Evolutionary transitions from outcrossing to selfing
The transition from outcrossing to selfing is very common in flowering plants and has important demographic, genetic and evolutionary consequences. We have worked on this problem using two main systems (Turnera, Eichhornia); both involve the evolutionary breakdown of heterostyly (58, 72). Most of our focus in recent years has been on the neotropical, annual, aquatic Eichhornia paniculata, where we have evidence of multiple independent transitions to selfing (115, 259). This species is an excellent study system as it is diploid, easily crossed and we can grow 2-3 generations a year in our glasshouses at Toronto. We have extensive collections of E. paniculata from throughout its range (Brazil, Jamaica, Cuba, Central America). Work by Brian Husband, Martin Morgan and Josh Kohn in the 90s demonstrated that the shift to selfing in E. paniculata results from the joint action of genetic drift and natural selection and represents one of the few cases that meet several of the key conditions in Sewall’s Wright’s shifting balance theory of evolution (97, 100, 102, 124).
Floral and mating system diversity in Eichhornia (Pontederiaceae) species (143). Eichhornia paniculata – A) a population in a Cuban rice field; B) inflorescence; C) outcrossing and selfing flowers from Brazil and Jamaica, respectively. Style morph frequencies in natural populations indicate the breakdown pathway to selfing from the 1:1:1 equilibrium in trimorphic populations. This results from the joint interaction of genetic drift and natural selection. The pathway involves three stages with sequential loss of morphs from trimorphism to dimorphism and finally to monomorphism (72, 259).