There are a number of plant crops that have lost some or all of their capacity to genetically recombine, either because of major genomic changes that prevent sexual reproduction but permit vegetative cloning, or long cultivation and resulting selection pressures. Bananas, peppermint, and orange daylilies are examples of the first group; true pepper, some varieties of potatoes, and garlic are among the latter.
Garlic, Allium sativum, is especially interesting. Many alliums produce by both seeds and bulb structures, and some also form aerial bulblets along with the flowers called ‘bulbils’. Garlic was no exception. But the process of growing flowers and producing seeds requires a lot of energy from a plant’s metabolism, and if bulb formation takes place at the same time as flowering, the plant tends to take resources away from the bulb and devote them to the flowers. Ancient peoples were aware of the general principle, and would often remove the flowering stalk to make the bulbs grow larger, as well as selecting the biggest bulbs to plant.
Over thousands of years, this resulted in varieties of garlic that were completely unable to reproduce sexually. ‘Softneck’ garlics don’t form flower stalks at all, and ‘hardneck’ garlics produce a flower stalk that is dominated almost completely by bulbils. Many plants are male-sterile, too, so even the few flower sections that develop normally are unlikely to produce viable seeds. There are hundreds of different varieties of garlic, but the plant has an unusual ability to adapt to local conditions after several seasons of growing in one place, and many of the characteristics that distinguish one variety from others are acquired responses to environmental factors. Genetic analysis suggests that there are only about ten distinct strains of garlic remaining, and the wild plant that crop populations were derived from no longer exists – probably driven into extinction through overharvesting.
This results in certain problems: without sexual recombination, the garlic plants have a harder time purging themselves of harmful mutations, and the limited population diversity means the species is vulnerable to new threats. Without the new combinations produced by sex, neither natural nor artificial selection pressures result in much adaptation. Garlic contains potent antibacterial and antifungal compounds, but they’re released only when the plant cells are crushed, and the species has known vulnerabilities to a variety of fungal diseases. Once enough spores build up in a plot of land, any garlic grown there will be withered and stunted – and because all of the plants are clones of their ancestors, resistance to the diseases doesn’t evolve.
In the sections of Asia where garlic originated, there are still closely-related wild populations, and some of the varieties grown there have many ‘primitive’ traits probably shared with the original progenitors of the species. Some researchers have managed to induce the production of fertile seed in plants of those varieties! Managing the task is tricky and tedious, though. See US patent 5746024. Once a few fertile seeds are produced, it becomes simpler and simpler to produce more with each generation, as the recombination makes it possible to select for fertility… assuming the population doesn’t carry sterility genes. And getting the first few seeds is quite difficult.
I’ve been attempting to reproduce the results with a strain of hardneck garlic I found growing wild in a nearby nature preserve. I don’t expect much for the first few years, but with a little luck, I might be able to produce new true varieties.