The concept of the “genepool” crops up in popular culture mainly as part of the set-up for jokes, but it has a very serious pedigree. It can be traced to the Russian geneticist Aleksandr Sergeevich Serebrovskii, who came up with the term genofond (“gene fund” in English) in 1926 to refer to the complete set of different genes found within a group of organisms. The genofond, or “genepool,” as the term was soon translated into English, thus represents a reservoir of diversity that can be tapped into by organisms to adapt to a changing environment, and by scientists for plant breeding and crop improvement.
The wild relatives of a given crop are said to be in its genepool because, although they may be different species, they can nevertheless exchange genes with their cultivated relation – even if they aren’t sure which fork to use for the salad. Not all wild relatives are equally ready to do this, unfortunately. That’s why two pioneers of the crop diversity conservation movement, Jack Harlan and Jan de Wet, decided it would be useful to divide up the genepool into different parts.[i] In their system, CWR species are classified into groups based on how easy it is for them to exchange genes with the cultivated species to which they are related. In this system, wild relatives are said to be in the crop’s primary, secondary or tertiary genepools.
It’s like concentric circles around the crop (or a triangle with several levels – see the diagram below). The primary genepool (GP1), the first ring, includes species that can be directly mated with the crop to produce lots of strong, fertile progeny. For example, the primary genepool of the sunflower consists of both cultivated and wild varieties of Helianthus annuus, as well as Winter’s Sunflower (Helianthus winterii), a perennial species found in the southern Sierra Nevada foothills of California. It’s easy for H. winterii’s genes to be brought into the cultivated sunflower. You could even call them different sub-species of the same species.
The secondary genepool (GP2) is composed of crop wild relatives that are distinct from the cultivated species, but which are still so closely related that they can cross with the crop to at least some extent to produce some fertile offspring. It’s the next ring around the bullseye. It is more difficult to use species from the secondary genepool, because reproductive barriers of different kinds are present between it and the crop. For example, Aegilops tauschii and Aegilops speltoides, two wild relatives in the secondary genepool of bread wheat (Triticum aestivum), are diploid. That means they have paired chromosomes, whereas bread wheat is hexaploid (six copies). Such mismatches create difficulties for breeders. In addition, some hybrids resulting from crosses with secondary genepool species are partly sterile or just weaklings.
The tertiary genepool (GP3) is made up of even more distantly related crop wild relative species. Think of a misanthropic, reclusive uncle who lives alone in a cabin in the woods. In order to get this sort of wild relative to come participate in the plant breeding hootenanny, they must be coaxed with the use of specific breeding techniques, such as embryo rescue or “bridging crosses” with members of the secondary genepool, for example. Such difficulties led plant breeder Harry Harlan to remark that it would be “easier to cross a plant breeder with a monkey than use wild species in crop improvement.”[ii] Even when the cross succeeds, the resulting progeny are often sterile – just like mules. This is the outer ring. Anything farther away and you’ll need biotechnology to transfer genes.
If a breeder is going to use crop wild relatives at all, she will obviously prefer to use those in the primary genepool. However, valuable genetic traits can, inconveniently, also be found in species as far out as the tertiary genepool. Fortunately, the recent development of advanced breeding techniques has made the use of even distantly related CWR species more feasible. For example, wild rice species Oryza coarctata, which lives on the banks of river estuaries in India, has been empirically shown to have one of the highest tolerances for saline water within the wild rice genepool.[iii] It takes a lot of work to get those genes over, though, and years of effort.
Sometimes, we don’t know how easy it would be to cross a species with its related crop. The experiment just hasn’t been done yet. In such cases, researchers use a proxy. They figure that evolutionary closeness is correlated with ease of crossing. Developed by Maxted et al. (2006), the “taxon group” concept uses existing taxonomic classifications, based on evolutionary relatedness, to predict which species will be easiest to use.[iv]