Speciation in the Lamiaceae
Background: Biology employs different species concepts - the classical morphological concept, introduced by Carl v. Linné, is based on similarity in shape and originally was conceived without any evolutionary model: Linné believed, as most scientists of his time, into species constancy, because God has created the species as they are. As a logical consequence of evolution theory, species were later defined as genetic entities - according to Mayr and Dobzhansky, a species is just a group of individuals that exchange genes and are separated from other groups by propagation barreers. As pointed out in the lecture, both concepts are not really representing the situation in plants very appropriately. The search for a third, unifying, species concept applicable to plants, is still ongoing. The challenge for each species concept are the transitions between species, the cases, where a new species is just in the process to be born (incipient speciation). The Lamiaceae are something like a test tube to investigate incipient speciation and therefore also to test and validate new concepts for plant species. With more than 7000 species, this family is one of the hardest nuts to crack for taxonomy, and taxonomy of this group has therefore been under continuous reconstruction. According to a bonmot by David Lindley, a British taxonomist, the taxonomic confusion of this family has become "a disgrace to Botany" (1828). The Lamiaceae belong to the most advanced groups of Angiosperms with complex, patterned, and dorsiventral flowers and a high specificity of pollination. This specificity is partially brought about by attraction of specific pollinator species, but it is also achieved by morphological mechanisms that ensure that different pollen transported by the same pollinator will nevertheless reach the stigma of a specific recipient. This "one postman - many letters" principle has been discovered first in Salvia (Sage), but also seems to be present in other genera. We currently investigate this in Dragonhead (Dracocephalum), a genus which is also medically relevant and harbours more than 40 species that are often sympatric. Within Dragonhead, there seem to be incipient lines: the moldavica cluster with long stamina that might employ the geometry of stamina versus stigma to provide specificity, in a way similar to Salvia, and the ruyschiana cluster, where stamina are shorter and hidden in the lip, but are protruding, when the pollinator sqeezes into the flower, a row of hairs at the upper lip will later brush off the pollen deposited at the neck of the animal. We hypothesize that this difference in pollination will act as a channeling force for speciation in this genus. Thus, the initial genetic difference may be very small and therefore might be overlooked in phylogenetic studies that are usually based on trait-unrelated genetic markers.
Task: To get to the core of incipient speciation, it is important to identify those few genes that launch reproductive channelling. These genes must act in the morphogenesis and development of flower organs. For instance, small mutations in the promotor of such morphogenetic genes might result in a slight delay of expression, which would have the consequence that the corresponding cellular process (for instance cell elongation in the filament of anthers) is slower in relation to other flower organs (for instance cell elongation in the stylum). The task is now to select three flower traits and to find out, what candidate genes might be relevant for the control of this trait, for these candidate genes, GenBank accessions should be searched - if possible for Dracocephalum, if not available for Dracocephalum, for other Lamiaceae. The delivered document should contain three traits with:
- name of the selected trait
- three candidate genes and a short statement why these candidates have been selected
- one GenBank accession for each of these candidate genes
Advice: If you need advice in constructing a search, please refer to this page and get the information, how to construct a search matrix, or how to enter Biosis. The relevant information has been studied in details in model plants, especially for Arabidopsis and tobacco, you will easily find review papers, where the processes involved in the development of flower organs are described.
- background on this topic (contains unpublished material, therefore protected by password). here
- a publication on the "one postman - many letters" principle in Salvia can be obtained under KIT licence here
- for advanced readers: how the staminal lever mechanism evolved several times in Salvia, publication can be obtained under KIT licence here