LOSS OF SEX IN CLONAL POPULATIONS OF A FLOWERING PLANT,DECODON VERTICILLATUS(LYTHRACEAE)

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Abstract
The loss of traits that no longer increase fitness is a pervasive feature of evolution, although detailed studies of the genetic, developmental, and evolutionary factors involved are few. Most perennial plants practice both sexual and clonal reproduction, and it has been hypothesized that populations with little sexual recruitment may lose the capacity for sexual reproduction by fixing mutations that disable one or more of the many processes involved in sex. The clonal, tristylous aquatic plant, Decodon verticillatus, exhibits 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 environmental conditions that inhibit pollination, fertilization, and seed maturation. 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 indicated that infertility is due to severe reductions in pollen tube numbers both early after pollination and later when pollen tubes were traversing the ovary, due primarily to the inability of pistils to support normal tube growth. A three-year greenhouse experiment comparing fertility, survival, and growth of F1 progenies produced from reciprocal crosses between plants from the infertile population and those from nearby fertile populations suggested that the genetic basis for infertility is simple and may involve a single recessive mutation. In addition, the results did not reveal any association 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 pleiotropy 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 populations under greenhouse conditions revealed substantial infertility in all but one monomorphic population. Populations varied somewhat in the stage at which infertility was expressed, however, pollen tube growth was impaired in all populations. These results provide strong support for the hypothesis that complex traits like sex are degraded by mutation when they no longer increase fitness.