Effect of inbreeding depression on outcrossing rates among populations of a tropical pine

• Inbreeding depression is common among plants and may distort mating system estimates. Mating system studies traditionally ignore this effect, nonetheless an assessment of inbreeding depression that may have occurred before progeny evaluation could be necessary. • In the neotropical Pinus chiapensis inbreeding depression was evaluated using regression analysis relating progeny F-values with seed germinability, the mating system was analysed in three populations with contrasting size, using isozymes, obtained a corrected outcrossing rate. • Selfing decreased seed viability by 19%, relative to an outcrossed plant. Multilocus outcrossing rates, t_m, varied widely among populations. In the two smallest populations t_m ≅ 1. Therefore, inbreeding depression did not affect the estimates, but overestimated t_m by 10% in the third population, which has a true mixed mating system (selfing was the major source of inbreeding), and an unusually low t_m for pines (t_m = 0.54, uncorrected, t _m = 0.49, corrected). • Inbreeding depression may be an uneven source of bias for outcrossing estimates even at the infraspecific level. Precision but not accuracy may be gained by including inbreeding depression in outcrossing estimates. Therefore, caution should be taken when comparing t _m among species or even populations within the same species.