The relationship between flower size, inbreeding coefficient and inferred selfing rate in British Euphrasia species

The genus Euphrasia in Britain comprises a taxonomically complex group of self-compatible, morphologically similar, hemi-parasitic, annual plant species of high conservation importance. The 19 diploid and tetraploid taxa currently recognised show striking variation in flower size. The objective of this paper is to determine whether a relationship exists between flower size and breeding system within Euphrasia. Following a survey of flower size variation among the 19 taxa, seven diploid populations, encompassing a broad range of flower sizes, were selected for detailed study. Four nuclear microsatellite loci were used to estimate the inbreeding coefficient Fis within each population. Fis values varied from to 0.17–0.77 and showed a significant, negative correlation with flower size. These results are best explained as the consequence of variation in selfing rate among the Euphrasia populations, with selfing rate increasing as flower size decreases. The potential factors influencing breeding system evolution in Euphrasia are discussed, together with the role of autogamy in generating taxonomic complexity and facilitating lineage differentiation within the genus.