1. ¹Royal Botanic Garden Edinburgh, 20A Inverleith Row, Edinburgh, EH3 5LR, Scotland, UK
2. ²School of Biological Sciences, University of Edinburgh, Kings Buildings, Edinburgh, EH9 3JR, UK
3. ³Biological Sciences, School of Engineering and Science, University of Paisley, Paisley, Renfrewshire, PA1 2BE, UK

Correspondence: PM Hollingsworth, PM Hollingsworth, Royal Botanic Garden Edinburgh, 20A Inverleith Row, Edinburgh EH3 5LR, Scotland, UK. E-mail: p.hollingsworth@rbge.org.uk

Received 23 June 2003; Accepted 18 June 2004; Published online 18 August 2004.

Abstract

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 F_is within each population. F_is 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.