Selective breeding is the process of breeding plants and animals for particular traits. Typically, strains that are selectively bred are domesticated, and the breeding is sometimes done by a professional breeder. Bred animals are known as breeds, while bred plants are known as varieties, cultigens, or cultivars. The cross of animals results in what is called a crossbreed, and crossbred plants are called hybrids. The term selective breeding is synonymous with artificial selection.
In animal breeding techniques such as inbreeding, linebreeding, and outcrossing are utilized. In plant breeding, similar methods are used. Charles Darwin discussed how selective breeding had been successful in producing change over time in his book, Origin of Species. The first chapter of the book discusses selective breeding and domestication of such animals as pigeons, dogs and cattle. Selective breeding was used by Darwin as a springboard to introduce the theory of natural selection, and to support it.[1]
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Animals with homogeneous appearance, behavior, and other characteristics are known as particular breeds, and they are bred through culling particular traits and selecting for others. Purebred animals have a single, recognizable breed, and purebreds with recorded lineage are called pedigreed. Crossbreeds are a mix of two purebreds, whereas mixed breeds are a mix of several breeds, often unknown. Animal breeding begins with breeding stock, a group of animals used for the purpose of planned breeding. When individuals are looking to breed animals, they look for certain valuable traits in purebred stock for a certain purpose, or may intend to use some type of crossbreeding to produce a new type of stock with different, and, it is presumed, superior abilities in a given area of endeavor. For example, to breed chickens, a typical breeder intends to receive eggs, meat, and new, young birds for further reproduction. Thus, the breeder has to study different breeds and types of chickens and analyze what can be expected from a certain set of characteristics before he or she starts breeding them. Therefore, when purchasing initial breeding stock, the breeder seeks a group of birds that will most closely fit the purpose intended.
Purebred breeding aims to establish and maintain stable traits, that animals will pass to the next generation. By "breeding the best to the best," employing a certain degree of inbreeding, considerable culling, and selection for "superior" qualities, one could develop a bloodline superior in certain respects to the original base stock. Such animals can be recorded with a breed registry, the organization that maintains pedigrees and/or stud books. However, single-trait breeding, breeding for only one trait over all others, can be problematic.[2] In one case mentioned by animal behaviorist Temple Grandin, roosters bred for fast growth or heavy muscles did not know how to perform typical rooster courtship dances, which alienated the roosters from hens and led the roosters to kill the hens after reproducing with them.[2]
The observable phenomenon of hybrid vigor stands in contrast to the notion of breed purity. However, on the other hand, indiscriminate breeding of crossbred or hybrid animals may also result in degradation of quality.
Plant breeding has been used for thousands of years, and began with the domestication of wild plants into uniform and predictable agricultural cultigens. High-yielding varieties have been particularly important in agriculture.
Selective plant breeding is also used in research to produce transgenic animals that breed "true" (i.e., are homozygous) for artificially inserted or deleted genes.
Selective breeding in aquaculture holds high potential for the genetic improvement of fish and shellfish. Unlike terrestrial livestock, the potential benefits of selective breeding in aquaculture were not realized until recently. This is because high mortality led to the selection of only a few broodstock, causing inbreeding depression, which then forced the use of wild broodstock. This was evident in selective breeding programs for growth rate, which resulted in slow growth and high mortality.[3]
Control of the reproduction cycle was one of the main reasons as it is a requisite for selective breeding programmes. Artificial reproduction was not achieved because of the difficulties in hatching or feeding some farmed species such as eel and yellowtail farming.[4] A suspected reason associated with the late realisation of success in selective breeding programs in aquaculture was the education of the concerned people – researchers, advisory personnel and fish farmers. The education of fish biologists paid less attention to quantitative genetics and breeding plans.[5]
Another was the failure of documentation of the genetic gains in successive generations. This in turn led to failure in quantifying economic benefits that successful selective breeding programs produce. Documentation of the genetic changes was considered important as they help in fine tuning further selection schemes.[3]
Aquaculture species are reared for particular traits such as growth rate, survival rate, meat quality, resistance to diseases, age at sexual maturation, fecundity, shell traits like shell size, shell colour, etc.
Salmonids
Gjedrem (1979) showed that selection of Atlantic salmon (Salmo salar) led to an increase in body weight by 30% per generation. A comparative study on the performance of select Atlantic salmon with wild fish was conducted by AKVAFORSK Genetics Centre in Norway. The traits, for which the selection was done included growth rate, feed consumption, protein retention, energy retention, and feed conversion efficiency. Selected fish had a twice better growth rate, a 40% higher feed intake, and an increased protein and energy retention. This led to an overall 20% better Fed Conversion Efficiency as compared to the wild stock (Thodeson et al.1999). Atlantic salmon have also been selected for resistance to bacterial and viral diseases. Selection was done to check resistance to Infectious Pancreatic Necrosis Virus (IPNV). The results showed 66.6% mortality for low-resistant species whereas the high-resistant species showed 29.3% mortality compared to wild species (Storset et al. 2007).
Rainbow trout (S. gairdneri) was reported to show large improvements in growth rate after 7-10 generations of selection (Donaldson and Olson 1957). Kincaid et al. (1977) showed that growth gains by 30% could be achieved by selectively breeding rainbow trout for three generations. A 7% increase in growth was recorded per generation for rainbow trout by Kause et al. (2005). In Japan, high resistance to IPNV in rainbow trout has been achieved by selectively breeding the stock. Resistant strains were found to have an average mortality of 4.3% whereas 96.1% mortality was observed in a highly sensitive strain (Okamoto et al. 1993). Coho salmon (Oncorhynchus kisutch) increase in weight was found to be more than 60% after four generations of selective breeding (Hershberger et al. 1990). In Chile, Neira et al. (2006) conducted experiments on early spawning dates in coho salmon. After selectively breeding the fish for four generation, spawning dates were 13 – 15 days earlier.
Cyprinids
Selective breeding programs for the Common carp (Cyprinus carpio) include improvement in growth, shape and resistance to disease. Experiments carried out in the USSR used crossings of broodstocks to increase genetic diversity and then selected the species for traits like growth rate, exterior traits and viability, and/or adaptation to environmental conditions like variations in temperature. Kirpichnikov et al. (1974) and Babouchkine (1987) selected carp for fast growth and tolerance to cold, the Ropsha carp. The results showed a 30-40% to 77.4% improvement of cold tolerance but did not provide any data for growth rate. An increase in growth rate was observed in the second generation in Vietnam (Tran and Nguyen 1993). Moav and Wohlfarth (1976) showed positive results when selecting for slower growth for three generations compared to selecting for faster growth. Schaperclaus (1962) showed resistance to the dropsy disease wherein selected lines suffered low mortality (11.5%) compared to unselected (57%).
Channel Catfish
Growth was seen to increase by 12 – 20% in selectively bred Iictalurus punctatus (Bondari, 1983). More recently, the overall response of Channel Catfish response to selection for improved growth rate was found to be approximately 80%, i.e., an average of 13% per generation (Dunham 2006).
Oysters
Selection for live weight of Pacific oysters showed improvements ranging from 0.4% to 25.6% compared to the wild stock (Langdon et al. 2003). Sydney-rock oysters (Saccostrea commercialis) showed a 4% increase after one generation and a 15% increase after two generations (Nell et al. 1996, 1999). Chilean oysters (Ostrea chilensis), selected for improvement in live weight and shell length showed a 10-13% gain in one generation. Bonamia ostrea is a protistan parasite that causes catastrophic losses (nearly 98%) in European flat oyster Ostrea edulis L. This protistan parasite is endemic to three oyster-regions in Europe. Selective breeding programs show that O. edulis susceptibility to the infection differs across oyster strains in Europe. A study carried out by Culloty et al. (2001) showed that ‘Rossmore’ oysters in Cork harbour, Ireland had better resistance compared to other Irish strains. A selective breeding program at Cork harbour uses broodstock from 3– to 4-year-old survivors and is further controlled until a viable percentage reaches market size (Culloty et al. 2004). Over the years ‘Rossmore’ oysters have shown to develop lower prevalence to B. ostreae infection and percentage mortality. Ragone Calvo et al. (2003) selectively bred the eastern oyster, Crassostrea virginica, for resistance against co-occurring parasites Haplosporidium nelson (MSX) and Perkinsus marinus (Dermo). They achieved dual resistance to the disease in four generations of artificial selection. The oysters showed higher growth and survival rates and low susceptibility to the infections. At the end of the experiment, artificially selected C. virginica showed a 34-48% higher survival rate.
Penaeid shrimps
Selection for growth in Penaeid shrimps yielded successful results. A selective breeding program for Litopenaeus stylirostris saw an 18% increase in growth after the fourth generation and 21% growth after the fifth generation (Goyard et al. 1999). Marsupenaeus japonicas showed a 10.7% increase in growth after the first generation (Hetzel et al. 2000). Argue et al. (2002) conducted a selective breeding program on the Pacific White Shrimp, Litopenaeus vannamei at The Oceanic Institute, Waimanalo, USA from 1995 to 1998. They reported significant responses to selection compared to the unselected control shrimps. After one generation, a 21% increase was observed in growth and 18.4% increase in survival to TSV. The Taura Syndrome Virus (TSV) causes mortalities of 70% or more in shrimps. C.I. Oceanos S.A. in Colombia selected the survivors of the disease form infected ponds and used them as parents for the next generation. They achieved satisfying results in two or three generations wherein survival rates approached levels before the outbreak of the disease (Cock et al. 2009). The resulting heavy losses (up to 90%) caused by Infectious hypodermal and haematopoietic necrosis virus (IHHNV) caused a number of shrimp farming industries started to selectively breed shrimps resistant to this disease. Successful outcomes led to development of Super Shrimp, a selected line of L. stylirostris that is resistant to IHHNV infection (Tang et al. 2000). Tang et al. (2000) confirmed this by showing no mortalities in IHHNV- challenged Super Shrimp post larvae and juveniles.
Selective breeding programs for aquatic species provide better outcomes compared to terrestrial livestock. This higher response to selection of aquatic farmed species can be attributed to the following:
Selective breeding in aquaculture provide remarkable economic benefits to the industry, the primary one being that it reduces production costs due to faster turnover rates. This is because of faster growth rates, decreased maintenance rates, increased energy and protein retention, and better feed efficiency (Gjedrem and Baranski 2009). Applying such genetic improvement program to aquaculture species will increase productivity to meet the increasing demands of growing populations.
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Babouchkine, Y.P., 1987. La sélection d’une carpe résistant à l’hiver. In: Tiews, K. (Ed.), Proceedings ofWorld Symposium on Selection,Hybridization, and Genetic Engineering in Aquaculture, Bordeaux 27–30 May 1986, vol. 1. HeenemannVerlagsgesellschaft mbH, Berlin, pp. 447–454.
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