Inside every sapota cell lies a huge book of DNA, and it's full of extra pages. These pages are mobile elements, bits of genetic code that can make copies of themselves and insert them in different places. Sapota has accumulated so many of these repeats that its genome became twice as large as that of its closest relatives, even though it gained almost no real genes.
One type, called Gypsy, multiplied more than all the others. About 90,000 years ago it set off a real explosion: it copied itself and inserted into the DNA, inflating the spaces between genes. The cell couldn't stop it, but it learned to live with it. It puts chemical marks on such snippets, the way a librarian sticks on a "do not lend" label. These marks, or methylation, keep mobile elements from stirring again.
Now that sapota's genome has been fully read, breeders can find the genes that make the fruit sweet and aromatic and develop new varieties. And the tree itself shows how plants cope with an invasion of mobile elements and even benefit from it.
The sapodilla genome is only 1.6 times larger than the human genome, yet it contains just about 35 thousand genes versus 20 thousand in humans: the difference in size is almost entirely due to repeats, not gene number.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.