Molecular Plant Breeding 2016, Vol.7, No.28, 1
-
18
5
M
h
m would be remain viable because an “m” sex chromosome is present in each genotype.
Recently, Ming et al., (2007) proposed that two genes (stamen suppressing gene in female flower and carpel
suppressor gene in male flower) play an important role for determination of sex forms. Stamen-suppressing gene as
the name indicates causes abortion of stamen before or at the stage of initiation of stamen primordia in female flower.
On the other hand, carpel suppressor or male fertility gene aborts the carpel at a later developmental stage in male
flower.
3.1 Suppression of recombination around sex determination locus
High-density genetic linkage mapping of the papaya genome was done for the purpose of cloning of sex
determination genes, which revealed severe recombination suppression around the sex determination locus (Ma et
al., 2004). This mapping result validated the Storey’s (1953) hypothesis stating that the region containing sex
determination genes behaves as a unique factor that does not undergo crossing over. A total of 1501 markers
including 1498 Amplified fragment length polymorphism (AFLP), morphological sex type, fruit flesh color and the
papaya ringspot virus coat protein markers were mapped onto 12 linkage groups (LGs) of papaya. Out of these 1501
mapped markers, 225 were found to be co-segregated with sex types. This linkage mapping has also demonstrated
that the genomic region around the sex determination locus possess high polymorphism.
3.2 Physical mapping of papaya genome
A high-density genetic map of papaya was constructed utilizing bacterial artificial chromosome (BAC) end
sequences derived microsatellite markers (Chen et al., 2007). This genetic map demonstrated that 12 LGs of papaya
were covered a total of 707 markers, containing706 microsatellite loci and one morphological marker (fruit flesh
color). These 12 LGs were made up of three minor and nine major LGs. The nine major LGs are equivalent to nine
chromosomes of papaya. LG1, one of the members of nine LGs is largest and present the sex chromosome of papaya.
The position of recombination suppression was observed around male specific Y chromosome portion (MSY) on
LG1as well as at the centromere portion of other LGs. Segregation distortion was observed on two LGs of papaya, i)
distortion on LG1 around the MSY was due to abortion of the homozygous YY genotype during post-zygotic
selection at 25-50 days after pollination and, ii) distortion on LG6 was due to an unknown reason.
In another study, the physical map was linked with genetic map of papaya. For integration purpose, a
sequence-tagged high density genetic map and BAC end sequences were utilized. This study revealed that the
location of recombination suppression is found across the genome as well as around the MSY on LG1.The size of
recombination suppressed portion i.e. MSY on LG1 was predicted about 8-9 Mb on the basis of integrated genetic
and physical mapping. The rate of recombination slowly rises as the distance from MSY portion increases and
rapidly rise at location about 10 Mb away from MSY to seven-fold of the genome coverage, then decreased again.
These rise and fall in rate of recombination in 10 Mb away from MSY and suppression of recombination in 8-9
Mb of MSY portion suggests that recombination rate in these portions gradually developed during the early stages
of sex-chromosome evolution (Yu et al., 2009).
More recently, physical maps for MSY region (Gschwend et al., 2011), and the hermaphrodite-specific Y
h
chromosome portion (HSY) and its X counterpart (Na et al., 2012) were constructed using BAC libraries. These
physical mapping results are significant to study the early events occurring during evolution of sex chromosome,
identify genes responsible for sex-determination and helpful in sequencing the sex specific regions of papaya.
3.3 Origin of sex chromosomes
The MSY portion possesses several types of chromosomal rearrangements such as insertions, deletions, inversions
and duplications. Expansion was identified on two regions of the MSY which suggested that at the molecular level,
homomorphic sex chromosomes (cytologically similar) are heteromorphic. The divergence between X and Y
h
were
estimated to be between 0.5 and 2.2 million years based on the gene found on HSY and X BACs, which indicates
the recent origin of the papaya sex chromosomes (Yu et al., 2008b).
