GEOGRAPHICAL VARIATION IN THE INBREEDING DEPRESSION OF SCOTS PINE

The magnitude of inbreeding depression caused by recessive mutations i n a population is dependent on the mutation rate and on the intensity of selection against the mutations. We studied geographical difference s in the level of early inbreeding depression of Scots pine in a commo n garden experiment. The mean abortion rate of experimentally self-pol linated seeds was significantly lower (75.4%) among trees that origina ted from northern populations (66-69 degrees N) than among trees from more southern (60-62 degrees N) populations (86.5%). Thus, the number of embryonic lethal equivalents was lower in the northern populations (4.5) than in the southern ones (6.9). The outcrossing rate at the mat ure seed stage was slightly lower in the northern populations (average 0.93) than in the southern one (0.99). The estimated selfing rate at the zygote stage varied from 0-0.28 in the populations. The reduction in the magnitude of inbreeding depression in the north may have been c aused by increased levels of self-fertilization in the northern popula tions. The proportion of self-fertilized seedlings and adults was very small in all populations (F approximate to 0), indicating high inbree ding depression also in later life stages. The high level of inbreedin g depression in the partially selfing Scots pine can be explained by m utation-selection balance only if the mutation rate is high.