Variation in inbreeding depression among families and populations of Clarkia tembloriensis (Onagraceae)

Abstract

The relationship between the self-fertilization rate of a population and the severity of inbreeding depression is difficult to predict because the underlying genetics may be complex. However, it is important to understand this relationship, and the degree to which inbreeding depression varies among families within populations, if we are to understand the evolution of plant mating systems. Inbreeding depression was studied in glasshouse trials using seed derived from two populations of Clarkia tembloriensis, Cantua Creek and Idria. These had very different rates of self-fertilization (s) and inbreeding coefficients (F) (s = 0.74, F = 0.77 in the CC-1 population; s = 0.16, F = 0.10 in the I-1 population). Outcrossing between these populations was equivalent to outcrossing within them; there was no evidence for outbreeding depression or increased heterosis from interpopulation crosses. The more self-fertilizing CC-1 population had fewer recessive lethal genes than the outbreeding I-1 population. However, cumulative inbreeding depression at the end of flowering was not significantly different between these populations. Inbreeding has seemingly purged the lethal genes from the CC-1 population but overall mutational load, especially in characters that are manifest late in the life history, is still substantial despite a history of inbreeding. Variation in inbreeding depression among families within these two populations was surprisingly large. The range of variation among families was five to seven times larger than the difference in inbreeding depression between populations. Variation in inbreeding depression among families should make these populations more susceptible to the invasion of genes which increase the rate of self-fertilization. © 1996 The Genetical Society of Great Britain.