Mating systems in Brassicaceae: Self-incompatibility system in the genus Capsella at the molecular and phenotypic level

Melanie Paetsch, Barbara Neuffer

In flowering plants, especially in short living plants, the reproductive phase is of particular importance. Beside seed dispersal the pollen transfer represents one opportunity for genes to move among individuals and populations. Therefore, mating strategies have diverse genetic consequences: outcrossing species may acquire intrapopulation variability more easily, whereas selfing species may increase colonisation ability and spreading out from their place of origin quickly and successfully often accompanied by bottle neck effects.  

Approximately 30 % of angiosperms are estimated to be predominantly selfing and it becomes obvious, that in some cases the advantages gained by self-fertilization outweigh the disadvantages as inbreeding depression and lack of gene recombination. Due to assurance of seed set, gene combinations which proved successful will stay together and certain genes will become fixed. The fixing of variants may have local adaptive advantage. 

Many genera in the Brassicaceae include both outcrossing and selfing species. Therefore, one has to assume that the transition from self-incompatibility (SI) to self-compatibility (SC) occured frequently and independently, whereas SI is thought to be the ancestral condition.

The genus Capsella comprises three species, two diploids and one tetraploid. The diploid C. grandiflora (Fauché & Chamb.) Boiss. represents the ancestral obligate outbreeding species from which C. rubella Reuter (2x) and C. bursa-pastoris (L.) Medik. (4x) originated.  The self-incompatible C. grandiflora is restricted to the western balkan, whereas the self-compatible C. rubella has colonised the Mediterranean climatic regions worldwide. In C. rubella the SI system broke down resulting in a predominantly selfing species with no change in the ploidy level. The breakdown of the SI system in Capsella coincides with reduction of flower size, colonising ability and a wider distribution range, dislaying ecotypic differentiation. We hypothesize that the breakdown of the SI system seems to be one of the key events in the evolution of the genus Capsella.  

	Inflorescences of Capsella species: The breakdown of the SI-system in Capsella rubella coincides with morphological changes, for example, reduction of flower size and loss of fragrance

In the Brassicaceae, outbreeding is genetically forced by a sporophytic self-incompatibility (SI) system. The genes responsible for recognition and rejection of self-pollen are located at the S-locus. We concentrate on the SRK gene because the publicised results allow us draw the conclusion that the reasons for the breakdown are likely to be found in the SRK gene itself.

The aims of the project are the identification and characterisation of SRK-genes in the genus Capsella, search for S-haplotypes in natural populations of diploid Capsella species,  establishing linkage to the S-locus controlling SI-phenotypes in natural Capsella populations in crossing experiments. We will gather sequence information of regions flanking the currently known part of the putative SRK in order to reveal the complete sequence of the gene, including transmembrane and kinase domain exons. Comparative analysis of SI to SC taxa then will show if mutations can be found which might be responsible for the breakdown of the SI system in C. rubella. Another reason for an SC phenotype may be due to dominance of SC over SI. This will be tested in crossing experiments.