Papaya Biology, Cultivation, Production and Uses

26 Papaya

and change independently. Papaya’s trioecious sex is determined by a pair of incipi-
ent sex chromosomes. Carica papaya has been found to have nascent sex chromo-
somes (Liu et al. 2004). The X and Y chromosomes are cytologically homomorphic,
but the Y chromosome has a recombination suppressed male-specific region (MSY)
which has lost gene content, resulting in the lethal YY genotype. Silene latifolia
has both a large male-specific non-recombining region on the Y chromosome, and
pseudoautosomal region that do still recombine with the X chromosome (Scotti and
Delph 2006; Delph et al. 2010). There are at least three regions in the male-specific
region of the Y relevant to sex expression, one that suppresses femaleness and two
that promote maleness (Zluvova et al. 2007). The sex chromosomes are heteromor-
phic, because the Y chromosome is larger than the X chromosome (Grabowska-
Joachimiak and Joachimiak 2002).
Six stages of sex chromosome evolution have been proposed to explain the varia-
tion of the sex chromosomes in angiosperms (Figure 2.5) (Ming et al. 2011). In the
first stage, a male-sterile mutation and a female-sterile mutation arise in close prox-
imity on a chromosome, but recombination still occurs in this region. Strawberry is

Autosome

Proto X (Z) Stage 1 Fragaria virginiana

Stage 2 Asparagus officinalis

Stage 3 Carica papaya

Stage 4 Silene latifolia

Stage 5 Cycas revoluta

Stage 6 Rumex acetosa

Key

Autosomal or pseudo-autosomal region

Male sterile and female sterile mutations

Degenerated male-specific region on Y

MSY corresponding region on X

Centromere

Proto Y (W)

X Y X Y X Y X Y X O

FIGURE 2.5 Six stages of sex chromosome evolution. (Adapted from Ming, R.,
A. Bendahmane and S. S. Renner, 2011, Annual Review of Plant Biology, 62: 485 –514.)