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Cover Female papaya trees with homogametic sex chromosome genotype XX. In this issue, four knob-like heterochromatin structures specific to the male-specific region of the "young" papaya Y chromosome are reported. The DNA sequences associated with these heterochromatic knobs are highly divergent and heavily methylated compared to those of its X chromosome counterpart, suggesting that DNA methylation and heterochromatinization play an important role in the early stage of sex chromosome evolution in papaya. (Photo: Qingyi Yu. [For details, see Zhang et al., pp. 1938-1943.])
