Sex Determination in Papaya

Papaya (Carica papaya) is diploid with 2n=36 chromosomes, made up of 17 pairs of ordinary autosomes plus one further pair that functions as a true sex-chromosome pair, discovered relatively recently by researchers in Hawaii. Unlike the highly differentiated X and Y chromosomes of humans, papaya's sex chromosomes are cytologically almost indistinguishable from its autosomes, because they evolved from an ordinary autosomal pair rather than diverging over a very long evolutionary time; nonetheless they are functionally distinct, since the Y chromosome specifically carries the genes needed for male flower-organ development while the X carries the genes needed for female flower-organ development, giving male plants the genotype XY and female plants XX. Overlaid on this X/Y system, papaya sex is further fine-tuned by a three-allele series (m, M1, M2) at a single additional locus: the homozygous recessive genotype mm restricts maleness and produces a female plant; the heterozygous genotype M1m induces maleness and produces a functional male; the heterozygous genotype M2m induces both sexes together, giving a rare bisexual plant; and any homozygous or h …