Papaya, Carica papaya - Identification of the hemaphroditic sex by molecular methods

Information 26-11-2017.

The papaya (Carica papaya) is a plant of the family Caricaceae, orginaria of tropical America and one of the main crops in tropical and subtropical regions of the World. This family includes a monoecious species (monooecius), 32 dioecious species (dioecius) and two trioic species (trioecius). The trioic species have three types of sex: hermaphrodite, female and male. Papaya diverges from its closest related monoecious, Vasconcellae monoica. The papaya sex is controlled by a pair of sex chromosomes.

There would be a genetic locus with three alleles: M (male), Mh (hermaphrodite) and "m" (female). The alleles M and Mh would be dominant over the "m". The "mm" females are homozygous recessive. The males (Mm) and the hermaphrodites (Mhm) would be heterozygous.

The hermaphroditic and male characteristics determined by two slightly different Y chromosomes, Y and Yh (alleles M and Mh), respectively. The genotype XX (mm) determines the female sex, XY (mM) determines the male sex and XYh (mMh) determines the hermaphroditic sex.

All the combinations of Y and Yh (YY, YYh, YhYh) are lethal embryonic, indicating that the Y and Yh chromosomes have degenerated, losing some genes necessary for embryonic development, so the only possible vital progenies are XX (mm) (female), XY (mM) (male) or XYh (mMh) (hemaphrodites). The Y and Yh chromosomes have a 98.9% similarity of their sequence, differing mainly in the intergenic and repeated regions. There would be at least two genes that would differentiate the Y and Yh chromosomes, one of which would control the long peduncle in male trees and another gene that would control carpel abortion (masculinizing gene) in male flowers. The abortion of the carpel occurs at a late stage due to the type of genetic regulation (downstream) since there is a carpel residue aborted in the structure of the male flower and even under certain conditions of optimal growth, some male flowers do not abort the carpel and they form fruits. In female flowers, there would be a stamen suppressor gene (feminizing gene). This regulation would be different (upstream) for the complete absence of stamen in female flowers and would take place before the beginning of the primordium of the stamen.

The male specific region Y (MSY: Male Specific Y), and the hermaphroditic specific region of Yh (HSY: Hermaphrodite Specific Y) are highly methylated and heterochromatized compared to the corresponding region of the X chromosome.

The female fruits have more seeds and less pulp than the hermaphrodites, so they have less interest. Hermaphrodite plants produce the best fruits, elongated with a small internal cavity, higher pulp-seed ratio and thick skin, which allows them to resist mechanical damage after harvesting,

There is no visual morphological evidence to differentiate the sexes from the plants until flowering, so the plants should be grown until flowering occurs. This means that all the plants are cultivated until flowering, and then all those that are not hermaphrodites are eliminated, with the consequent economic loss due to the work that their care requires. After evidencing sex, only the hermaphroditic plants are maintained.

Hence, one of the needs is to identify the nursery seedlings to conserve only those identified.

Different molecular identification methods have been developed based on the findings of the comparisons (SCAR markers) obtained after the comparisons of their genomic sequences with the RAPD (Random Amplified Polymorphic DNA) and AFLPs (Amplified Fragment Length Polymorphism) methods.

Recommended tests for diagnosis:

  • Molecular identification of the Mh locus (Yh gene), specific for hermaphrodites.

Tests carried out in IVAMI:

  • Molecular identification of the Mh locus (Yh gene), specific for hermaphrodites.

Recommended sample:

  • Small leaf sample (3-4 mm pieces).

Conservation and shipment of the sample:

  • Refrigerated (preferred) for less than 2 days.
  • Frozen: more than 2 days.

Delivery of results:

  • Molecular diagnosis (PCR): 48 to 72 hours.

Cost of the test: