Mixed-mating systems in Dalechampia scandens: is herkogamy a predictor of the magnitude of inbreeding depression?

Abstract

Mixed mating systems in plants occur when both out crossing and self-fertilization occur in the same population. Costs of inbreeding and benefits of insuring reproduction for example in absence of pollinator are expected to maintain mixed-mating system. Plants can avoid selfing through herkogamy and/or dichogamy the separation in space and time of the male and female organs. On the other hand, inbreeding depression can be lowered by repeatedly exposing deleterious genes to selection via repeated events of inbreeding, a mechanism referred to as purging. In this study, I tested whether auto-fertilization rate was negatively correlated with the level of herkogamy and whether populations with a low level of herkogamy (sASD) showed lower level of inbreeding depression due to higher opportunity for purging. I analyzed the effect of different level of herkogamy on the auto fertilization rate between two populations of the Neotropical vine Dalechampia scandens (Euphorbiaceae). I then compare the performances of inbred and out bred crosses for several fitness components between both populations. Increased herkogamy decreased the rate of auto fertilization. Seeds from inbred crosses were lighter than the ones from out bred crosses in both populations. However, there was no difference in germination rate between cross types in either population although germination rate was much lower in the low herkogamy population. Vigour at two weeks was lower for inbred seedlings in the high herkogamy population, but this effect was not stronger in harsh environment. Furthermore, this effect disappeared at four weeks. These results suggest that differences in seed mass and vigour at two weeks were most likely due to differential maternal investment between the inbred and out bred crosses and not to inbreeding depression.