Sporophytic self-incompatibility in diploid and tetraploid races of Rutidosis leptorrhynchoides (Asteraceae)

Controlled pollinations were used to examine the mating system of diploid and tetraploid individuals of the endangered grassland herb Rutidosis leptorrhynchoides F.Muell. Crosses among unrelated plants gave 1.5-2 times as many fruit as crosses between plants that were half-sibs, while selfed crosses generally resulted in no fruit. Three classes of compatibility reaction were observed within outcross treatments: (1) reciprocal compatibility, (2) one-way compatibility and (3) reciprocal incompatibility. This is diagnostic of sporophytic control of self-incompatibility, which is characteristic of the Asteraceae. This is supported by the occurrence of a dry stigma and trinucleate pollen. Analysis of the behaviour of self- and outcross pollen on the stigma by using fluorescence microscopy shows that rejection of self-pollen does not all occur at one point but at a number of stages, with cumulative reductions in the adherence of pollen to the stigma, pollen germination, pollen tube penetration of the stigma and fertilisation. On the basis of both fruit set and pollen behaviour data, for any level of relatedness, tetraploid plants are about 20% less likely to be compatible with each other than diploid plants. This presumably reflects the greater likelihood of matching S alleles given the greater potential for polymorphism at the individual level. The occurrence of two plants, one diploid and one tetraploid, that set large amounts of fruit on selfing shows that self-incompatibility can break down.