| The evolution of reproductive isolation is a critical step in speciation through limiting gene flow between divergent species.Self-incompatibility(SI)is an intra-species reproductive strategy in flowering plants to prevent self-fertilization and thus encouraging outcrossing and allogamy.Because the cocurrance of SI and the radiation of angiosperms in Cretaceous Period,it is considered as a major driving force for rapid speciation of angiosperms.A switch in mating system from SI to self-compatibility(SC)could also lead to strong premating reproduction isolation.Unilateral incompatibility(UI)usually occurred between pairs of the closed related SC and SI species(SI×SC rule),when pollen from SC species is rejected by the stigma of SI species,while the reciprocal pollination is compatible.UI has been reported among species with gametophytic SI systems and the molecular mechanism is emerging.However,less is known in species with sporophytic self-incompatibility systems.Here we investignated the characteristics of UI responses between the closed related Arabidopsis,focusing on exploring the role of UI in speciation and clarifying the relationship between SI and UI.The major results are as follows:1.Pollination between two species of the Arabidopsis or/and Capsella genus,show that the strength of UI response was various with the species pairs and with floral development.A positive correlation between the UI strength and genetic divergence of the species pairs indicates that UI is an important prezygotic reproductive barrier.2.UI in both stigma and pollen side UI is a qualitive-quantitive trait by analysis of phenotype in BCF1(Backcross first generation).3.We identified five candidate QTLs(quantitive trait locus),which are located on chromosome 1,2,3,5 and 7 by whole genome sequencing of DNA.4.QTL analysis showed the major QTLs underlying SUI were mapped to a interval of 3.92-5.0 Mb on chromosome 3 and 16.1-18.1 Mb on chromosome 2,respectively.5.Genetic analysis showed that a pseudo self-compatibility A.lyrata in a spatenous mutant is caused by the incompatibility of self-pollen rejection but not recognition at late floral stages.Intriguingly,UI response changed was also greatly weakened when SI response was diminished at late floral stages.UI and SI,therefore,share common mechanism in pollen rejection.Our findings show that UI could have been an important prezygotic reproduction isolation barrier.UI has evolved by recruiting the pollen rejection mechanism of SI and obtaining the mechanism of recognition heterospecific pollen.Our findings thus provide a mechanistic explanation of the concurrence of the evolution of SI and radiation of flowering plants in the Cretaceous Period. |
