Why are there no perennial grain crops? That’s the provocative question posed by a recent paper in Evolutionary Applications written by three scientists working at The Land Institute.1 Whose institutional mission, of course, is to breed just this sort of crop, on the assumption that they “could reduce soil erosion while maintaining production of food staples.”
So what’s the answer, and what can be done about it? The authors start by pointing out that if you plot life form against net annual reproductive effort for angiosperms there’s a gap in the graph where herbaceous perennial crops with big, plentiful seeds and fruits should be. You have annual grain crops, of course, and fruit and berry cultivars, but nothing in between. Could it be that this particular “morphospace” is impossible on logical grounds? Or that there has not been enough time for the combination to develop?
After looking at a number of different possibilities, the authors come up with a very stark statement:
We suggest that the simplest explanation for the absence of perennial herbs with high reproductive effort is that, while biophysically possible, this lifeform could not have evolved by natural selection.
Let’s unpack that a bit. The authors point out that wild, out-crossing perennials are great at generating genetic diversity because
…somatic mutation generates heterozygosity in long-lived individuals … and allogamic recombination ‘destroys the associations built by, and favored by, selection’ … yet inefficiently purges deleterious recessive alleles.
This means that high genetic load results (the build-up of lethal recessive mutations), and therefore, the authors argue (with plenty of evidence to back them up), low seed set. Now, couple that with the fact that perennials tend to be ecologically dominant. What do you end up with?
You end up with a bunch of plants which are “poor candidates for rapid natural selection” in the new, open agricultural environments of the Neolithic: slow to colonize, still connected by geneflow to surrounding “wild” populations, and able to spread by vegetative propagation once established. This made “rapid domestication by sexual cycles unlikely.” Rapid domestication by vegetative propagation, yes, but that’s another story.
No wonder that
…domestication of annuals … under natural selection would have been faster than—and probably pre-empted—the domestication of perennials.
So, if you want perennial grain crops, you have to use artificial selection where natural selection has “failed.” The key is to minimize genetic load. You can do that by prioritizing for domestication perennials that are self-pollinating, which tends to be better at getting rid of deleterious mutations. Or by developing inbred lines by selfing and then re-combining the “purged” lines to restore heterozygosity, including in hybrid varieties. Or by selecting forcefully and strictly for high seed set (which has been working, say the authors). Crosses between annual grains and their perennial wild relatives can bring together “domestication traits and the perennial life history,” as has begun to be done for rice, wheat, rye, sorghum and sunflower.
Definitely worth a try. Because, as the authors conclude:
Adding herbaceous grain type crops to the inventory of existing, mostly tree-like, perennial food crops would give farmers additional options for balancing humanity’s demand for both nutritional and ecological services. We predict that artificial selection will open previously inaccessible regions of plant morphospace to agriculture and will reveal that some promising taxa were under-sampled. The grass family, for example, has given us our most valuable grain crops, but 70% of the 8000 grass species are rhizomatous perennials (Crepet and Niklas 2009) and were almost certainly overlooked in the early rounds of domestication which relied on natural selection.