Cabbages are the dogs of the crop world, trotted out whenever a point about diversity needs to be made. ((Photo by Paul Williams, found here.)) Brussels sprouts, Siberian kale, kohl rabi and good old boring Savoys are all members of a single species, Brassica oleracea, just as chihuahuas and great Danes are all Canis familiaris. If anything, though, brassicas are more complicated, because different species also interbreed moderately freely, much more so than the occasional doggie-style hybrid. One of the products of that promiscuity is the species known as oilseed rape, B. napus. Cabbage and turnip (B. rapa) apparently did their thing spontaneously “some centuries ago”. But they did so only a few times, and as a result the genetic diversity of oilseed rape is rather limited, depending as it does on the diversity present in those few original crosses between cabbage and turnip.
An obvious solution (obvious to plant breeders at any rate) is to reproduce those original crosses and resynthesize oilseed rape by arranging crosses between a more diverse bunch of cabbages and turnips, and indeed that is now old hat. Various programmes since the 1990s have created scores of “new” varieties of oilseed rape. The problem is that many of these have all the drawbacks of old oilseed rape, before the seeds were cleaned up of their nasty biochemicals and sanitized as canola. Merely crossing resynthesized oilseed with existing varieties in the hope of getting some of the better qualities of the new varieties into the improved cultivars is thus fraught with the risk of introducing bad stuff as well. The answer is backcrossing, breeding the offspring lines with their parents while selecting for the good qualities and against the bad qualities. As the authors of a new paper ((Girke, A., Schierholt, A., & Becker, H. (2011). Extending the rapeseed genepool with resynthesized Brassica napus L. I: Genetic diversity Genetic Resources and Crop Evolution DOI: 10.1007/s10722-011-9772-8)) note, this is
“a labor intensive approach and a long-term perspective, and appropriate resynthesized lines would have to be selected carefully.”
If you’re going to get into that, you may as well choose the parents for your breeding programme to be as different as possible from what you already have, to maximise the chances of finding something good and useful. Which is what the paper is all about. Andreas Girke and his colleagues measured the genetic distances within three distinct groups of oilseed rape and a collection of resynthesized varieties. They found, more or less as expected, that the spring, winter and Asian oilseed rapes were more similar to one another while the synthetic rapes were more diverse. Furthermore, and reassuringly, while the three groups tended to cluster as groups in a more detailed analyses of their DNA diversity, the synthetic rapes were roughly equally spread among the three clusters. The detailed analyses will give brassica breeders some of the information they need to breed better oilseed varieties. But a crucial question remains unanswered. How do offspring of the synthetic rapes crossed with established varieties perform in the field? The researchers crossed 44 synthetic rapes with two cultivars and planted out the offspring to measure how well they do and whether there is any relationship between the genetic distance between the two parents and performance in the field. But don’t get too excited; “The result of these studies will be presented in a separate publication.”
P.S. Many of the crops on which humanity depends are genetically interesting inter-specific hybrids, and re-doing the original, accidental crosses not only throws light on their evolution but also is of practical value to breeders. The whole business of resynthesized lines, which as far as I know began with CIMMYT’s synthetic wheat lines, is little known outside the esoteric world of plant breeding. It would be great to have a readable survey of the field, but I have not been able to find one. Anyone know of anything? And has resynthesis even been attempted for the banana? Answers, please.
We had a fairly extensive programme of resynthesis of Brassica napus at Scottish Crops Research Insitute in the 1980s. It started, if I remember correctly in the 1970s, as an effort to broaden the base of forage rapes (the things you feed to cows in Scotland in the winter). Ian MacNaughton was the pioneer who led the team that enabled it to be done on sufficient scale to make it workable from a breeding point of view. A major objective was club root resistance (still a key breeding objective in oilseed rapes). We also transferred a variety of self incompatible genes as a prelude to making F1 hybrids using self incompatibility (but male sterility turned out to be commercially more reliable).
A general point is the importance of pre-breeding as part of the overall crop improvement process. It’s also worth commenting that the idea of resynthesis and hybrid transfers of useful genes in Brassicas was part of the breeding process at Scottish Plant Breeding Station, SCRI and in a number of European and American groups from the late 1960s on. There was quite a lot of interesting work on the relative merits of crossing at different ploidy levels, on the tissue culture processes to use and on the consequences of making the crosses in different directions and the results from subsequent backcrosses.
I hear that there is currently an interest in biofuel rapeseed. In this case the nasty biochemicals wouldn’t matter much while heterosis might be the real interest. Almost back to the start of the resurgence of rapeseed in the second world war in Canada – as a source of high quality (high erucic) oil for aeroplanes.