Botany and Improvement 29
sex determination locus. 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. It was reported that two sex determination genes control the
sex determination pathway. One, a feminising or stamen suppressor gene causes
stamen abortion before or at flower inception, while the other, a masculinising or
carpel suppressor gene causes carpel abortion at a later flower developmental stage.
Detailed physical mapping is beginning to reveal structural details about the sex
determination region and sequencing is expected to uncover candidate sex deter-
mining genes. The cloning of sex determination genes and understanding the sex
determination process could have profound application in papaya production (Ming
et al. 2007).
Sex determination in papaya is controlled by a recently evolved XY chromo-
some pair, with two slightly different Y chromosomes controlling the development
of males (Y) and hermaphrodites (Yh). For the study of early sex chromosome
evolution, Wang et al. (2012) sequenced the hermaphrodite-specific region of the
Yh chromosome (HSY) and its X counterpart, yielding an 8.1-megabase (Mb) HSY
pseudomolecule and a 3.5-Mb sequence for the corresponding X region. The HSY
is larger than the X region, mostly due to retrotransposon insertions. The papaya
HSY differs from the X region by two large-scale inversions, the first of which likely
caused the recombination suppression between the X and Yh chromosomes, followed
by numerous additional chromosomal rearrangements. Altogether, including the X
and/or HSY regions, 124 transcription units were annotated, including 50 functional
pairs present in both the X and HSY. Ten HSY genes had functional homologs else-
where in the papaya autosomal regions, suggesting movement of genes onto the HSY,
whereas the X region had none. Sequence divergence between 70 transcripts shared
by the X and HSY revealed two evolutionary strata in the X chromosome, corre-
sponding to the two inversions on the HSY, the older of which evolved about 7.0
million years ago. Gene content differences between the HSY and X are greatest in
the older stratum, whereas the gene content and order of the collinear regions are
identical (Wang et al. 2012).
2.5.2 Cytogenetics of Sex Chromosomes
Papaya has nine chromosome pairs, in which seven are metacentric and the remain-
ing two pairs are sub-metacentric (Ming et al. 2008). Papaya chromosomes are
small and uniform in morphology, making them hard to differentiate using length,
arm ratios, or banding patterns, which led to difficulties in identifying sex chromo-
somes in early studies of papaya sex determination (Wai et al. 2010). In an early
investigation, precocious separation was observed between a chromosome pair dur-
ing anaphase I of meiosis of a pollen mother cell in males and hermaphrodites.
Recombination occurs throughout the homologous regions of the sex chromosomes,
but it is suppressed in the sex specific region of the chromosome pairs (~13% of the
sex chromosomes) (Zhang et al. 2008). Recombination rate recovers and elevates
in the border regions of the HSY (Yu et al. 2009). To link chromosomes to their
genetic sequences, genetic mapping of the papaya genome was carried out and