Molecular Plant Breeding 2016, Vol.7, No.28, 1
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Yu et al. (2008a) studied the characteristics of dioecious X and Y BACs and a comparison was done in between the
sequences of both dioecious and gynodioecious X, Y and Y
h
BACs chromosomes. Several chromosomal
rearrangements including insertions, duplications, deletions, and inversions were detected between the X and
Y-specific BACs and expansion was predicted on the Y BAC due to suppression of recombination in this region.
Both Y and Y
h
-specific BACs shared high degree of sequence identity in DNA. X-specific BACs were found to be
almost identical in both dioecious and gynodioecious. Y and Y
h
chromosomes were diverged approximately 73,000
years ago which suggested that a common ancestral Y chromosome is responsible for the evolution of both Y and Y
h
chromosomes.
3.4 Features of male-specific portion of the Y chromosome
A pachytene chromosome-based cytogenetic mapping of MSY was done to study the features of MSY. The MSY
region constitute 13% portion of the Y chromosome. A high level of methylation and centromere is present within
MSY of the Y chromosome. This methylation and centromere play a valuable role in silencing the gene and
suppression of recombination in evolution of Y chromosome (Zhang et al., 2008).
3.5 Evolution of sex chromosomes
Expansion of both Y chromosome (Yu et al., 2008a) and X chromosome (Gschwend et al., 2012) occurs which is the
common feature during early stages of evolution of sex chromosomes. The recently originated papaya X
chromosome was compared to its homologous autosome in close relative monoecious
Vasconcellea monoica
for
revealing the evolutionary history of the X chromosome. The
V. monoica
genome size (626 Mb) was found 41%
larger than the papaya genome (372 Mb) which suggested the expansion in X chromosome of papaya. The reason
behind the expansion of papaya X chromosome is due to the higher accumulation of repetitive sequences as
compared to the autosomal sequence.
3.6 Gene responsible for sex determination
Urasaki et al., (2012) performed a digital transcriptome analysis to identify sex determining gene in papaya. This
analysis was done by utilizing floral samples from male, female and hermaphrodite papaya plants. 312 unique tags
were located to sex chromosomes (Y
h
and X; most of them mapped on X chromosome), and 30 were mapped on
both Y
h
and X chromosomes. In addition, Y and Y
h
chromosome-specific gene i.e. MAD-box gene was identified. It
regulates the expression of other distant genes such as increased the expression of genes in the female or reduced the
expression in the male which plays role in sex-determination.
3.7 Sequencing of sex chromosomes
The sequencing of the sex chromosomes HSY and its X counterpart were done. The main purpose of sequencing
was to study the events occurring at the time of early stages of HSY and X chromosome evolution. This study
revealed that HSY differs from X regions in respect of size of chromosomes, physical size of inversions and gene
content etc. A high amount of retrotransposons are present in HSY portion, which makes HSY portion larger in
size than X. HSY and X possess two main inversions namely, inversion 1 and 2. A similarity was found in the
physical size of inversion 1 portion of HSY and X chromosome regions, as they possess similar amount of
repetitive sequence (80.7% in HSY and 76.5% in X chromosome, respectively). Since recombination suppression
accumulates high amount of repetitive sequences in HSY region (80.2% in HSY against 60.5% in the X) therefore
the size of inversion 2 in HSY chromosome was more than twice that of X chromosome. A total of 16
transcripts-encoding sequences (nine HSY-specific genes and seven pseudogenes) were identified in HSY which
is lesser in number as compared to X containing 28 transcript-encoding sequences (24 X-specific genes and four
pseudogenes). In addition, the sequencing of sex determining regions HSY and corresponding X regions yields 8.1
Mb and 3.5 Mb pseudomolecules, respectively. The emergence of sex chromosomes were reported about 7.0 million
years ago (Wang et al., 2012).
More recently, Vanburen et al. (2015) did the sequencing and resequencing of MSY and HSY portions using
BAC-by-BAC approach. This study reported that high similarity in gene content was predicted in both MSY and
