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Ginkgo (Ginkgo biloba L.) is the only living representative of a plant lineage that has existed for at least 270 million years. However, the evolutionary relationship of gingko to other living seed plants has long been controversial and highly debated. Six different placements have been proposed for gingko in relation to other divisions of non-flowering seed plants. Recently, a research team from the Biodiversity Research Center using chloroplast phylogenomic studies and examining the reasons for discrepancies in previous classifications, has once again reported evidence that ginkgo and cycads are sister groups. Their research was published in the journal Genome Biology and Evolution on January 12, 2013.
Molecular phylogenetics is the study of evolutionary relationships between groups of organisms based on evidence from molecular sequencing data. Through data analyses, phylogenetic trees are established and organisms are grouped into several "clades". Organisms within a clade come from a common ancestor. There are several possible relationships between clades: nested (a clade within a larger clade), sisters (clades with an immediate common ancestor) or basal (if one clade branches off another before the establishment of the first member of a new clade).
Seed-bearing plants are classified into gymnosperms (or "naked" seed-bearing plants) and angiosperms (flowering plants with seeds inside fruits). Within the living gymnosperms, there are four divisions: cycads (Cycadophyta), ginkgo (Ginkgophyta), gnetophytes (Gnetophyta), and conifers (Pinophyta). Among these, the classification of gingko, which unprecedentedly has only one living member -- G. biloba -- has long been controversial.
The research team, led by Distinguished Research Fellow Shu-Miaw Chaw used analyses of chloroplast genomes to re-examine the position of gingko in the evolutionary tree of life. They constructed and analyzed the largest and most diverse gymnosperm chloroplast genome database to-date, including 25 gymnosperm chloroplast genomes, with 35,994 nucleotides or 11,998 amino acids, and carefully examined the reasons for discrepancies in previous classifications. They found that in the evolutionary trees inferred from the nucleotide sequences, the phylogenetic position of the ginkgo is significantly influenced by five factors: the breadth of the sampled species, evolutionary tree construction methods, codon sites (codon position), the position of gnetophytes, and whether or not gnetophytes are included in the data sets. Their analyses finally reaffirm that the ginkgo and cycads are sister groups.
In 1997 Dr. Shu-Miaw Chaw's group hypothesized that ginkgo and cycads were sister groups. This hypothesis, however, was based only on an analysis of the 18S ribosomal RNA gene sequences and has been questioned. It is hoped that the findings from the current study will help to quell the debate.
The full article entitled "Chloroplast Phylogenomics Indicates that Ginkgo biloba Is Sister to Cycads" is available on the Genome Biology and Evolution website at: http://gbe.oxfordjournals.org/content/5/1/243.short.
The complete list of authors is: Chung-Shien Wu, Shu-Miaw Chaw, and Ya-Yi Huang.
About ginkgo
The genus Ginkgo is believed to have existed for 270 million years. Until 150 million years ago it was still distributed worldwide. Only in China did ginkgo survive the extreme cold of the Pleistocene; elsewhere it died out. Gingko is commonly known as a living fossil. It was introduced to Europe and America in the late 17th century. Ginkgoes are able to survive severe air and soil pollution, and its trunk is pest-resistant. When soil erosion and mudslides threaten the survival of the plant, ginkgoes can generate new sprouts from their roots. Ginkgo is widely cultivated as a roadside and garden tree in many areas because of its unique biological significance and its beautiful golden autumn leaves. Ginkgo seeds are also used in Chinese and Japanese cuisine and in Chinese herbal medicine. Extracts of ginkgo leaves are used to treat cardiovascular disease.
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