Variation in hybrid fertility among the disjunct populations and species of Sullivantia (Saxifragaceae)

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Abstract
Crosses were made in all combinations between the four allopatric taxa of Sullivantia. Artificial hybridizations between any two Sullivantia plants almost invariably yielded viable seed that produced vigorous F1 hybrids. The observation that Sullivantia species are highly interfertile is in agreement with similar findings for many other north temperate perennial plants. In most cases, the average pollen fertility of interspecific hybrids is reduced compared with the fertility of parental plants and intraspecific hybrids. Since no meiotic aberrations were observed in hybrids, gross chromosomal rearrangements apparently are not responsible for the reductions in hybrid fertility. Within Sullivantia there is a general correlation between geographical distance separating parental populations and reduction in hybrid fertility. Similar correlations have been reported for several other angiospermous species, but the present study is the first report of such a geographical distance – fertility correlation within an entire genus.