Again and again Charles Darwin found inspiration in the cucumber and its fellow cucurbits.

The first trait to strike him was the unplantlike motility of the vine's tendrils, organs adapted to the habits of climbing and running. Use resulted in adaptation, disuse in diversification and loss.

In the varieties which grow upright or do not run and climb, the tendrils, though useless, are either present or are represented by various semi-monstrous organs, or are quite absent.

Then he noted the diversification of marrow, gourd and melon fruit forms under agricultural selection and pondered the irreducible essence of species identity. He decided to trust the biological species concept, namely that different species cannot produce fertile offspring.

If we were to trust to external differences alone, and give up the test of sterility, a multitude of species would have to be formed out of the varieties of these three species of Cucurbita.

Having a biological definition of species identity, Darwin was then able to unravel the relationship between species and apparently stable, taxonomically important traits without fear of arguing in a circle. He contrasted these traits with variable features found within a species. He was also able to identify convergent evolution under selection of the fruit morphologies of distinct species of melons and cucumbers (C.R. Darwin, The Variation of Animals and Plants under Domestication 1st edn., 2nd issue, vol. 1, John Murray, London, 1868).

Now, on p 1275, Sanwen Huang et al. report the de novo assembly and annotation of the 243.5-Mb genome of the “Chinese long 9930” inbred line of cucumber and the use of a linkage map in the assembly process to tie the assembled contigs to the chromosomes. The Illumina GA technology has proven practical, so now many diverse lines can be rapidly sequenced to enable marker-assisted breeding of high-yielding, disease-resistant, and fresh green-scented cucumbers, along with melons, squash and pumpkins.

Cucumber and melon diverged 4–7 million years ago, and C. sativus carries chromosome fusions that distinguish the cucumber karyotypes from those of melon (C. melo) and a more distant relative, the watermelon (Citrullus lanatus). Were he here today, Darwin could see that these sets of chromosomes physically reinforce the biological species barrier to fertility, were the (widely varying) sexual systems of the plants to permit crossing.

What would Charles do next, equipped with genomes? No doubt he would be most intrigued to compare the genesis of the woody and non-woody tendrils of grapevine and cucumber, respectively. Then he might scan for signatures of plant-human coadaptation during the domestication processes of early agricultural humans. Then he might travel to investigate the adaptations contributing to the success of Cucumis dipsaceus, the wild spiny cucumber originating in Eastern Africa that is now invading the Galapagos Islands he once explored.