Dynamic transposable element accumulation in the nascent sex chromosomes of papaya

From their inception, Y chromosomes in plants and animals are subjected to the powerful effects of Müller's ratchet, a process spurred by suppression of recombination that results in a rapid accumulation of mutations and repetitive elements. These mutations eventually lead to gene loss and degeneration of the Y chromosome. Y chromosomes in mammals are ancient, whereas most sex chromosomes in plants and many in insects and fish evolved recently. Sex type in papaya is controlled by a pair of nascent sex chromosomes that evolved around 7 million years ago. The papaya X and Yh were recently sequenced, providing valuable insight into the early stages of sex chromosome evolution. Here we discuss the fruits of this work with a focus on the repeat accumulation, gene trafficking and promiscuous DNA sequences found in the slowly degenerating Yh chromosome of papaya.