Evolution of meiosis timing during floral development

Meiosis divides the haploid and diploid portions of the life cycle in all s exual organisms. In angiosperms meiosis occurs during flower development, t he duration of which varies widely among species and is affected by environ mental conditions within species. For 36 species representing 13 angiosperm families, we determined the time at which meiosis ceased in the anthers as a fraction of the total time from floral primordium initiation (beginning of development) to flower opening (end). It was found that this fraction, r ather than being continuously distributed among species, occurred in three discrete classes despite wide variations within and among species in absolu te developmental durations. Each species was characterized by a single timi ng class. For all species within a given timing class, therefore, the durat ions before and after the end of microsporocyte meiosis existed in constant ratio. Each timing class was found in phylogenetically distant species; co nversely, a plant family often contained more than one class. Timing class was not related to ploidy level, inflorescence architecture, pollination sy ndrome or mating system. These findings show that either the durations befo re and after microsporocyte meiosis are regulated by the same exogenous pro cess, or one duration determines the other. They further imply that the und erlying developmental processes have evolved in a limited number of ways am ong flowering plants.