A tale of reduction and reversion –
In the Paleozoic Era, but more particularly in the succeeding Mesozoic Age, plants related to the modern ginkgo were distributed worldwide, and had diversified into many families and genera. With the sole exception of the popular Ginkgo biloba, all representatives of this once dominant plant type are now extinct. A study of ginkgo fossils by Thilo Fischer of LMU Munich, Rainer Butzmann (Munich), Barbara Meller of Vienna University and Evelyn Kustatscher of the Museum of Nature South Tyrol now sheds light on an important step in the evolution of the fruiting bodies of these ancient plants. In the extant G. biloba tree, the seeds develop on a bipartite stalk. In contrast, the newly discovered fossils suggest that the seeds of ancestral forms were directly attached to leaves. “This supports the old hypothesis that the modern stalked seed form evolved from seed-bearing leaves by a reductional transformation,” says Fischer. “The finding is based on well preserved fossil leaves and seeds of ginkgo plants, which we were lucky enough to discover in the Bletterbach Gorge in the Dolomites of South Tyrol a few years ago. Such specimens are extremely rare.” The project was made possible by the support of the Museum of Nature South Tyrol in Bolzano and the Geoparc Bletterbach. (BMC Evolutionary Biology, November 2010)
The Bletterbach Gorge, together with eight other striking formations in the Dolomites, was designated as a World Heritage Site by UNESCO in 2009. The ravine owes this distinction to its geological setting. Here, huge volumes of rock have been progressively removed by erosion, leading to the exposure of strata that provide a window into millions of years of Earth history. Many significant fossil finds have come to light in the Bletterbach Gorge. These include now the approximately 700 specimens of fossil plants which were recovered from a virtually inaccessible site by the authors of the new study in the years 2003 to 2009.
The most important component of the collection consists of well preserved macroscopic remains of ginkgo leaves – the first fossil ginkgophyte leaves to be found in the Dolomites. And some of these leaves turned out to be special in another respect. In the extant ginkgophyte G. biloba the seeds form on a short, dichotomously divided stalk, but some of the new fossils have seeds that appear to arise directly from the leaf margin. The fossil seeds were carefully examined by light microscopy and scanning electron microscopy, and were found to be closely similar to those of the modern ginkgo in other respects.
During the 19th century, some botanists had already noticed that, in rare cases, seed formation in ginkgo trees could give rise to an unusual and highly characteristic type of reproductive structure, which is directly attached to the leaf. In Japan, modern ginkgo trees that display this aberrant class of seed structure are known by the name “o-ha-tsuki” (“seed on leaf”) and they enjoy special protection as national treasures. “Some botanists suggested early on that the o-ha-tsuki variant might actually represent the ancestral form of the reproductive structure in ginkgos“, says Fischer. “It might be the result of a reversion mutation that restores the evolutionarily older state. According to this hypothesis, the seed form that we find in the modern ginkgo arose by a reductional transformation of the leaf. The fossils found in the Bletterbach Gorge are compatible with this long-standing idea.“
"Permian Ginkgophyte fossils from the Dolomites resemble extant O-ha-tsuki aberrant leaf-like fructifications of Ginkgo biloba L."
BMC Evolutionary Biology 2010, 10:337
Prof. Dr. Thilo Fischer
Chair of Biochemistry and Physiology of Plants
Großhaderner Str. 2 - 4