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mating combinations, TS x TS, and TS x T7 and T7 x T7 and their F1 plants to check the segregation pattern of
papaya sex. The first two mating combinations i.e., TS x TS, and TS x T7 yielded 2:1 ratio of hermaphrodites vs
dioecious females offspring which was supported by chi-square test but T7 x T7, resulted in all F1 plants
hermaphrodite with no females. On the basis of the results obtained they concluded that a lethal recessive gene
could be linked to T7 cultivar and this lethal allele causes the fatality in female offspring.
SSR markers have also been developed in several other plants such as in hemp (Rode et al., 2005), hop (Jakse et
al., 2008) and date palm (Elmeer et al., 2012; Maryam et al., 2016). Rode et al., (2005) identified first time
sex-linked SSR markers in hemp. Ten SSRs were found to be polymorphic in population 00/50. Out of these, three
SSR markers CS301, CS308 and CS501 were identified as sex-linked markers. In CS308 SSR marker, three
different marker alleles were detected, 185 bp and 192 bp in the male parent and 183 bp and 185 bp in the female
parent, respectively. Jakse et al., (2008) utilized microsatellite marker HlAGA7 which produced an allele of 165
bp size in all males, it indicated a tight linkage between male characters in hop
.
Two groups (Elmeer et al., 2012;
Maryam et al., 2016) utilized microsatellite markers to differentiate between male and female in date palm. First
group reported that the primer mPdCIR048 produced one locus with the size of 160/190 bp reoccurred in 4 male
samples but not detected in any of the female samples. Similarly second group observed that SSR Primer
mpdCIR48 produced a specific locus (250/250) in all male samples only. Primer DP-168 produced a locus of
300/310 bp reoccurred in 5 date palm male samples, which indicated that these are potential markers to identify
sex at early seedling stages in date palm. SSR marker utilization in other plant species highlighted that these
marker could be developed to identify sex types in papaya (Table 2).
5.5.2 Single nucleotide polymorphism (SNP)
SNP is new generation marker based on the principle of the single nucleotide change (A, T, C or G) in DNA
sequences of different individuals of species of genome. They are commonly present in animals and plants. The
frequency range of SNP is one SNP every 100-300 bp in plants. Distribution of SNP in coding and non-coding
region of genomes is hetergenous. They are generated by either transition: purine to purine or pyrimidine to
pyrimidine exchanges (A or G to C or T and vice-versa) or transversion: purine to pyrimidine or pyrimidine to
purine exchanges (A or T to C or T and vice-versa). They possess several advantages such as co-dominant and
biallelic nature, often linked to gene, highly polymorphic and showing high reproducibility which makes them
highly efficient marker system over RAPD, ISSR and AFLP (Jiang, 2013). These markers are evolutionarily
stable due to low mutation rate. Polymorphism in SNPs arises due to insertion and deletion with respect to single
base in the genome. They cannot be resolved by conventional methods like agarose, and polyacrylamide gel
electrophoresis. Their detection includes sequenced genomes and next-generation sequencing technologies
(Martin et al., 2010), capillary electrophoresis (Drabovich et al., 2006) and mass spectrometry (Griffin et al., 2000)
etc. They are important in detection of functional polymorphism if present in coding region because change in
amino acid sequence resulting altered phenotype (Singh and Singh, 2015).
No SNP study has been done yet in papaya for sex identification, but recently sex-linked SNP marker was
identified in
Pistacia vera
(Kafkas et al., 2015) using restriction site-associated DNA (RAD) sequencing (Table 2).
Thirty eight putative sex-linked SNP markers were produced from 28 reads by RAD sequencing and further
validation of these sex-linked markers were done by SNaPshot analysis. This study demonstrated that eight SNP
loci could effectively differentiate sex types. Further, high-resolution melting (HRM) analysis along with real-time
PCR was done by utilizing these eight SNP loci. Out of these eight SNP loci, only four SNP loci
(SNP-PIS-133396, SNP-PIS-136404, SNP-PIS-167992, and SNP-PIS-174431) were successfully separating sex
in all 166
Pistacia
plants. Similarly, SNP could be utilized for identification of gender in
C. papaya
due to their
abundance in the genome and highly polymorphic nature.
Conclusion
The purpose of present article is to reflect the genetics of sex determination of economically and medicinally
important papaya fruit crop and to know the current approaches employed for identification of sex at
