BIPARENTAL INBREEDING DEPRESSION IN THE SELF‐INCOMPATIBLE ANNUAL PLANT GAILLARDIA PULCHELLA (ASTERACEAE)

Biparental inbreeding is thought to be a common feature of plant populations with restricted pollen dispersal. It is generally assumed that the inbreeding depression frequently observed to accompany self‐fertilization can be extrapolated to the lesser degrees of consanguinity involved in biparental inbreeding, but this is virtually untested. To test this assumption, seeds collected from a single natural population of the self‐incompatible annual Gaillardia pulchella were used to generate full‐sib families derived by crossing either noninbred full‐sibs (inbred families) or noninbred nonrelatives (outbred families). Members of each family were divided between high‐stress and low‐stress treatments that differed in soil volume and nutrient level. Inbred seedlings had a lower chance of survival, were more likely to be morphologically abnormal, and grew more slowly than outbred seedlings, indicating the presence of biparental inbreeding depression. Stress treatment had no significant effect on inbreeding depression, and no family stress‐environment interactions were detected. Inbreeding did not increase the among‐family variance in growth rate, suggesting that inbreeding depression of growth rate is caused by many genes with small individual effects. Relative to direct estimates of inbreeding depression, observed levels of near‐neighbor outcrossing depression, presumed to be biparental inbreeding depression, are surprisingly high in many plant species.