The number, dominance relationships and frequencies of self-incompatibility alleles in a natural population of Sinapis arvensis L. in South Wales

Abstract

The incompatibility genotypes of a further 25 plants from the South Wales population of Sinapis arvensis studied by Ford and Kay (1985) have been determined, and an additional 21 S-alleles have been found. This brings the overall number of plants analysed from the population to 35, and the total number of S-alleles found also to 35. It is estimated that there are 52 S-alleles in the population.Dominance interactions between S-alleles have been established in a total of 42 different heterozygotes (pooling data obtained during the present study with that of Ford and Kay (bc. cit.)). Co-dominance of alleles in both pollen and stigma occurred in 18 (42.8 per cent), dominance of one allele in the pollen occurred in 21(50 per cent), dominance of one allele in the stigma occurred in two (4.8 per cent), and dominance of one allele in both pollen and stigma occurred in one (2.4 per cent). There is a minimum of three levels in the pollen dominance hierarchy, and two in the stigma dominance hierarchy; dominance interactions between S-alleles in the stigma may be non-linear. There is some indication that pollen dominant alleles tend to be found in lower frequencies than pollen recessive alleles.Preliminary data show that at least some S-alleles in S. arvensis have a very wide geographical distribution.

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

“…In general, non-isoplethy results in underestimation of population S allele number, since some S alleles will be rare and less likely to be represented in samples (Campbell andLawrence, 1981, Lawrence, 2000). Unequal S allele and S phenotype frequencies have previously been observed in natural populations of SSI Sinapis arvensis (Stevens and Kay, 1989) and GSI Papaver rhoeas (Campbell and Lawrence, 1981). Some of the possible reasons for non-isoplethy, which were thoroughly investigated in P. rhoeas, include: populations not being at mating system equilibrium; stochastic variation in frequencies at mating system equilibrium; and selection at loci linked to the S locus (Brooks et al, 1996;Lawrence, 2000).…”

“…The resulting value is a proportion ranging from zero (as many different S alleles identified as S alleles sampled) to one (the minimum number of S alleles possible for a SSI system (ie, 2) identified in the entire sample). (Stevens and Kay, 1989) Testing the equality of S phenotype frequencies (isoplethy) Since the dominantly expressed S alleles identified in our sample also correspond to the S phenotypes, a test of the prediction of isoplethy at mating system equilibrium was appropriate. Isoplethy was tested using the 2 test described in equation 6, which is itself a modified version of the 2 test developed for use in GSI studies (Campbell and Lawrence, 1981) so that it deals with single S allele genotype data.…”

“…The assumption of equal S genotype and S allele frequencies at mating system equilibrium is effectively replaced with the assumption of isoplethy at mating system equilibrium. The performance of the modified Paxman (1963) estimator relative to its unmodified counterpart was tested using Monte Carlo simulations of published diploid S genotype datasets and associated population S allele number estimates for natural SSI populations of other species (Stevens and Kay, 1989;Kowyama et al, 1994), where the modified estimator was calculated from 1000 resampled haploid datasets consisting of single randomly chosen S alleles. (Lawrence, 1996) Measuring the thoroughness of the SI study Repeatability (R) is a standard measure of the thoroughness with which an S allele assay has been carried out and was calculated according to equation 5.…”

“…Strict codominance between S alleles is necessary for functional GSI, whereas in SSI there are no restrictions on the evolution of complex dominance interactions between S alleles and dominance hierarchies between S alleles form an intrinsic part of most SSI systems (Hiscock and Kues, 1999). In SSI systems, dominance interactions allow for the natural occurrence of individuals homozygous for recessive S alleles, which are absent in normally functioning GSI systems (Stevens and Kay, 1989). 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).…”

Number of incompatibility alleles in clover and other species

“…Superscripts on the species names indicate the sources of data and estimates which are as follows: 1, Emerson (1939Emerson ( , 1940 6, Fearon et al, (1994);7, Ockendon, (1985); 8, Stevens & Kay, (1989);9, Kowyama et a!. (1994).…”