"The hybrid gives us a natural controlled experiment," said Fei. "Both versions of each gene sit in the same cells and receive the same regulatory signals. If one version is more active than the other, we know the difference is written into the DNA right next to that gene."

The researchers measured gene activity in three fruit tissues — the fleshy outer wall, the placenta, and the jelly around the seeds — at up to four stages from early development through ripening.

The results were strikingly consistent. In every species, tissue and stage examined, cis changes were the main driver of differences in gene activity, affecting up to 23.5% of active genes, compared with no more than 4.6% affected by trans changes. The number of genes with regulatory differences roughly doubled in the more distant wild relatives compared with S. pimpinellifolium, the closest relative of cultivated tomato. Many of these switches were also highly specific, working in only one tissue at one moment of fruit development.

Decoding color, sweetness and bitterness

The map explains several familiar fruit traits. In the green-fruited species, cis changes boost genes that steer the pigment pathway away from lycopene — which is why those fruits stay green.

Sweetness comes down to a matched pair of sugar-related genes. A gene that breaks down sucrose and another that blocks this process are regulated in a coordinated way in the green-fruited species, allowing those fruits to retain more sucrose.

The data also reveal a developmental switch for bitterness. Early in fruit development, trans regulation maintains high levels of bitter, defensive compounds called glycoalkaloids in young fruit. Later, cis changes in cultivated tomato convert those compounds into non-bitter forms as the fruit ripens — a pattern the study describes as "defense early, edibility at maturity."

"Breeders have always known wild tomatoes hold valuable traits, but their use in breeding introduces undesirable traits as well, necessitating time- and labor-intensive cleanup," said Giovannoni. "What they've lacked is a way to tell which of thousands of genetic differences is behind each trait to make selection more targeted. Now we can point to specific genes for selection, with effects in specific tissues, at specific moments in fruit development and ripening."

An open resource

The findings come with caveats typical of gene-regulation studies: the map identifies targets and mechanisms, but breeding tastier varieties still requires years of crossing and selection, and introducing wild traits can still disrupt yield, size or shelf life.

As Catala put it: "Wild tomatoes are an enormous reservoir of diversity for flavor, nutrition and resilience, but bringing those traits into modern varieties can come at the expense of other traits that breeders have spent decades improving. This can be countered with precise information on how the genes responsible for such traits are controlled."

To speed that work along, the team has made its data — including two newly assembled wild tomato genomes — freely available, giving breeders and researchers worldwide a head start on bringing the best of wild tomatoes back to the table.