THE DISTRIBUTION OF POSTMATING REPRODUCTIVE ISOLATING GENES IN POPULATIONS OF THE YELLOW MONKEY FLOWER,MIMULUS GUTTATUS

Abstract

Postmating reproductive isolating barriers are generally believed to arise as the chance by-product of genetic differentiation. The classical view is that these barriers normally involve differentiation at many loci, and therefore require long periods of allopatric isolation. The formal genetics of, and the distribution of genes responsible for, such barriers are known in very few cases. This paper examines the distribution of the genes responsible for two different postmating barriers in 18 populations of the yellow monkey flower, Mimulus guttatus. The genetic relatedness of the populations was measured by a morphometrical analysis. Widespread polymorphism was found for three of the four components of the two genetic systems responsible for the two barriers, with at least 13 populations possessing genes for one or both of the barriers. In one system (the C7/U8 system; Christie and Macnair, 1984), the distribution of the two component genes was correlated with the morphometrical similarity and geographical location of the populations. This pattern could be produced by a historical association or by an adaptive response. In the other system (the Cerig/C10 system; Macnair and Christie, 1983), the genes were more widely dispersed, and there was no obvious morphometrical or geographical association. Populations possessing the complementary factors causing partial reproductive isolation are not always widely separated geographically. These results indicate that the spread of postmating reproductive isolating genes through drift, selection, or hitchhiking could readily cause reproductive isolation to evolve in this species.>.

Indirect Evolution of Hybrid Lethality Due to Linkage with Selected Locus in Mimulus guttatus

“…For seven generations, only tolerant plants were selected to be backcrossed to Stinson Beach (Figure S1). The Stinson Beach population is nontolerant and lacks any intrinsic incompatibilities with Copperopolis [23]. We used multiple outbred Stinson Beach lines to avoid the development of inbreeding depression in our backcross lines.…”

“…Furthermore, this population sampling is designed to explicitly test for an effect of habitat-mediated selection on a single genomic region with known phenotypic effect, Tol1 / Nec1 loci, and not a genome-wide analysis. Lastly, this design will not be confounded by the effect of hybrid inviability because these off-mine populations lack the incompatible allele found in the Cerig population [23].…”

“…This variance is likely caused by two factors—segregating variation at multiple incompatibility loci in our tester line, Cerig10 , and variation in temperature in the greenhouses in which the F1 BC7 × Cerig10 offspring were grown. There are multiple incompatibility loci segregating within the Cerig population that interact with the Copperopolis allele at the locus we have now identified as Nec1 [22],[23]. Our tester line, Cerig10 , was chosen because it gave the highest level of incompatibility, but it may be heterozygous at some incompatibility loci, inflating variation in hybrid lethality.…”

“…Subsequent experiments have demonstrated that the Cerig hybrid lethality factors are geographically widespread and segregate in three California populations. In contrast, the Copperopolis lethality factor is geographically restricted; no incompatible plants were identified in test crosses with plants from 15 other populations ( N  = 10 plants/population) [23]. The lethality factor is at high frequency in the Copperopolis population; 16 Copperopolis genotypes (five of which are from this study, see details below) have been tested and all of them produce lethal offspring when crossed to Cerig tester lines [22],[23].…”

“…In contrast, the Copperopolis lethality factor is geographically restricted; no incompatible plants were identified in test crosses with plants from 15 other populations ( N  = 10 plants/population) [23]. The lethality factor is at high frequency in the Copperopolis population; 16 Copperopolis genotypes (five of which are from this study, see details below) have been tested and all of them produce lethal offspring when crossed to Cerig tester lines [22],[23]. Hybrid lethality in this system is consistent with the BDM model and was not observed to be genetically de-coupled from the copper tolerance phenotype [22],[24].…”