Low levels of inbreeding depression and enhanced fitness in cleistogamous progeny in the annual plant Triodanis perfoliata

The maintenance of outcrossing in cleistogamous plants that produce both open, facultatively outcrossing chasmogamous (CH), and closed, obligate selfing cleistogamous (CL) flowers is puzzling because CL reproduction is thought to be more reliable and less costly. A possible explanation for the maintenance of CH flowers is the avoidance of inbreeding depression. However, inbreeding depression for cleistogamous species has rarely been quantified. In this study, we estimate levels of inbreeding depression in plants from three populations of Triodanis perfoliata (L.) Nieuwl., a dimorphic cleistogamous annual, under greenhouse conditions. Estimates of inbreeding depression at multiple life stages in all three populations were low and often not different from zero. Inbreeding depression at specific life stages varied, with two populations showing later-acting inbreeding depression, which is also found in other selfing species. In two of the study populations, selfed CL progeny outperformed selfed CH progeny, indicating a flower-type effect. The low levels of inbreeding depression and the superior fitness of CL compared with selfed CH flowers that we observed make the maintenance of CH flowers in this system surprising, and suggest that other advantages of outcrossing CH flowers are likely responsible for maintaining mixed mating in this species.