Genetics Key to TiLV Resistance

A new study has found that resistance to Tilapia Lake Virus\r\n(TiLV) appears to be due to differences in genes between families of the same\r\nfish, reports Bonnie Waycott.

Hopes are high that these results will greatly improve the\r\nprotection of farmed tilapia stocks. Since its detection in Israel in 2014,\r\nTilapia Lake Virus (TiLV) has ravaged tilapia populations in 16 countries\r\nacross Asia, Africa and South America, and caused mortalities of up to 90%.
\r\nArguably the biggest disease threat to tilapia farming and one of the biggest\r\nfor global aquaculture, several stages of the tilapia life cycle from\r\nfingerlings to adults can be affected. Both wild and farmed fish are\r\nsusceptible, with clinical signs including behavioural changes, discolouration,\r\nloss of scales, skin haemorrhages, open wounds, eyeball protrusion and\r\nabdominal swelling. Fish can also be lethargic at the surface or be found\r\ngasping.

There is currently no cure for viral diseases in\r\naquaculture. Although vaccines and selective breeding have proved successful in\r\nreducing the severity of some, there are currently some knowledge gaps relating\r\nto TiLV with no effective or affordable vaccines available.

Now, a new study published in the journal Aquaculture has\r\nfound that there is substantial genetic variation in the resistance of tilapia\r\nto the virus, with some families showing no mortalities to TiLV and others\r\nshowing very high mortalities. The finding is offering hope that breeding\r\nprogrammes could be key to tackling one of the worst diseases to hit the global\r\nfish farming industry in recent years.

With funding from the UK government and through the\r\nframework of the CGIAR Research Program on Fish Agri-Food Systems, researchers\r\nfrom the University of Edinburgh's Roslin Institute and WorldFish analysed the\r\ngenes of Genetically Improved Farmed Tilapia (GIFT), in an attempt to estimate\r\nthe levels of genetic variation for resistance to TiLV.

"The Roslin Institute has been working in partnership\r\nwith WorldFish to improve selective breeding for Nile tilapia, " said\r\nProfessor Ross Houston, lead author of the study and personal chair of\r\naquaculture genetics at the Roslin Institute.

"This has included developing genomic tools and looking\r\nat new target traits for genetic improvement. One of these traits is disease\r\nresistance. In this case, we took advantage of an outbreak in a tilapia pond\r\nwhere many different families of tilapia were affected. Using the data from\r\nsurvivors and mortalities from this outbreak, and how this varies across\r\nfamilies, we were able to show that resistance to TiLV is highly\r\nheritable."

Using data from 1821 pedigreed fish from 124 different\r\nfamilies of tilapia collected during and after the pond outbreak, Houston and\r\nhis team defined resistance using binary survival and days-to-death traits.\r\nThey were able to record mortality levels because the fish contained electronic\r\ntags, which were used to assign individuals to specific families. Using the\r\npedigree for the fish affected by the outbreak, it was also possible to\r\nseparate how much of the variation in mortality was due to genetics, and how\r\nmuch was due to other reasons such as environmental factors.

TiLV resistance highly heritable

The results showed that resistance to TiLV was highly\r\nheritable, with about 50-60% of the variation in mortality due to genetics\r\n(this is particularly high for a disease resistance trait). Some families of\r\ntilapia had zero mortality while others had a 100% death rate, meaning that\r\nselective breeding for improved resistance is likely to be highly effective.\r\nThe team was also able to show that selecting for improved resistance to TiLV\r\nis unlikely to have any impact on the growth of the fish and may also benefit\r\nfarmers' yields. The study also stated that using data from a natural pond\r\noutbreak, as opposed to a controlled experiment, better reflected the method of\r\ninfection in terms of time of exposure and spread within the population.

The selective breeding of fish with genes that are resistant\r\nto TiLV is said to be one way of limiting the damage of this disease, but there\r\nare some technical and legislative barriers to overcome, according to Ross\r\nHouston.

"The technical barriers are related to the fact that a\r\nreliable and repeatable TiLV experimental challenge model would be required to\r\nmaximise the potential for breeding for TiLV resistance," he said.

"There is a lot of research on developing such\r\nchallenge models and with some promising results to date, so this is likely to\r\nbe overcome soon. This would avoid having to rely on pond outbreak data to use\r\nfor breeding, where it is difficult to predict where and when outbreaks will\r\noccur."

Genomic tools could also be extremely useful to identify\r\ngenetic markers that could be used to predict TiLV resistance breeding values,\r\neven in the absence of disease challenge data on close relatives, he added.

Key farmed species

Nile tilapia are among the key species farmed worldwide\r\ntoday. According to FAO data, the production of tilapia reached around 6.2\r\nmillion tonnes in 2016. The species is also one of the major sources of animal\r\nprotein, particularly in developing countries in Asia, South America and Africa\r\nand can be farmed under a variety of systems from extensive backyard ponds to\r\nlarge, commercial operations, with little environmental impact. It's also an\r\naffordable source of food for many people and a healthy source of nutrients and\r\nessential fatty acids.

Factors such as these make it all the more important for\r\ntilapia producers to consider a host of options to increase the chances of\r\navoiding TiLV. In addition to considering disease resistance and using specific\r\npathogen-free tilapia stocks for breeding programmes, investing in advanced\r\nmolecular diagnostics will enable farmers to determine whether individuals are\r\nvirus-free at different stages of the tilapia life cycle. Other key preventive\r\nsteps include screening live tilapia entering a country as seed or broodstock\r\nfor aquaculture purposes, contingency plans for possible outbreaks and regular\r\nbiosecurity audits and disease surveillance programmes.​​

"Improvements in biosecurity are always important and\r\nthis can be related to transparency and communication," said Ross Houston.

"There is other research that is targeting the\r\ndevelopment of vaccines, which would also be very useful, but in addition to\r\ncost issues they are very difficult to administer to young tilapia which can be\r\nbadly affected by TiLV. In this sense, genetics and breeding are definitely\r\nmajor opportunities here."

Together with WorldFish, the Roslin Institute is part of an\r\nambitious and active research programme where genomic tools are being developed\r\nto find genes and markers linked to disease resistance. Hopes are high that the\r\nstudy by both organisations could lead to new opportunities to improve breeding\r\nfor disease resistance. Further research will also be required to evaluate the\r\ngenetic architecture of host resistance to TiLV and evaluate the possibility of\r\nmarker-assisted or genomic selection to expedite the breeding of tilapia\r\nstrains with improved resistance to the virus, according to the study.

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"It would be very interesting to find out the reasons\r\nunderlying why some families are seemingly fully resistant, and others fully susceptible\r\nin our study," Ross Houston commented. "This is both scientifically\r\nimportant and also has potential applications to provide new solutions to\r\ntackle the disease."

Source: World Fishing