QUANTITATIVE GENETICS OF MATING SYSTEM DIVERGENCE IN THE YELLOW MONKEYFLOWER SPECIES COMPLEX

To study the genetic basis of phenotypic changes in floral characters associated with mating system divergence in the yellow monkeyflowers ( Mimulus, section Simiolus), all pairwise crosses were conducted betwee n four taxa, of which two were independently evolved inbreeders. The e ffective number of genetic factors differentiating taxa for each of si x characters was estimated from backcross and F-2 segregation variance s, using a model that incorporates dominance. The number of factors, a veraged over taxa pairs, ranged from 5.3 (for stigma-anther separation ) to 12.8 (for pistil length). Dominance was slight to moderate for mo st characters, with a slight bias for inbreeding characteristics to be dominant over outbreeding traits. F-2 segregation patterns indicate s trong genetic correlations among floral size characters but variable c orrelation of these characters with flowering time and little correlat ion with stigma-anther separation. Crosses between the two selfing and two outcrossing taxa did not lead to the recovery of extreme transgre ssive segregants in the F-2. These results suggest that either homolog ous loci are involved in repeated shifts to selfing or that the likeli hood of appearance of novel recombinations is limited by the large num ber of loci, each with small effect. The implications of these results are discussed in the context of recent models of mating system evolut ion.