PAOMU METASEQUOIA: THE LONELY SURVIVOR OF A LIVING FOSSIL

Through morphological investigations on leaves collected from all known native Metasequoia glyptostroboides (Dawn Redwood) trees (n=5396) in southwest-central China, we found that an isolated tree living in the remote Paomu Village, Luota Township, Longshan County of Hunan Province (Paomu tree) possesses distinguished leaf cuticle characteristics that were different from all other living trees (common trees). Further examinations on cuticles from fossil Metasequoia leaves of different ages shown that the Paomu tree cuticle characters were similar to those of fossils. We proposed that the Paomu tree represented a relict of Metasequoia cuticle type that was once widespread in the ancestral population. Thus the Paomu tree should be viewed as the true “living fossil” of the genus; other representatives of the native living population of Metasequoia in China are the result of geologically recent mutation(s). Genetic analyses using rDNA ITS and 5.8S coding region have shown that the Paomu tree is genetically different from those of all other trees, and phylogenetic analysis indicates that the Paomu tree is basal to all other common trees so far sampled in the living population. Observations on other anatomical characters by various techniques, e.g., of clearing leaf, epidermis stripping, and thin sectioning, have further shown that the Paomu tree is different from common trees by having evident hypodermis which are lacking in common trees but found in all fossils by far observed. Physiologically, the Paomu tree also shows high water use efficiency as indicated by more negative values of δ¹³C of bulk leaf samples in comparison with all other common trees, consistent with morphological and anatomical results. The genetically different and morphologically unique Paomu tree increases the gene diversity of the endangered Metasequoia population. The ancestral cuticle type found in the Paomu tree provides material for a better calibration of the relationship between stomata index and CO₂ concentration.