Resistance to diseases
New sources of resistances are often found in landraces from far-off places. Scientists maintain these in national and international collections, gradually discover their useful traits and breed new varieties from them.
For example, the USSR began breeding for durable late blight resistance in potatoes for their farmers around 1980 using South & Central American landraces and wild species held at its N.I. Vavilov All-Russian Institute of Plant Genetic Resources in St Petersburg. After the USSR collapsed, its Hungarian programme leader, Dr Istvan Sárvári, continued the work on his home farm, creating Sarpo Mira, one of the most blight resistant potato variety in the world. 
Sarpo Mira gives huge yields of potatoes on my allotment without any fungicides! Désirée, an older variety, is typically completely killed by late blight.
Old varieties of tomato that I and many others used to grow outdoors are now usually destroyed by late blight. Several modern varieties now incorporate the Ph-2 and Ph-3 resistance genes from the wild tomato, Solanum pimpinellifolium. Primabella (see above) is a modern, highly resistant, cherry tomato developed by a German PPB project.
Downy mildew-resistant onions and shallots are even more recent and based on one resistance gene derived from a wild species Allium roylei. The specimens used were from the Centre for Genetic Resources at Wageningen University in The Netherlands and were originally collected in the Himalayas. Fortunately, the needs of commercial growers and gardeners for keeping onions more-or-less coincide and I have grown the resistant onion variety, Santero F1, with superb results. I have also grown mildew-resistant Oregon sugar pod peas, a variety developed in the USA and released there in 1985; resistance was absolute.
We actually owe a lot to early plant hunters. I have been reading The Riddle of the Tsangpo Gorges by Frank Kingdon Ward (!885-1958), about his 1924-25 expedition to the bleak uplands of Tibet. Clearly, he enjoyed challenges but despite his understatements, with a little imagination, it is possible to appreciate how tough the conditions actually were and how much we should be grateful for those wonderful Rhododendrons, Meconopsis poppies etc he brought back.
Scientific selection
The ‘Father’ of plant heredity, Gregor Mendel, identified mathematical ratios for how major genes controlling contrasting traits like flower colour, dwarfness etc, are inherited; modern science has shown how this occurs through genes comprised of ‘strings’ of DNA (Deoxyribonucleic acid) molecules on paired chromosomes.
He ignored small differences, now recognise as being controlled by minor genes. Yet these are more numerous and very important. Modern statistical methods allow them to be distinguished from background genetic ‘noise’ and to ‘stack’ them, each contributing in a small but additive way. Minor genes are particularly important in determining yield. Wheat in the UK yielded about 2.5 tonnes/hectare in the 1940s but now yields 8 tonnes, a staggering achievement. A few major genes particularly those controlling plant height were important in this, improved crop management also, but the greater part came from successfully stacking innumerable beneficial minor genes.
To facilitate all this, modern varieties are now be bred in research station fields, glasshouses and laboratories, not on-farm and certainly not in gardens!
F1 hybrids
Most modern varieties are F1 hybrids and these are created in the following way. Parents are selected based on specific desired traits, for example, earliness, high yield and perhaps resistance. A series of breeding lines are created, each with one or more of the desired traits; each of these are inbred for several generations. Any abnormal plants created by inbreeding bringing together deleterious genes are rejected. Lines with the desired traits are then crossed to create an F1 (filial 1) generation. With luck, one or more of the hybrids have all the specific desired traits along with good vigour etc. The plant breeder then selects the best – a new F1 hybrid variety is born. Its parental lines are kept and multiplied in order to produce the seed of the new F1 hybrid.
Having highly inbred parents, F1 hybrids also receive a near-identical set of chromosomes from each. So, all the seed of this F1 hybrid generation have more-or-less identical sets of chromosomes (although each set is different), resulting in uniformity. Flower gardeners may value this uniformity for providing dramatic displays but commercial vegetable growers also value it hugely because the uniform growth facilitates cheap mechanical harvesting.
F1 hybrids are also generally vigorous and so high yielding, because abnormal plants caused by deleterious genes have been able to be rejected. Gardeners and growers alike value this. In the USA, maize F1 hybrids dominated, reaching almost 95% by the mid-1950s (Crow, 1998) with yields almost doubling to about 2.7t/ha. This increased yield was due largely to the extra vigour resulting from hybridity.
Yet maize yields have continued to increase since then, quadrupling despite almost all crops already being F1 hybrids. Instead, this is largely due to breeders identifying new and old beneficial minor genes using modern statistical methods and stacking them using conventional crossing followed by selection.
Admittedly, this partly still because they are F1 hybrids, because it has allowed the huge investments in plant breeding because their inbred parental lines remain unavailable to competitors. This has allowed seed companies effective monopolies. The resultant large investments have driven the increased yields.
Similar investments in breeding open-pollinated varieties might have achieved similar improvements. Yet such investments couldn’t be made, because of their lack of protection. Many, however, would argue that the loss of seed sovereignty has been too high a price to pay – see Disadvantages of modern varieties.
I have obtained increased vigour by crossing two modern maize varieties, Mezdi and Damaun, bred specifically for organic gardeners and growers. The seedlings of the F1 hybrid seem more vigorous and the yield better than either parent. But the opportunity to produce this hybrid is free to all gardeners and growers so there is no big financial benefit to companies and is unlikely to be marketed commercially.
‘My’ F1 hybrid sweetcorn
Genetically modified crops
GM crops currently are not allowed in the UK or in any countries in the European Union. Nonetheless, some mention needs to be made, not least because modern methods of engineering plant genomes seem likely to be allowed in the fairly near future in the UK.
The advantages of genetically manipulated crops for our health are often touted, for example, the opportunity to introduce or raise vitamin content, especially vitamin A. Such biofortification is a common aim of public institutions. For example, ‘golden rice’ has been genetically engineered by the International Rice Research Institute (IRRI). Yet such institutes are each often focused on just a few crops; if IRRI wasn’t focussed on rice, might it have otherwise tried persuading consumers simply to eat more fruit and vegetables?
But transgenic disease resistance may be more important because such a dramatically different source of resistance has sometimes proved vary durable: virus-resistant transgenic squash and papaya have been grown in the USA for over 20 years. Resistance can also be fashioned so as to also provide resistance against a spectrum of virus strains and even against several related virus species.
Patents are also commonly associated with GM genes and GM technologies. Like ownership of the parental lines of F1 hybrids, these have encouraged companies to invest heavily in such a protected breeding environment.
Ease of creation
One final but often-unremarked feature of modern varieties is that they are many. This is because it is particularly easy to create series of varieties based on one or other of the parental lines of F1 hybrids. Most may be unsuitable for gardeners (see next section and Home Page) but their sheer numbers mean some are. This can benefit gardeners if they are enabled to make informed decisions based on unbiased, easily understood and available information.
Conclusions
Most of the above benefits are very well illustrated by maize yields in the USA. They had been flat at about 1.5 tonnes/hectare until about 1940 but they almost doubled to 2.7t/ha by the 1950s, probably due largely to maize varieties changing from ‘old-fashioned’ open-pollinated varieties to F1 hybrids during that period. Doubling the yield is fantastic achievement.
But maize yields have continued to rise, now reaching 10.0t/ha. This further quadrupling of yield occurred with no increase in the proportion that were hybrids; they already dominated! These benefits were achieved largely by improved crop management, ensuring useful genes are retained and new useful genes discovered in plants held in national and international collections were added (Duvick, 1999 & 2005) and by modern breeding methods enabling these genes to be accumulated.
And the bulk of the genetic improvements were achieved by the slow process of iteratively cross-pollinating superior parents, selecting the best progeny, cross-pollinating these etc etc, not by F1 hybrid creation.
The GM traits introduced (herbicide and pest resistance) mostly have not affected yield – but they have made the crop easier to grow.
Similar increases in yield have been achieved by modern varieties of all our staple crops, though not always so spectacularly. And this includes self pollinating crops such as wheat for which the production of F1 hybrids is effectively impossible. So, science-led, iterative conventional crossing of selected parents by pollination followed by selection of the progeny is still the main driver of crop improvement!
But it shouldn’t be forgotten that all this was largely enabled by protected ownership of genetic material ensuring an avalanche of investment: without this, would these benefits have been achieved? Are the resulting monopolies, high seed prices and loss of seed sovereignty are too high a price to pay.
But, now you’ve read about the advantages, click here to read about the disadvantages and decide which benefits are worth the price we might be paying. Then click to discover how participatory plant breeding and varietal selection promote the inclusion of traits beneficial to gardeners in modern varieties.