Evolution

Postdispersal selection following mixed mating in Eucalyptus regnans.

The fate of inbred progenies strongly influences the evolution of natural populations undergoing mixed mating. While the response to selection following mixed mating may be quite different to that under random mating (Allard et al. 1968; Wright and Cockerham 1985; Hedrick 1990), inbreeding depression prior to reproduction will reduce the contribution of inbreds to the final mating population. If few self-progeny survive, evolution will effectively follow that of a randomly mating population (Lande et al. 1994). Inbreeding depression has been studied extensively in many plant species (reviewed in Charlesworth and Charlesworth 1987; Husband and Schemske 1996; Ritland 1996; Williams and Savolainen 1996). However, many of these studies have been short term and conducted in greenhouses or gardens, which may underestimate the intensity of inbreeding depression due to the absence of competition (Schmitt and Ehrhardt 1990; Ritland 1996). Further, while early acting inbreeding depression appears to be well documented, there is a lack of information of later-age inbreeding depression in long-lived angiosperms, as noted by Husband and Schemske (1996) and Ritland (1996).

The study of long-lived plants is of particular interest as the rate of mutation per generation may be much higher than for small annual species, suggesting differences in the expression of inbreeding depression (Lande et al. 1994; Husband and Schemske 1996). Long-lived conifers exhibit severe inbreeding depression for seed set and for later age growth and moderate levels for survival in the field and reproductive traits (Husband and Schemske 1996; Williams and Savolainen 1996). However, conifers are wind pollinated and out-crossing rates are generally close to one (Aide 1986). In addition, conifers possess polyembryony, which enables selection against inferior embryos without reducing reproductive output (Sorenson 1982). Here we report on late-acting inbreeding depression and selection in progenies from mixed mating after competition has developed between inbred and outcross progenies of a long-lived forest-tree angiosperm, Eucalyptus regnans Muell.

Eucalyptus regnans is the tallest angiosperm species in the world, attaining heights of up to 90-100 m (Ashton 1981). It is a common dominant of the forests of southeastern Australia, generally forming pure, even-aged stands on rich, fertile soils in high rainfall areas (Ashton 1981). The species is mass flowering, producing up to 1 million small, hermaphroditic, flowers per tree during a single season (Fripp et al. 1987). Pollination occurs via a variety of nonspecific insect and other animal (Ashton 1975; Griffin 1980) vectors. Although individual flowers are protandrous (Griffin and Hand 1979), the flowering phenology provides ample opportunity for selfing via geitonomy (Griffin 1980). Prezygotic self-in-compatibility is not important with equivalent numbers of fertile self and outcross ovules present at 16 wk after controlled pollination (Sedgley et al. 1989). However, lower seed production under controlled self-pollination compared to cross-pollination (Griffin et al. 1987) indicates that self-pollinated ovules suffer inbreeding depression after this date. Moreover, in mixed pollinations, outcross ovules out compete selfs, resulting in higher outcrossing rates than predicted based on the fitness of the cross-types determined from the independent pollination treatments (Griffin et al. 1987). Overall, 18% to 77% of the mature seeds produced by E. regnans under natural pollination are selfs (Fripp et al. 1987; Griffin et al. 1987; Moran et al. 1989), although clearly the rate of actual self-fertilization is much higher.

Mature seed remains stored in woody capsules in the canopy for several years (Ashton 1975), but it is only following wildfire that successful regeneration typically occurs (Ashton 1981). Fire kills the mature canopy, prepares a receptive seed bed, removes competing vegetation, and promotes seed shed (Ashton 1981). The seeds of E. regnans are small (Boland et al. 1980). Seed dispersal is mainly by wind or gravity and is virtually limited to approximately twice the tree height (Cremer 1966). The longevity of seed in the soil is low (Ashton 1979), and seed fall at other times is either removed by ants (Ashton 1979) or is unable to grow under the unfavorable conditions beneath an unburnt mature canopy (Ashton 1981). While millions of seed per hectare are shed following fire, only 10% ([approximately] 200,000 per hectare) of these seeds germinate, and only 0.5-1.0% ([approximately] 20,000 per hectare) produce seedlings (Cunningham 1960). Deaths continue as the stand ages until the density stabilizes between 40-80 stems per hectare (0.002% of the initial seed rain) around 150 years postfire (Ashton 1976). If wildfire is absent for longer than about 300 years, the eucalypts will senesce, thin out, and be eventually replaced by climax temperate rainforest (Jackson 1968). The enormous loss of progeny with stand growth provides the opportunity for the development of intense competition (Barber 1965). This study addresses the dynamics of postdispersal selection and the fate of inbred progenies in the competitive environment of regenerating forests.

MATERIALS AND METHODS

The mating design and field trial have been described in Griffin and Cotterill (1988). Thirteen mature E. regnans individuals from two populations of native forest (six from Narracan and seven from Thorpdale; Victoria, Australia) were crossed in two disconnected factorials within populations to produce 12 Thorpdale and nine Narracan families of controlled outcross seedlings (Table 1). Controlled self- and naturally open-pollinated (OP) progeny were also obtained from all parents (Table 1). A field trial was established in August 1979 with families planted as single tree plots in 36 complete randomized replications at a spacing of 2 m by 3 m among seedlings. Survival was recorded at 11 (1 yr), 21 (2 yr), 30 (3 yr), 43 (4 yr), 115 (10 yr), 152 (13 yr), and 176 (15 yr) mo after planting. Diameter at breast height was measured from 3 yr onward and used to calculate individual tree basal area.

The average proportion of planted seedlings surviving at each age (cumulative survival) was calculated for each cross-type (outcross, OP, and self), population, and parent. …

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