Widespread in the Mediterranean area, autumn squill looks similar wherever it is found, be it in Greece, Turkey or Spain. However, a closer look at the cellular level reveals that the obvious morphological uniformity strongly contrasts with an extraordinarily high variability concerning chromosome number and structure. The greatest diversity can be found in Crete as well as around Spain, Morocco and Algeria, a fact that led to the hypothesis that Prospero autumnale originated (or survived) in these areas, from where it later made its way through Europe. Up to now it was difficult to test this hypothesis as well as to assess the evolutionary role of the enormous chromosomal variability, but rapid technological advancements in recent years allow addressing these questions.
In the FWF-project "Origin and genome evolution of chromosomal races in the polyploidy complex of Prospero autumnale" Hanna Schneeweiss and her team are addressing evolutionary questions concerning origins, mechanisms and frequencies of chromosomal changes, which will allow a better understanding of the role of chromosome change in race formation and eventually speciation. In close collaboration with researchers from the United Kingdom, the Czech Republic and Austria, around 600 plants, currently growing in the greenhouses of the Botanical Garden of the University of Vienna, will be analyzed chromosomally and genetically using a number of different approaches.
Why is this plant special?
Autumn squill is exceptional in many ways. Not only do populations often differ in their chromosome numbers (usually multiples of the chromosome base numbers of five, six or seven), but virtually every type of known chromosomal mutation has been reported from this plant. Furthermore, polyploidy - the presence of more than the normal two chromosome sets -- is frequently observed in Prospero autumnale. A change in the genomic constitution after polyploidization provokes a major challenge for the fine-tuned functioning of a cell. As a by-product of such changes, populations may become reproductively isolated, which is the hallmark of speciation.
Major drivers of chromosomal and genomic changes are repetitive sequences – part of the DNA that usually doesn't carry genetic information. Among those are "retrotransposons", ubiquitous and frequent constituents of plant genomes. They possess their own genes, which enable them to make copies of themselves, and insert them elsewhere in the genome. Apart from direct effects on genome size, this may strongly affect the evolution of genes, as Hanna Schneeweiss explains: "Sometimes, when making new copies of retroelements, adjacent plant genomic regions may be copied as well and inserted, together with the new copy of retrolement, elsewhere. Or new copies may be inserted within a functional gene, thus deactivating it."
This copy-paste mechanism of retroelements is usually activated by stress, such as environmental (for example draught, wounding by herbivores) or genomic stress after polyploidization. Changes caused by retroelement activity, such as alterations of gene activity or chromosomal rearrangements, do not necessarily have to be bad. Certain changes may be beneficial for the plant when coping with a changing environment.
How to paint the genome
To detect and analyse chromosomal changes and to assess their evolutionary significance in Prospero, Hanna Schneeweiss and her team use different methods. One of them is called FISH (Fluorescence in situ hybridization). In this process, known pieces of DNA (probes) are labeled with fluorescent dye, applied to chromosomes and allowed to bind with genetically similar regions in chromosomes. Finally, the chromosomal location of such probes is detected by the emitted fluorescent light. This technique allows direct visualization of different probes relative to each other in the chromosomes."Seeing in which chromosomes the probes localize, helps understanding how the chromosomes in different species are rearranged", explains Schneeweiss.
Sound interpretation of chromosomal and genomic diversity as well as questions concerning how Prospero autumnale dispersed or where it originated requires knowledge on the evolutionary relationships of populations. Such relationships can be inferred from analyses of selected nucleotide sequences (DNA sequences) and are shown as a phylogenetic tree. "When species diverge from one another their DNA, which constantly changes due to mutations, evolves independently. By performing statistical analysis one can deduce how they are related". (dh)
Mag. Dr. Hanna Schneeweiss from the Department of Systematic and Evolutionary Botany is team leader of the three-year long FWF project "Origin and genome evolution of chromosomal races in the polyploidy complex of Prospero autumnale" which started in October 2009. |
