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.

Occurrence and Physiological Breakdown of Self-incompatibility in Easter Cactus

The existence of self-incompatibility (SI) was demonstrated in `Britton' and `Rose' Easter cactus (Rhipsalidopsis). In a full diallel cross among five clones, 18 out of 20 outcrosses resulted in 68% to 100% fruit set, whereas reciprocal crosses between two of the clones and all five self-pollinations failed to set fruit. Pollen tube growth was greatly inhibited in styles of selfed pistils, but there was no evidence of pollen tube inhibition in compatibly crossed pistils. Easter cactus exhibited characteristics typically found in sporophytic SI systems (trinucleate pollen, papillate stigmas, and scant stigmatic exudate) together with those associated with gametophytic SI systems (stylar inhibition of pollen tube growth and absence of reciprocal differences in outcrosses). Additional experiments were performed to determine the effects of bud pollinations, growth regulators (BA, GA3, and NAAm), and high temperatures (0- to 48-h exposure at 40C) on the SI response. Heat treatments were more effective than either bud pollinations or growth regulators in overcoming SI, and yielded an average of 7.2 viable seeds per treated flower when plants were incubated for 12 h at 40C and selfed immediately after incubation. Isozyme analysis of the S0 parent and putative S1 progeny confirmed that selfing had occurred following heat treatments. Using S1 progeny in breeding programs may extend the flower color range and lead to a greater diversity in other plant characteristics than presently exists in cultivated germplasm. Chemical names used: N-(phenylmethyl)-1H-purin-6-amine [benzyladenine (BA)], gibberellic acid (GA3), and α-naphthaleneacetamide (NAAm).