Factors Affecting Clonal Diversity and Coexistence

Recent genetic studies of asexually reproducing fishes in thegenus Poeciliopsis (Poeciliidae) revealed abundant variationin the form of multiple sympatric clones. Recurrent hybridizationsbetween sexual species provides the principal source of clonalvariation. The hybrids are spontaneously endowed with a clonalreproductive mechanism that perpetuates a high level of heterozygosity.Migration within and between river systems, and mutations, alsocontribute to clonal diversity in these fish. Coexistence amongdifferent clones and with the sexual ancestors depends in partupon specializations characteristic of individual clones. Clonalreproduction is an efficient mechanism for freezing a portionof the niche-width variation contained in the gene pool of themore broadly adapted, sexual ancestors. Multiclonal populationsachieve significantly higher densities relative to the sexualforms than do monoclonal populations. This relationship is afunction of the clonal variability upon which natural selectioncan act and upon the capacity of a multiclonal population tobetter exploit a heterogeneous environment through niche diversification.In all-female organisms such as Poeciliopsis, which are dependentupon sexual species for insemination, competitive abilitiesprobably are at a premium in the densely populated pools andarroyos of the Sonoran Desert. Competitive abilities are probablyless important for truly parthenogenetic clones which rely oncolonization abilities to escape from their sexual ancestorsand from other clones.