Composite cross population

A composite cross population (CCP) is created by crossing a number of plants from different lines, and subsequently bulking seeds from the resulting offspring. This makes a CCP a population of plants with a lot of inherent genetic diversity, in contrast to monocultures where all plants are clones and homozygous at all loci (fully inbred). In recent years CCPs have been proposed as a way to create modern landraces of wheat, barley and oats. Research is done to explore whether they are better suited for organic farming than the modern cultivars. They are suited for participatory breeding of crops, which is in contrast to cultivars owned by big breeding companies.[1]

The idea of using CCPs in plant breeding was initially published in 1956.[2] Yield data for 4 different populations for 8–28 years were presented in the article and after 8–15 years of repeated breeding under natural selection, the populations out-yielded the reference cultivar.

Stages

Creating a CCP involves 3 steps: initiation, multiplication and mixture. The population then goes into the maintenance phase.[1]

Initiation

A number of lines, generally 7-30, with interesting properties, such as yield or baking quality, are selected and all possible crosses of them are done. If many lines of different genetic background are used, a huge amount of genetic diversity will be present.

Multiplication

Seeds from crosses are sown out and harvested separately for a growing season or two until enough seeds are available.

Mixture

All seeds are mixed in equal portions to produce the first CCP generation.

Maintenance

The population is grown repeatedly and possibly changes due to natural selection. Each year seeds are saved after harvest, and used as seed for the next growing season. Plants that are successful under the prevailing growing conditions will give more seeds and contribute more to the next generation, compared to less successful plants. Disease will cull susceptible plants and the population will over time become resistant to the common diseases, but only if the initial population has resistance genes present.

Pros

When genotypes grown together in a diverse population have different profiles of resource use they complement each other in the exploitation of the limiting resource and therefore are subject to smaller between-plant competition.

In case of disease or environmental change some plants will take over when others fail. Yield stability over years and environments can be better than pure lines due to compensation.[1]

Participatory plant breeding (PPB) methods represent alternatives aimed to improve local adaptation breeding, to promote genetic diversity, to empower farmers and rural communities.[3] In PPB farmers are actively participating in developing new cultivars or populations, e.g. by performing selection.

Cons

Different genotypes may compete with each other in a way that is not beneficial for the population.[1] A pure line optimized for a certain environment or usage outperforms the CCP under these specific conditions.

There is no reason to believe that the Darwinian selection will work in the desired direction for desired traits such as baking quality.

Common Bunt is a seed borne disease in wheat. In conventional farming it is controlled by fungicide treatment of seeds. In organic farming seeds can be cleaned by brushing before sowing, but it is also desirable that plants have genetic resistance. A CCP, including crosses of resistant cultivars, was grown with heavy common bunt infection for 5 years and it appeared to get more resistant, but the common bunt's virulence appeared to change at least as fast. The overall result was that infection levels went up.[4]

Genetic resource

CCPs can be a valuable resource of genetic material. They can be used to preserve valuable genetic diversity for future use, or as base for new breading programs. Promising plants can be selected and multiplied, generating new varieties.[5]

EU Seed Law

It is legal under EU law to sell and buy seeds from a composite cross population as an experiment in the period 1 March 2014 to 31 December 2018.[6]

The ORC Wakelyn winter wheat CCP .[7] has been approved for sale

References

  1. 1 2 3 4 Döring, Thomas F.; Knapp, Samuel; Kovacs, Geza; Murphy, Kevin; Wolfe, Martin S. (17 October 2011). "Evolutionary Plant Breeding in Cereals—Into a New Era". Sustainability. 2011 (3). doi:10.3390/su3101944. Retrieved 20 January 2018.
  2. ↑ Suneson, Coit A. "An Evolutionary Plant Breeding Method" (PDF). Retrieved 14 December 2017.
  3. ↑ Desclaux, Dominique. "Participatory Plant Breeding Methods for Organic Cereals" (PDF).
  4. ↑ Borgen, Anders. "Improving quality and health in diverse populations of wheat" (PDF). Retrieved 18 December 2017.
  5. ↑ "Morphological diversity in a barley composite cross-derived population evolved under low-input conditions and its relationship with molecular diversity: indications for breeding". 21 August 2016. Retrieved 20 January 2018.
  6. ↑ "COMMISSION IMPLEMENTING DECISION of 18 March 2014 on the organisation of a temporary experiment providing for certain derogations for the marketing of populations of the plant species wheat, barley, oats and maize pursuant to Council Directive 66/402/EEC". EUR-lex. 20 March 2014. Retrieved 9 December 2017.
  7. ↑ "Launch of ORC Wakelyns Population". Retrieved 18 September 2018.
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