Sex Expression in Papaya
Authors: Anjali Soni, Preeti Singh and Jai Prakash
Division of Fruits and Horticultural Technology
ICAR-Indian Agricultural Research Institute, New Delhi-12
Corresponding author email: anjali00soni@gmail.com


Papaya (Carica papaya L.) is an edible tropical fruit crop which has several medicinal and nutritional benefits. Propagation of papaya by seed is still the most practical method of raising the crop. There are several reasons why a desirable sex type of papaya plant needs to be identified prior to planting? Generally, the number of male plants outnumbers the females in a plantation, which renders it unproductive. The grower is thus confronted with the problem of selecting the right type of plant material for commercial plantation; but unfortunately growers cannot identify the productive and unproductive plants at nursery stage, so as to have of male and female plants. The papaya generally flowers 75 to 150 days after transplanting and identification of the desirable plants at seedling stage would help in raising the orchard with appropriate design. In subtropical region the dioecious varieties like Pusa Nanha and Pusa Dwarf are preferred in over gynodioecious once due higher number of stable female plants in a population, besides their dwarf stature and high yields.

Genetics of Sex Expression

Sex expression in papaya is controlled by a single gene, with three alleles which have a pleiotropic effect. In papaya, females possess homogametic X chromosome (XX), while male and hermaphrodite have heterogametic XY and XY hchromosomes, respectively. The sex inheritance in papaya depends on three alleles including a recessive "m" allele for females and a dominant "M" allele for males and "M h" allele for hermaphrodites. Female (homozygous recessive; mm), male (heterozygous dominant; Mm) and hermaphrodite (heterozygous dominant; Mhm) are the three viable genotypes. The dominant homozygous combinations (MhMh, Mh M, and MM) would be lethal and therefore non-viable. It was assumed that dominant alleles M and Mh represent genetically inactive regions of "sex chromosomes" which slightly vary in their length and functional genes are lost in these regions. Therefore, these homozygous dominant genotype would be lethal, while Mm and Mhm would be remain viable because an "m" sex chromosome is present in each genotype. With the advancement of the science and techniques new facts has been explored, i.e.physical mapping and sample sequencing of the non-recombination region led to the conclusion that sex determination is controlled by a pair of primitive sex chromosomes with a small male-specific region (MSY) of the Y chromosome. We now postulate that two sex determination genes control the sex determination pathway (Ming et al., 2007).

Importance of Sex Determination Study in Papaya

Papaya is trioecious species with three sex types: male, female and hermaphrodite. Sex type identification is valuable in papaya because sex of the papaya plant cannot be predicted morphologically at early seedling stages. Among these sex types, hermaphrodite plants are preferred for commercial cultivation due to their pyriform shaped fruits (Storey, 1941). Female plants are grown mainly for papain production and male plants are not useful for economic purposes as they do not produced fruits and hence they should be removed from the field which increases production cost. . However, female trees require presence of small number (6-10%) of male trees in the field for fruit production. Sex type cannot be identified phenotypically at early seedling stages until the plant flowers (i.e. 3-4 months after germination). This process increased the cost, labor and waste time. In order to save this it is necessary for farmers that the sex type of this crop is identified before transplanting.

Methods for Identification of Sex Types

Papaya sex identification at early plant development stage has been a problem since long back which led to the researchers to develop many marker techniques such as

(1) Morphology based- On the basis of traits such as leaf or root morphology, rate of growth and seed coat color etc.

(2) Biochemical markers- Isozyme (biochemical) markers

(3) Polymerase chain reaction (PCR)-based markers- It includes RAPD, AFLP and ISSR

(4) Sequencing-based markers- It includes simple sequence repeats (SSR) and single nucleotide polymorphism (SNP).

The failure of morphological tags, cytological evidences and isozyme markers to determine the sex types in papaya at the seedling stage has led to use of DNA markers for determining the sex differences. Markers for sex types in papaya that could be generated through DNA analysis using PCR technology is seen as a reliable strategy. However, knowledge of the sex types in papaya is important in selecting parents for use in hybridization. Crosses between females and hermaphrodites will give all fruit-bearing progenies. In addition, for micropropagation, the early detection or identification of the sex type of papaya seedlings would be advantageous, since the desired sex type can be selected prior to micropropagation.

References

Ming, R., Yu, Q. and Moore, P. H. (2007) Sex determination in papaya. Seminars in Cell and Developmental Biology 18(3): 401-408.

Storey, W.B. (1941) The botany and sex relations of the papaya.Hawaii Agriculture Experiment Station Bulletin, 87: 5-22.





About Author / Additional Info:
I am a Research Scholar at IARI New Delhi.