Mixed mating in the ‘obligately outcrossing’Tolpis (Asteraceae) of the Canary Islands

Abstract

The genus Tolpis is largely restricted to the Canary Islands, where all of the endemic species except one are self-incompatible perennials (as indicated by levels of selfed seed set). A recent study suggested that self-incompatibility is leaky in some populations (Crawford et al. 2008, Int. J. Plant Sci. 169: 782–791). In the present study, allozyme markers were used to examine the mating systems in eight populations of Tolpis from two different islands in the Canaries. Two of the eight populations yield t (outcrossing rate) estimates close to 0.5, indicating a mixed mating system. The other populations are predominately, although probably not exclusively, outcrossing. The results for Tolpis are relevant to the broader issue of the breeding systems of the colonizing ancestors of oceanic lineages because some self compatibility would allow one propagule from a generally self-incompatible outcrossing source population to set at least some seed from selfing. This, along with the typically higher genetic diversity of individuals from outcrossing populations, may have facilitated the radiation of Tolpis in the Canaries. The present results provide the first evidence for mixed mating in what has previously been interpreted to be an ‘obligately outcrossing’ island lineage. In a more general context, the present study suggests that more detailed investigations of the breeding systems of putatively obligate outcrossing species are needed to test some of the long-held assumptions that the results would invariably show high outcrossing.