University of Science and Technology, Pingtung, Taiwan, 912, Republic of China. yuchung@mail.npust.edu.tw
Hung
Kuo-Hsiang
KH
Moore
Shann-Jye
SJ
Ge
Xue-Jun
XJ
Huang
Shong
S
Hsu
Tsai-Wen
TW
Schaal
Barbara A
BA
Chiang
Ty
T
D002681:China; D029784:Cycas / Q000235:genetics*; D018742:DNA, Chloroplast / Q000235:genetics*; D004272:DNA, Mitochondrial / Q000235:genetics*; D018744:DNA, Plant / Q000235:genetics; D021903:DNA, Ribosomal Spacer / Q000235:genetics; D019143:Evolution, Molecular; D017343:Genes, Plant; D005828:Genetics, Population; D005843:Geography; D006239:Haplotypes; D010802:Phylogeny; D011110:Polymorphism, Genetic; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA
D018742:DNA, Chloroplast; D004272:DNA, Mitochondrial; D018744:DNA, Plant; D021903:DNA, Ribosomal Spacer