Crops and cultivars as we know them today, did not exist in the wild many years ago. Different cultivars were developed through domestication and the selection of specific traits such as bigger kernels, shattering resistance and loss of seed dormancy.
Wheat domestication began as early as 7 500 BC in the Fertile Crescent, which is now known as the region in the Middle East spanning Iraq, Syria, Lebanon, Israel, Palestine, Jordan, Egypt, the south-eastern region of Turkey and the western borders of Iran. In Mexico, native people domesticated maize as early as 10 000 BC.
The domestication of crops limited their genetic diversity, which means they are susceptible to biotic and abiotic stresses, hence the necessity for cultivar development. The main crops cultivated in South Africa include maize, wheat, sugar cane and sunflower.
The cultivar development process
Cultivar development is a lengthy process that involves numerous steps. These steps include setting objectives, creating genetic variation, multiplying and distributing seed, as well as selecting, evaluating and releasing new varieties.
To develop a new cultivar, a breeder needs to improve or incorporate a specific trait that will enhance the crop for the producer and/or consumer. Any new trait must be identified in germplasm before it can be incorporated – germplasm is limited due to domestication, but alternative methods are available to expand genetic diversity.
Unconventional breeding enables breeders to create more genetic variability such as mutagenesis and the incorporation of genes from wild relatives or other organisms. These breeding techniques usually extend the time frame of a breeding programme compared to conventional breeding that only requires hybridisation.
Once genetic variation has been achieved, parental varieties need to be identified and selected. The next step is to produce offspring, which is then evaluated to identify the best candidates in relation to the specific objective. Offspring is assessed over a minimum of two growing seasons in different locations.
Passing the test
Before a breeder can release a new cultivar, it needs to pass the distinctness, uniformity and stability (DUS) test, which is performed by the South African National Seed Organisation (SANSOR). The DUS test ensures that the new candidate cultivar differs from any other registered cultivar, that its morphological variation is within limits as described by the breeder, and that the new cultivar is stable.
Once the cultivar passes this test, seed can be produced and distributed to producers.
Timeframe of cultivar development
Although it is possible to release new cultivars within five years of starting a breeding programme, it can take as long as 15 to 20 years if pre-breeding is required to develop new germplasm.
Incorporating new genes such as disease resistance into germplasm, has a significant fiscal impact on breeding programmes. Therefore, breeding companies must ensure that this new trait will have a large enough impact on the new cultivar. The incorporation of new genes obtained from mutations or other organisms can, however, also influence crops negatively as the gene segment also contains other genes.
One example is the stem rust resistance gene, Sr26, identified in wheatgrass. Although it has a positive rust-resistant trait, it negatively affects quality and yield in wheat. To avoid problems such as these, the influence of foreign DNA material must be evaluated to determine its overall effect. These evaluations are usually done in later generations when genetic variation has stabilised. The incorporation of new genetic material means that the crop loses some traits and the breeder has no control over the outcome.
Developing a base population
Once genetic variation has been achieved, plant breeders make selections and crosses to produce offspring that serve as the base population for the breeding programme. They use specific criteria to select all the offspring that meet the requirements. This offspring will be developed, evaluated and selected for the next generation until genetic variation has stabilised.
Genetic lines will only be evaluated for agronomic traits, quality and yield after genetically stable generations have been bred. Usually only one of these lines will be selected for the DUS test and by this stage of the process, breeders have obtained plant breeders’ rights for their product.
Although crop breeding is time-consuming, it is always a race against time. While newer technologies have been developed over time, they do not necessarily lessen the time spent on developing new cultivars, as it just broadens the genetic germplasm for breeders to make better selections to improve and develop new cultivars.
The ins and outs of hybrids
Hybrid seed is the first generation of a cross. This seed is developed to exploit hybrid vigour in order to get the best out of the two parents. Hybrid seed is genetically unstable and therefore cannot be used for the follow-up season’s plantings, as segregation and uniformity will be visible among the plants.
Different varieties, existing cultivars or lines previously developed by pre-breeding programmes, are selected. These plants are crossed with one another to create different families. These families can be evaluated to identify the best cross combination that will improve the specific characteristics of existing cultivars.
Hybrid evaluation is challenging because enough seed must be produced to perform all the analyses. These crosses are usually made at more than one location, which means the team and breeder must strictly control them. Hybrid trials are performed and evaluated over two or more years.
Main focus of cultivar development
The two primary traits breeding programmes focus on are quality and yield, and these traits need to meet specific standards. Nowadays many breeding programmes for various crops also focus on drought and salt tolerance as changes in climatic conditions affect production.
Famine and malnourishment are already affecting many countries. It is predicted that the world population, which currently stands at 7,8 billion people, will reach 9,7 billion in 2050. This trend predicts that yield also needs to increase. Alternative traits such as disease or drought tolerance need to be optimised to ensure production.
In light of this, various breeding programmes are focusing on higher nutrient levels. One example is ProVA biofortified maize, which increases provitamin A levels, as vitamin A deficiency leads to blindness in children. Nutritional studies indicate that ProVA biofortified maize is an effective source of vitamin A.
Another new trend in breeding is the production of durable disease resistance in crops to prevent or limit the use of chemical fungicides. This type of resistance focuses on combining several resistance genes in a cultivar to avoid boom and bust cycles.
Pathogens and insects are living organisms that adapt continuously and overcome resistance within crops. It is therefore essential to combine different resistance genes that can support one another and limit disease infections.
Maize breeding programmes focus on developing hybrids for the production of bioethanol, as fossil fuel energy is no longer sustainable. Studies are also underway to determine whether other parts of the plant are suitable for the production of bioethanol.
Challenging but rewarding
One challenge breeders experience is the constant inconsistency that characterises breeding programmes. As it takes several years to develop new cultivars, factors such as fluctuating climatic and environmental conditions, disease or pest outbreaks, and budget constraints can lead to unforeseen challenges.
Despite these challenges, plant breeding is rewarding as it allows plant breeders to contribute to society. Although plant breeding may be risky, the prospect of the results that await plant breeders in the next season, is exciting. Data analysis and evaluations are always concluded with much anticipation. – Dr Ansori Maré, Department of Plant Sciences, University of the Free State
For more information, send an email to Dr Ansori Maré at MareA@ufs.ac.za.
