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On the trail of heterosis genes – how hybrid breeding can profit from genomic research

The high-yield cultivation of maize has for a long time since been unthinkable without hybrid plant breeding. The phenomenon used by plant breeders to achieve high crop yields nevertheless harbours many secrets. Science is using the methods of modern genome research to solve the mystery of heterosis in order to speed up hybrid plant breeding and establish heterosis in many other agriculturally important crop species.

Prof. Albrecht Melchinger speaks of a “fascinating phenomenon” as he refers to a model on the table in front of him: four maize cobs attached vertically one beside the other on a wooden base. The two cobs in the middle clearly stand out from the others in size and diameter; they are the progeny of the two cobs to their left and right and are referred to as hybrids. The “fascinating phenomenon” to which they owe their appearance is referred to as heterosis. And the specific use of heterosis in agriculture is referred to as hybrid plant breeding. In scientific terms, heterosis is also known as hybrid vigour or outbreeding enhancement. “It describes the possibility of obtaining a genetically superior individual by combining its parents’ virtues,” explains Melchinger, Professor of Applied Genetics and Plant Breeding at the University of Hohenheim. And this superiority can be enormous. As far as maize is concerned, the hybrids are superior to their parents in that they produce yields that are 100 times higher than those produced by the parent plants.

New opportunities in heterosis research

Prof. Dr. Albrecht Melchinger © University of Hohenheim

The American botanist and plant geneticist George Harrison Shull was the first scientist to describe heterosis in detail in 1908. Hybrid breeding rapidly became established in the USA as a result of droughts in the 1930s. The scientific background of heterosis was quite unknown at the time, although it was of "great national interest," recalls Melchinger. The first heterosis conference in 1950 went on for as long as four weeks. Nowadays, almost 60 years later, heterosis is still not fully understood, but there are completely "new ways of investigating the phenomenon of heterosis".

Efficiency through modern marker technology

The new ways of investigation Melchinger is referring to are genomic research methods. “This does not mean genetic engineering,” said Melchinger highlighting that genomic research focuses on “elucidating the genome”. Modern DNA marker technology has also become affordable for the plant sciences thanks to the intensively promoted and funded field of human genetics. “180 dollars is enough to pay for the analysis of 60,000 data points; this is a high-density coverage of the genome,” said Melchinger. What does science expect to gain from the enhanced application of genomic analyses? Maize hybrid breeding battles with a quantity problem. Every year, approximately 10,000 maternal and paternal inbred lines are created for the purpose of producing hybrids. The entire area of Baden-Württemberg would not be sufficient to cultivate the crosses of these inbred lines’ 100 million progeny for testing purposes,” explained the plant breeder. “Using DNA chip technology, the scientists are already able to assess in the laboratory whether the seedlings carry the sought-after genes or not. This is a huge gain in efficiency.”

Methods of hybrid breeding

In scientific terms, heterosis describes the possibility of obtaining genetically superior individuals (the two cobs in the middle) by combining their parents’ virtues (left and right maize cobs) © Devezi

Hybrid plant breeding is already firmly established for maize. Farmers in developed nations exclusively use high-yield maize hybrids. One reason for this is that nearly all maize grown in industrial nations exhibits heterosis and the other reason is that maize hybrids can be generated very easily. Maize has female and male flowers (silk) on one and the same plant. In order to produce hybrids, the maternal line is castrated so that its stigma has to be pollinated with the pollen of the paternal line. The hybrid seeds can then be harvested from the maternal line.

It is far more difficult to produce wheat, barley, rye or rice hybrids. "This is because pollination happens inside the blossoms in these plants," explains Melchinger. The maize flowers can be castrated using a pair of tweezers; however the effort needed to do this by no means justifies the benefit. That is why plant breeders like to use the genetic mechanism of cytoplasmic male sterility (CMS), i.e. the total or partial male sterility.

As a result of infertility, the plants in the maternal line need to be pollinated by the pollen of the paternal line. "But no pollen no grains," said Melchinger referring to another important pre-requisite for the creation of hybrids. "It is therefore necessary to restore the plants' fertility." This is done using so-called restorer genes that are crossed into the sterile maternal line with the pollen of the paternal line, a process that also involves state-of-the-art marker technology.

Hybrid breeding in rice corresponds to 20 years of breeding progress

Despite the relatively complicated breeding method, rice hybrids are on the advance. After all, heterosis in rice leads to a 30 per cent increase in yield. In view of rising population numbers and the increasing scarcity of agricultural land, heterosis seems to be a convincing argument for increasing crop yields. Conventional breeding procedures just lead to an annual increase in yield of 1.5 per cent, “which means that successful hybrid plant breeding corresponds to 20 years of breeding success,” said Melchinger. And all this without using genetic engineering. “The balance is shifting towards genomic research,” said Melchinger highlighting his personal view with regard to financial funding.

Yield has once again become a breeding objective

Melchinger also highlighted that a few years ago people were very reluctant to give harvest yield as the objective of breeding. However, in times when agricultural overproduction has ceased to exist, plants are exposed to the effects of climate change and agricultural products can also be used for the production of energy, the vacuum in food and feed needs to be filled by a greater yield per unit of area. Melchinger is convinced that this can be achieved more quickly with hybrid breeding than with conventional methods. In the last six years, the German Research Foundation (DFG) has declared “Heterosis in plants” one of its major priority programmes. The results have recently been presented at a three-day conference held at the University of Hohenheim. It is known that the heterosis phenomenon involves a large number of genes, but “how the orchestra actually plays together is something that still needs to be discovered,” said Melchinger. If everything goes as planned, then hybrid plant breeding will become even more efficient and become established in other important culture crops such as wheat, barley, millet (sorghum) and pearl millet.

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