Palaeopolyploidy in fossil plants: evidence from 200 million year old pollen tetrads
Friday seminar by Wolfram M. Kürschner
Polyploidy, or whole genome doubling is regarded as an important evolutionary phenomenon. It is a key mechanism for plant speciation leading to new evolutionary lineages for many angiosperms and may be the spark for the dawn of flowering plants. It has been also suggested that polyploid plant species coped better with environmental stress than their diploid relatives, improving their survival chances during periods of dramatic biodiversity decline such as during the End-Cretaceous mass extinction, some 65 million years ago.
So far, however, the fossil record of plants has remained largely unexplored as a source of confirming information on ancient polyploidization. As cell size correlates with the DNA content stomatal size of living and fossil leaf remains has been used to infer ploidy history in angiosperms during the Cenozoic. Similar to the stomatal size, pollen size and volume also varies with the ploidy level of a plant.
In my talk I will present data from the fossil pollen “Classopollis” extracted from sediments of the Triassic – Jurassic transition, which is characterized by a major environmental crisis about 200 million years ago. Tetrad morphology and the variation in size of Classopollis show an exceptional high variability such as the occurrence of aberrant pollen tetrads, triads and dyads and the distinct variation of pollen size. We interpret the variability in pollen size and tetrads as the formation of unreduced (2n) gametes, which is one of the main mechanisms in modern polyploid formation. The pollen size pattern suggests the presence of ancient whole genome doubling events in the conifer family Cheirolepidiaceae. Polyploid speciation may explain the high variability of growth forms and adaptation of these conifers to different environments / resistance to extreme growth conditions. We suggest that polyploidy may have also reduced the extinction risk of these conifers during the Triassic - Jurassic biotic crisis.
Dr. Wolfram M. Kürschner
Department of Geosciences and Centre for Earth Evolution and Dynamics (CEED), University of Oslo