But What About the Plants?
or How to Avoid an Overdraft at the Gene Bank
How long does a plant have to be cultivated before it can be considered a crop rather than a wild species? Is the process of domestication rapid or a long drawn out affair, in which genes flow freely between wild and cultivated populations over many generation? Just as there are days when we all feel a little more ‘feral’ than others, the answer to these questions probably varies between species.
You might imagine that the process of selecting the very first individuals from the wild to cultivate occurred so far back in the mists of time that it’s irrelevant to modern farmers and growers. However, it seems likely that first steps along the domestication road may have unintentionally influenced its final destination.
Far from being ancient history, collecting seeds from wild crop relatives is an important job that gene-bankers do routinely for virtually all our modern crops. When you are tasked with finding and capturing truly undomesticated genotypes, you really start to appreciate how difficult it is pluck something from the wild, without producing a whole raft of surprising consequences. In the biblical account of the flood, Noah did something similar by collecting all the animals in pairs. Noah’s ark can be considered the world’s first gene-bank. However, small this collection, there is no mention of the subsequent effects of inbreeding, genetic bottle necks or random genetic erosion.
One of the most important factors that causes genetic change during domestication is generation upon generation of selecting preferred types. Having said that, unintentional random choice right at the start of the process may radically limit the subsequent choices available.
Many plants reproduced by both seeds and vegetatively. Some crop species are grown to harvest their seeds, others for their vegetative storage organs. Given there appears to be some biological truth behind the expression – ‘you cannot have everything in life’ here lies the rub. If our ancestors were effective at identifying the plants that were most prolific in producing root tubers, then in subsequent generations, they may struggle to produce seeds. Even more striking, similar examples include selecting for seedless grapes and citrus. These may be much easier to eat without those fiddly seeds. But what happens when you want to cross breed them for disease resistance?
In the wild, not all plants are not the same size and it’s not always easy to see where one individual stops and the next one starts. In grasslands and woods plants can grow through each other. Even those apparently separated, may in fact be the same genetic individual, where the inter-connecting sections have died. Such complexity make it really tricky if you are trying to select plants truly randomly and not over collect the few really large individuals. This is really important if you are to avoid losing genes that might be really useful in future breeding programmes.
The first few generations are also likely to be disproportionately important in introducing bias into the gene pool and slashing crop genetic diversity. I came across a great example of this on a recent visit to NIAB in Cambridge. They have been investigating why wild flower seed mixes designed to produce flowers for pollinators all summer long, were only flowering for a brief period in the middle of summer. They soon discovered that the seed mix which had been chosen to contain early, mid and later flowering species was in fact being harvested for seed on a single occasion in mid-season. Thus in one single generation of domestication, the later flowering species had been selected to flower in mid-summer. Many of the late flowering genes had been lost by accident in a single season. This can hardly be described as domestication, but it beautifully illustrates how genetic change can be rapid and unintentional as species transition from wild to cultivated forms.