INHERITANCE OF MATING-SYSTEM MODIFIER GENES IN EICHHORNIA-PANICULATA (PONTEDERIACEAE)

The evolutionary breakdown of tristyly to predominant self-fertilizati on in Eichhornia paniculata (Pontederiaceae) is associated with the sp read and fixation of floral variants of the mid-styled morph (M) with modified short-level stamens. Reduced stigma-anther separation in the variants results in a high degree of self-fertilization. To investigat e if the genetic basis of mating-system modification differs among flo ral variants from different parts of the geographical range of the spe cies we performed controlled crosses on genotypes from six populations from N.E. Brazil and Jamaica. Measurements of floral traits in self a nd F, crosses grown under uniform glasshouse conditions demonstrated t hat the gene(s) modifying stamen position in the M morph are largely r ecessive, have no apparent phenotypic effects on other floral characte rs and are only expressed in the M morph. Crosses between modified gen otypes from northern and southern parts of N.E. Brazil resulted in F, progeny with unmodified flowers, indicating that different recessive g enes are responsible for stamen modification in the two regions. Cross es between modified genotypes from Jamaica and the two regions of N.E. Brazil indicated a more complex basis for stamen modification in Jama ica, and confirmed that the genetic basis of selfing differed between the two regions of Brazil. Our results provide evidence that the evolu tion of self-fertilization in E. paniculata has occurred by different genetic pathways. In addition, the data suggest that the sequence of e vents commences with the fixation of recessive alleles affecting only filament length, followed by evolution at modifier loci responsible fo r larger phenotypic changes to floral architecture.