The mating system in an experimental population of rye (Secale cerenle
L.) cv. Merced was studied using eight polymorphic isozyme loci as ge
netic,markers and two progeny samples from a large population: 20 emas
culated mother plants (obligately cross-pollinating) and 100 non-emasc
ulated mother plants. Merced rye showed a significant frequency of sel
f-fertilization (s = 1-t) in a previous generation, but in the generat
ion under study the outcrossing frequency was not significantly differ
ent from t = 1. This fact suggests that environmental factors can affe
ct the rate of outcrossing vs. selfing (self-fertilization); In spite
of the outcrossing rate of t = 1, the large progenies from emasculated
plants (100 seeds per plant) demonstrated that pollination was not co
mpletely at random. This represents a violation of the generally accep
ted mixed-mating model on which most methods used to estimate mating p
arameters are based. Temporal heterogeneity and the differential abili
ty of gametophytes to fertilize are possible causes of the non-randomn
ess of mating. It is concluded that, although violations of the assump
tion of the mixed-mating model can bias the estimates, most of the est
imated outcrossing values clearly differ from complete outcrossing.