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Sex Expression in Papaya: Morphological Marker, Molecular Genetics and Environments

Farzana Mustafa Era
Department of Genetics and Plant Breeding, Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur 1706, Bangladesh

Mohammad Sharif Raihan
Department of Genetics and Plant Breeding, Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur 1706, Bangladesh

A K M Aminul Islam*
Department of Genetics and Plant Breeding, Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur 1706, Bangladesh

Abstract/ Executive Summary:

Papaya (Carica papaya L.) is a polygamous species and the plants are extremely 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 dispersed 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 (artificial) 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 exhibited 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 number 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.

Keywords: Carica papaya, Polygamous, Gynodioecious, Hermaphrodite, Pistillode, Sexual lability

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Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Papaya

Objectives:

Self-pollination in males, crosspollination between males, and cross-pollination between males and hermaphrodites, can all be done using the sexually ambivalent males (SAMs) that produce perfect flowers during certain periods of the year. Male and hermaphrodite plants undergo various degrees of sex reversal, depending on seasonal changes and climate (Awada, 1958a). The female plant is the most stable form.

Methodology:

2.1 Types of flowers Papaya is polygamous species and possesses three sex forms viz., female, male and hermaphrodite (Yu et al., 2008). The flowers are grown on the inflorescence called Cyme (Storey, 1969), slightly fragrant, fleshy and waxy, and yellow to cream in color. The inflorescence type varies according to the sex of the plants. Flowering occurs generally within 3-6 months after germination but it could be extended up to 9-12 months in some cases (Vashistha et al., 2016). 2.2 Anthesis of flowers The flower opening starts from 6 am to 12 pm (Azad and Rabbani, 2004). The time of anthesis could be changed due to environmental difference as well as interaction of genotype. 2.3 Dehiscence of anthers The anthers of C. cauliflora started dehiscence after 8 am. Azad and Rabbani (2004) reported the maximum anther dehiscence in C. cauliflora at 11-12 pm but minimum dehiscence was noticed in C. papaya cv. Shahi at the same time in Bangladesh. The anthers dehiscence of all species was increased with increasing the temperature up to 12 pm. These results revealed that high temperature speed up the dehiscence of anthers. Sharma and Bajpai (1969) reported that optimum anther dehiscence occurred at 12 to 1 pm for papaya. Khuspe and Ugale (1977) reported that maximum number of anthers dehisced between 9 am to 12 pm in C. papaya cv. Washington. 2.4 Pollen viability Pollen remains viable for 2 days before and after anthesis, with maximum viability on the day of anthesis. At room temperature, and 50% relative humidity, pollen remains viable for 48 hrs. Garrett (1995) reported that 90% of freshly dispersed pollen grains were viable in summer but that in winter, viability dropped to about 45% in some lines and as low as 4.5% in others. Allan (1963) found that extremes of humidity reduce the storage life of papaya pollen which, under ideal (artificial) storage conditions, potentially remains viable for about 5-6 years. Allan (1963) also found that temperatures below 10ºC significantly affect pollen viability, possibly as a consequence of degenerated pollen mother cells. 2.5 Stigma receptivity Azad and Rabbani (2004) observed that the stigma become receptive two days before anthesis and continued up to five days after anthesis. Significant increase of number of fruit set was recorded on the day of anthesis. But it began to decline gradually upto five days after anthesis in all the species. No fruit set was found in all species at six days after anthesis and stigmas became dried. 2.7 Fertilization and fruit setting Abundance of pollen or pollinator efficiency may affect fertilization and fruit setting ultimately fruit production. On an average 1,000 seeds are found in a single fruit, indicating that 1,000 viable pollen grains may fertilized receptive stigma. The more seeds in a fruit, the larger the fruit grows (McGregor et al., 1976). 5 Sex determination in plants There are no universal models supporting sex determination in plants. There are lots of hidden mechanisms for development of sex organ in the plant.

Source of Information: Fundamental and Applied Agriculture Vol. 4(4), pp. 978–994: 2019

Article Link (Online Journal): doi: 10.5455/faa.48652

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Achievement/Findings:

Identification of desirable plant population at early stage is very important and crucial for papaya cultivation. Several morphological characters such as seed coat colour, root morphology etc. found to be related with the sex types in papaya. Black and dark brown seed coat color exhibited higher frequency of the female and hermaphrodite plants. Petiole thickness and stems color also worked as a morphological marker for sex determination in papaya. Almen reagent test, Ammonium molybdite test, Titanous chloride test also used to identify sex in papaya. Plant growth regulators such as Ethrel, Kinetin and IBB gave the most expected number female flower in papaya but excessive use of Ethrel may cause higher number of male. Environment may also affect the sex expression of papya. Recently molecular markers are also used to identify the sex type of papaya. The knowledge on sex expression and sex determination is important for papaya breeding as well as cultivation.

Knowledge Management: Journal