Apomictic Parthenogenesis and the Pattern of the Environment

Advantages of obligatory apomixis are modeled, and curculionidweevils are used as examples. It is suggested that the allegedtwofold number of female offspring in relation to sexual parentalsshould not be exaggerated. Instead, specific circumstances facilitatingthe origin of especially heterotic genotypes capable of apomixisshould be sought; crosses between differentiated (long-isolated)populations (preferably followed by a population flush) seempromising. Optimal strategies in temporally heterogeneous environmentsare studied, and situations favoringsexual reproduction bothin fine-grained and coarse-gained environments seem common (apomictsare capable only for individual adaptability, but sexual populationsmay track even one-generation changes by evolving). In apomicts,diapause determination and other seasonal constraints need specificattention, as they may interfere with geographical dispersal,and only life-cycles of one or more years seem possible. Inrich biotic communities, which evolve continuously, evolutionaryrigidity of apomicts may lead to their extinction. This showsthat the dichotomy between immediate and long-term advantagesof apomixis needs revision; if the number of generations neededfor competitive exclusion between asexual and sexual organismsis relatively high, the greater evolutionary potential of sexualpopulations must be considered (there are directional changesboth in the physical and biotic environments). Apomixis thenseems most advantageous in low-competitive, early successionhabitats which are recurrent and change relatively slowly (increasedcolonization ability of apomicts is here advantageous). Also,apomixis may succeed in spatially coarse-grained environmentin stable patches small enough to exclude high-competitive communityat the site. Then reduced vagility (loss of flight) may facilitatethe origin of apomixi