The degree of early inbreeding depression determines the selfing rate at the seed stage: model and results from Pinus sylvestris (Scots pine)

Abstract

Early inbreeding depression, i.e. embryonic recessive lethals, eliminates a large proportion of selfed progeny during embryo development. A model of early inbreeding depression suggested that in most conifers the variation between genotypes in the number of lethals rather than the variation in the actual rate of self-fertilization accounts for the variation between selfing rates at the seed stage. Polyembryony, the formation of several embryos per ovule in conifers, diminished the fitness cost of embryonic lethals and allowed embryo competition. We studied variation in the outcrossing rate at the seed stage in an experimental population of Scots pine. Despite extensive variation, pollen production of the trees, which is expected to predict the probability of self-fertilization, did not account for the low selfing rate variation at the seed stage. The genotypes having lowest numbers of embryonic lethals had the highest selfing rates at the seed stage. Early inbreeding depression maintains a very low selfing rate at the seed stage and masks the correlation between the rate of selffertilization and the selfing rate at the seed stage. This is typical of most conifers and can also be common among perennial angiosperm species.

Joint analysis of spatial genetic structure and inbreeding in a managed population of Scots pine

“…Based on the parentage analysis, the results of which support a total of 19 fathers and 15 mothers and only nine pairs of trees that were either full-or half-sibs, we can conclude that the trees do not form a single full-sib family. The high overall inbreeding coefficient contrasts with the efficient mechanisms for purging inbreeds described in Scots pine (Muona et al, 1987;Kärkkäinen and Savolainen, 1993). On the other hand, this apparent contradiction could be explained if some of the trees are the result of mating between already related parent trees, as supported by the parentage analysis.…”

Variation of selfing rate and inbreeding depression among individuals and across generations within an admixed Cedrus population

“…There are only a few studies that have estimated the individual selfing rates and ID under natural mating systems, by calculating the proportion of empty seeds and with biparentally inherited molecular markers such as isozymes, which need more complicated statistical estimators (Kärkkäinen and Savolainen, 1993;Ritland and Travis, 2004;Bower and Aitken, 2007). In this study, we took benefit of the high diversity and paternal inheritance of cpSSRs markers in Pinaceae species (Parducci et al, 2001;Ribeiro et al, 2002;Naydenov et al, 2005) and particular Cedrus (Fady et al, 2003;Terrab et al, 2006).…”

“…In most conifers, during early embryogenesis, ID acts more or less severely depending on the number of embryonic lethal alleles in the parental genome (see Introduction). Kärkkäinen and Savolainen (1993) suggest that polyembryony, which occurs frequently in Pinaceae, can diminish the cost of embryonic lethals and make embryo competition possible, leading to an underestimation of the primary selfing rate. In this regard, the genus Cedrus shows simple polyembryony, derived from the fertilization of about half of three to six archegonia in each female gametophyte, and cleavage polyembryony, which leads to genetically identical embryos (Wilson, 1923;Favre-Duchartre, 1970).…”

“…In Pinus taeda L., Remington and O'Malley (2000b) showed that embryonic lethals do not contribute to ID later in the life cycle. Beyond seed abortion, ID delays seed germination (Sorensen, 2001), increases juvenile mortality (Koelewijn et al, 1999), depresses growth (Sorensen and Miles, 1982;Bower and Aitken, 2007), causes morphological abnormalities later in the life cycle (Wilcox, 1983), and decreases seed production and fertility in young trees (Kärkkäinen and Savolainen, 1993;Durel et al, 1996). At this later stage mildly deleterious mutations or polygenic mutations, which difficult to purge, are likely the main actors (Husband and Schemske, 1996).…”

Genetic information from progeny trials: a comparison between progenies generated by open pollination and by controlled crosses

“…In several conifer species, Scots pine and Norway spruce being no exception, such selfed progenies have been eliminated already at the seed stage due to early and extreme inbreeding depression (Kärkkäinen and Savolainen, 1993;Koski, 1971;Williams, 2007). Furthermore, eucalypts are naturally insect pollinated and paternal reproductive success may therefore be highly uneven.…”