Mate availability in small populations of plant species with homomorphic sporophytic self-incompatibility

Abstract

Plants of a self-incompatible species, which occur in small populations, may have reduced fitness due to the limited availability of compatible mates. Self-incompatibility decreases inbreeding by allowing successful mating to occur only with individuals which differ by at least one-allele at the S-locus. A computer simulation model was developed to test the effect of small population size upon the diversity and the relative frequency of the S-alleles which determine the number of available mates. In a large population at equilibrium, the greater the number of S-alleles the greater the frequency of available mates for all individuals in the population. In small populations (less than 50 individuals), they are unable to maintain a high diversity of S-alleles and therefore there is a decrease in the frequency of available mates. In addition, in small populations there is an increase in the variance of available mates. The number of mates in these populations depends on the genotype of a particular individual. Two patterns would be expected in a small population of incompatible species: (1) a lower seed set per individual due to limited mates, and (2) an increase in variation of seed set among individuals due to the variance in available mates. Lower seed set would lead to a decrease in fitness of particular genotypes and could increase the possibility of local extinction of the species.

The population genetics of sporophytic self-incompatibility in Senecio squalidus L. (Asteraceae) I: S allele diversity in a natural population

“…The possibility of selectively altering domi-nance interactions between S alleles in SSI systems provides an extra dimension of 'flexibility' to the SI system during times of population perturbation of the kind experienced by S. squalidus. In general, increased dominance between S alleles will result in increased MA relative to that predicted under a codominant SSI system (Byers andMeagher, 1992, Schierup et al, 1997). This is a consequence of individual mating phenotypes being reduced to that of their dominant S allele alone, thereby increasing the MA by the proportion of plants in the population expressing the recessive S allele.…”

“…Mating system consequences of low S allele diversity Since S. squalidus appears to have fewer S alleles than are generally found at evolutionary mating system equilibrium for SSI, mate availability (MA) may restrict seed-set (Byers and Meagher, 1992). The fact that non-isoplethy of S phenotypes is a feature of the Oxford population does not appear to be reducing MA much further than low S allele number alone, since if the MA value calculated for the Oxford sample (77.6%, Table 4) is representative of the entire Oxford population of S. squalidus, then it is close to the deterministic MA value (83.3%) predicted for SSI populations consisting of six S alleles interacting to form a simple dominance series (Schierup et al, 1997).…”

“…The resulting value is a proportion ranging from zero (all individuals inter-incompatible) to one (all individuals intercompatible). (Byers and Meagher, 1992) …”

“…In SI systems, mate availability (MA, defined as the average proportion of compatible mates in a population) is maximised and mating system equilibrium reached, at equal S phenotype frequencies (isoplethy) (Finney, 1952). For GSI systems, where dominance interactions between S alleles are absent, isoplethy is equivalent to equal S allele frequencies, whereas in SSI systems, isoplethy results in S allele frequencies that are dependent on the dominance interactions between the S alleles, such that the more recessive alleles occur at higher frequencies than dominant S alleles (Byers and Meagher, 1992;Schierup et al, 1997).…”

Population genetics of sporophytic self-incompatibility in Senecio squalidus L. (Asteraceae) II: a spatial autocorrelation approach to determining mating behaviour in the presence of low S allele diversity

“…Therefore, it follows that SSI species with few S alleles may not be mating with optimal efficiency and may experience adverse fitness consequences. The threat to reproductive assurance caused by low S allele number can be quantified in terms of mate availability (MA), the proportion of compatible mates in SI populations (Byers and Meagher, 1992). Reduced seed set and complete reproductive failure due to low S allele number has been demonstrated in the threatened and rare Asteraceous species, Hymenoxys acualis var glabra (DeMauro, 1993), and Aster furcatus (Reinartz and Les, 1994).…”