Phenotypic and Genetic Diversity of Papaya

Abstract

Knowledge of the diversity within any species is necessary for understanding evolutionary relationships, developing efficient strategies for preservation of genetic resources, and effectively directing the breeding and selection for genetic improvement. Common papaya (Carica papaya L.) is a member of the small family Caricaceae, which is comprised of six genera some of which have changed taxonomic rank over time. Notably, the genus Carica has always included the common papaya but has at times also included highland papayas of the genus Vasconcellea as a section within the genus. However, based on genetic polymorphism, Vasconcellea was recently restored as a genus comprised of 21 species and the genus Carica has been reduced to the single species C. papaya. As a consequence, papaya is now recognized as being less diverse than when it included Vasconcellea spp.

Common papaya shows moderate to high phenotypic variation for the morphological characters of leaf shape and size, types of inflorescences and flowers, and fruit shape and size. Phenotype diversity in papaya is broad for the horticulturally important traits of plant stature, length of the juvenile phase, stamen carpellody, carpel abortion, fruit characters, and reactions to pests and pathogens. The most distinctive, and economically most significant, phenotypic traits of papaya varieties are related to flower and fruit characteristics.

The major advantage for analysis of DNA polymorphism over phenotyping for characterizing plant diversity is the greater resolution at the molecular level and that chromosomal loci sequences can provide a direct link to diagnostic genes. Genetic polymorphism is the primary source for variation of the morphological and physiological phenotypes. Natural selection for or against a particular phenotype leads to evolutionary changes within and among species and eventually to the level of genetic diversity in the species, which in turn provides the basis for adaptation to changes in environmental conditions. The magnitude and structure of the genetic diversity of a population determine the ability of that population to adapt to its environment through natural selection of the particular combination of genes conferring greatest fitness.

Collectively, these diversity analyses suggest that C. papaya diverged from Vasconcellea spp. early in its evolution and the wild South American Carica species are more closely allied with a member of the related genus Jacaratia than to C. papaya. Genetic diversity studies within cultivated papaya indicate that cultural preference and geographic isolation have forced selection of cultivated papaya from a relatively narrow genetic base to result in low genetic diversity. The potential for obtaining greater diversity within papaya from its relatives indicates a need to introgress desirable traits from those wild relatives.