Sex expression in papaya: morphological marker, molecular genetics and environments

Abstract

Papaya (Carica papaya L.) is a polygamous species and the plants are ex- tremely diverse in their sexual systems. Three sex types are available in papaya viz., male, female and hermaphrodite and are controlled by a single gene with three alleles (m, M1, M2). The genotypes represent gynoecious, androecious and hermaphrodite individuals. Ninety percent of freshly dis- persed pollen grains were viable in summer but viability dropped to about 45% in some lines and as low as 4.5% in others in winter. The extremes of humidity reduce the storage life of papaya pollen but under ideal (arti- ficial) storage conditions it potentially remains viable for about 5-6 years. The stigma become receptive two days before anthesis and continued up to five days after anthesis but began to decline gradually upto five days after anthesis. Papaya plants produce fruit either through cross-pollination or self-pollination or parthenocarpy depending on their sex types. On an average 1,000 seeds are found in a single fruit, indicating that 1,000 viable pollen grains may fertilized receptive stigma. The sex types in papaya is found to be related with several morphological characters. Seed coat color, petiole thickness, stems color worked as a morphological marker for sex determination in papaya. The black and dark brown seed coat color exhib- ited higher frequency of the female and hermaphrodite plants. The range of 54–60° petiole orientation, 3.7–4.2 cm petiole thickness and 9.4–10.4 cm petiole length gave higher percentage of female and hermaphrodite plants. On the other hand, unique purple stem color was reported to express as hermaphrodite plants. In case of chemical identification of sex in papaya Almen reagent test, Ammonium molybdite test, Titanous chloride test gave 71%, 60% and 55% accuracy of femaleness respectively. Ethrel gave the most expected number (46.67%) but excessive Ethrel may also cause higher num- ber of male. In case of Kinetin and IBB 100 ppm and 200 ppm gave higher percentage of female. Environment may also affect the sex expression of papya. In along with these, several molecular markers may also be used to identify the sex type of papaya. Among them SSR and RAPD is mostly familiar and successful.