Loss of sex in clonal populations of a flowering plant, Decodon verticillatus (Lythraceae)

The loss of traits that no longer increase fitness is a pervasive feature o f evolution, although detailed studies of the genetic, developmental, and e volutionary factors involved are few. Most perennial plants practice both s exual and clonal reproduction, and it has been hypothesized that population s with little sexual recruitment may lose the capacity for sexual reproduct ion by fixing mutations that disable one or more of the many processes invo lved in sex. The clonal, tristylous aquatic plant, Decodon verticillatus, e xhibits marked geographical variation in sexual recruitment. Populations at the northern limit of the range are usually monomorphic for style length, consist of single genotypes, and produce almost no seed, due, in part, to e nvironmental conditions that inhibit pollination, fertilization, and seed m aturation. Controlled crosses in a greenhouse provided evidence for greatly reduced sexual capacity in an exclusively clonal, monomorphic population. Plants from this infertile population produced only 3-18% as many seeds per pollination as fertile populations. Observations of pollen tube growth ind icated that infertility is due to severe reductions in pollen tube numbers both early after pollination and later when pollen tubes were traversing th e ovary, due primarily to the inability of pistils to support normal tube g rowth. A three-year greenhouse experiment comparing fertility, survival, an d growth of Fl progenies produced from reciprocal crosses between plants fr om the infertile population and those from nearby fertile populations sugge sted that the genetic basis for infertility is simple and may involve a sin gle recessive mutation. In addition, the results did not reveal any associa tion between infertility and other aspects of survival and vegetative vigor . The infertile genotype was likely fixed in the population through founder effect rather than indirect selection resulting from antagonistic pleiotro py or direct selection of advantages associated with reduced investment in sexual reproduction. A broader comparison of sexual fertility in 15 clonal, monomorphic populations and five genotypically diverse, trimorphic populat ions under greenhouse conditions revealed substantial infertility in all bu t one monomorphic population. Populations varied somewhat in the stage at w hich infertility was expressed, however, pollen tube growth was impaired in all populations. These results provide strong support for the hypothesis t hat complex traits like sex are degraded by mutation when they no longer in crease fitness.