Summary: The self‐incompatibility (SI) status of 571 taxa from the Asteraceae was identified and the taxa were scored as having SI, partial SI or self‐compatibility (SC) as their breeding system. A molecular phylogeny of the internal transcribed spacer (ITS) region was constructed for 211 of these taxa. Macrophylogenetic methods were used to test hypotheses concerning the ancestral state of SI in the Asteraceae, the gain and loss of SI, the irreversibility of the loss of SI and the potential for partial SI or SC to be terminal states. The ancestral breeding system in the family could not be resolved. Both maximum likelihood and parsimony analyses indicated that transitions among all breeding system states provide the best fit to the data and that neither partial SI nor SC is a terminal state. Furthermore, the data indicated that the loss of SI is not irreversible, although breeding system evolution has been more dynamic in some clades than in others. These results are discussed within the context of evidence for the gain and loss of SI, the evolutionary role of partial SI and methodological assumptions of tests of breeding system evolution.

New data are presented on the distribution and frequency of self-sterility (SS) – predominantly pre-zygotic self-incompatibility (SI) systems – in flowering plants and the hypothesis is tested that families with self-sterile taxa have higher net diversification rates (DRs) than those with exclusively self-compatible taxa using both absolute and relative rate tests. Three major forms of SI systems (where pollen is rejected at the stigmatic, stylar or ovarian interface) are found to occur in the oldest families of flowering plants, with times of divergence >100 million years before the present (mybp), while post-fertilization SS and heterostyly appear in families with crown ages of 81 and 87 mybp, respectively. It is also founnd that many (22) angiosperm families exhibit >1 SI phenotype and that the distribution of different types of SS does not show strong phylogenetic clustering, collectively suggesting that SS and SI systems have evolved repeatedly de novo in angiosperm history. Families bearing self-sterile taxa have higher absolute DRs using all available calibrations of the angiosperm tree, and this affect is caused mostly by the high DR of families with homomorphic SI systems (in particular stigmatic SI) or those in which multiple SS/SI phenotypes have been observed (polymorphic). Lastly, using sister comparisons, it is further demonstrated that in 29 of 38 sister pairs (including 95 families), the self-sterile sister group had higher species richness and DR than its self-compatible sister based on either the total number of taxa in the clade with SS or only the estimated fraction to harbour SS based on literature surveys. Collectively, these analyses point to the importance of SS, particularly pre-zygotic SI in the evolution of flowering plants.

Selection may favour a partial or complete loss of self-incompatibility (SI) if it increases the reproductive output of individuals in the presence of low mate availability. The reproductive output of individuals varying in their strength of SI may also be affected by population density via its affect on the spatial structuring and number of S-alleles in populations. Modifiers increasing levels of self-compatibility can be selected when self-compatible individuals receive reproductive compensation by, for example, increasing seed set and/or when they become associated with high fitness genotypes. The effect of variation in the strength of SI and scrub density (low versus high) on seed set, seed germination and inbreeding depression in seed germination (δ_germ) was investigated in the partially self-incompatible species Flourensia cernua by analysing data from self-, cross- and open-pollinated florets. Examination of 100 plants in both high and low scrub densities revealed that 51% of plants were strongly self-incompatible and 49 % varied from being self-incompatible to self-compatible. Seed set after hand cross-pollination was higher than after open-pollination for self-incompatible, partially self-incompatible and self-compatible plants but was uniformly low for strongly self-incompatible plants. Strongly self-incompatible and self-incompatible plants exhibited lower seed set, seed germination and multiplicative female fitness (floral display × seed set × seed germination) in open-pollinated florets compared with partially self-incompatible and self-compatible plants. Scrub density also had an effect on seed set and inbreeding depression: in low-density scrubs seed set was higher after open-pollination and δ_germ was lower. These data suggest that (a) plants suffered outcross pollen limitation, (b) female fitness in partially self-incompatible and self-compatible plants is enhanced by increased mate-compatibility and (c) plants in low-density scrubs received higher quality pollen via open-pollination than plants in high-density scrubs.

• Backgrounds and aims Flourensia cernua is a partially self-incompatible, wind-pollinated shrub that grows in two scrub types of contrasting densities. It was anticipated that differences in plant density would affect the amount of genotype availability, and thus higher outcrossing rates and less genetic differentiation would be found at high-density sites. • Methods At five high-density sites and at five low-density sites, 11 allozyme loci were analysed in adults. Outcrossing rates were estimated using five allozyme loci sampled from eight families from each scrub type. • Key results High levels of genetic variation were found at all sites (ranging from P = 82–100 %, H_e = 0·33–0·45, and H_o = 0·4–0·59). Heterozygotes were found in excess (F_IS = −0·15 ± 0·06 s.d.), suggesting that natural selection favours heterozygosity, and there was little differentiation between sites (F_ST = 0·08 ± 0·02 s.d.). Life history attributes, such as long-lived habit and wide geographic distribution, as well as the presence of a self-incompatibility system may explain these results. Outcrossing rates did not differ from 1·0 in both scrub types, and there was no genetic differentiation between scrub types (F_ST = −0·01 ± 0·004 s.d.). • Conclusions The high rate of outcrossing favoured by partial incompatibility may generate unrestricted gene flow between scrub types and thus may explain the lack of differentiation between them. High heterozygosity could be expected in long-lived plants of arid zones as they confront a variable and stressing environment.