ploids and autopolyploids are expected to have reduced inbreeding depression (Charlesworth and Charlesworth, 1987, except under their overdominance model, and Hedrick, 1987), and the magnitude of inbreeding depression is negatively correlated with selfing rates in diploid angiosperms and gymnosperms (Husband and Schemske, 1996). Unfortunately, few studies have addressed levels of inbreeding depression in polyploids empirically; most of the available data come from ferns.
Inbreeding depression in ferns (often referred to as genetic load in these studies) has been estimated by taking advantage of the life cycle that involves a free-living, haploid gametophyte generation that can, in most cases, self-fertilize to produce a completely homozygous diploid sporophyte. These studies have involved culturing gametophytes in isolation, in sib pairs, and in non-sib pairs. The number and survival of sporophytes resulting from these treatments are recorded, and these data can be used to estimate inbreeding depression, outcrossing depression, and the number of lethal equivalents per genome. If a greater number of normal sporophytes is produced by non-sib pairs of gametophytes than by either sib pairs or isolated gametophytes, then the population or species is considered to exhibit inbreeding depression.
Masuyama and Watano (1990) reported two studies of inbreeding depression in diploid and tetraploid pairs of ferns. In Phegopteris, 30–60% of selfed gametophytes of the diploid race formed sporophytes, and nearly 100% of all selfed gametophytes of the tetraploid race formed sporophytes. In Lepisorus, only 4% of selfed gametophytes of the diploid race produced normal sporophytes, whereas 98–100% of the gametophytes of the tetraploid race formed sporophytes. These data were interpreted as evidence for reduced inbreeding depression in the tetraploid, with the lower inbreeding depression allowing for increased selfing rates.
There are few estimates of selfing rates in polyploid fern species, largely because polyploid fern populations often lack sufficient levels of segregating allozyme markers; however, selfing rates have been estimated in a few diploid-tetraploid pairs. In Polystichum, the allotetraploid Polystichum californicum has a selfing rate of 0.236, whereas selfing rates in the two diploid progenitors, Polystichum dudleyi and Polystichum imbricans, are only 2–3% (Soltis and Soltis, 1990). In tetraploid Pteris dispar, selfing rates are 0.84, much higher than the rate of 0.01 estimated for the diploid race (Masuyama and Watano, 1990). Limited evidence for ferns suggests reduced inbreeding depression and higher selfing rates in tetraploids than in diploids.
Comparisons of outcrossing rates and levels of inbreeding depression in diploid and polyploid angiosperms also are rare. Both outcrossing rates and inbreeding depression have been estimated for diploid and autotetraploid populations of Epilobium angustifolium (Husband and Schemske,