Introduction
In past decades, the global poultry industry has made enormous progress due to the rapid development of breeding technology. In layers, total egg numbers have exceeded 300 within the past century (Flock et al., Reference FLOCK, LIU, CAO and LU1992). In broilers, weights can reach up to 2.352 kilograms (Huo, Reference HUO2010), with many countries now producing marketable carcasses at 35 d of age. However, with the increase of productive performance, some physiological functions have been severely limited, such as adaptability and disease resistance. Van der Zijpp (Reference VAN DER ZIJPP1983) found that high production induced high susceptibility to MD (Marek's Disease) and Lamont (Reference LAMONT1998) found a negative correlation between laying performance and the resistance for anti-MD. On the whole, high-yielding poultry reared intensively are now more susceptible to diseases.
Although they have played a crucial role in preventing and treating diseases, traditional methods have become increasingly problematic and unacceptable in many markets. Antibiotics which were once available against the majority of bacterial infections, have been under more scrutiny because of potential fears from consumers and the overuse of antibiotics in general (Cho et al., Reference CHO, ABD EI ATY, GOUDAH, SUNG, YI, SEO, KIM, SHIM, JEONG, LEE and SHIN2008; Nikolaidou et al., Reference NIKOLAIDOU, SAMANIDOU and PAPADOYANNIS2008; Vass et al., Reference VASS, HRUSKA and FRANEK2008). Current regular vaccines have been suffering powerful challenges because of the variation of pathogens (Islam et al., Reference ISLAM, RASHID, HOQUE, JULI and KHATUN2008; Johansson and Eichelberger, Reference JOHANSSON and EICHELBERGER2010).
Disease resistance breeding is an important way to solve this problem. After being first reported by Roberts and Card (Reference ROBERTS and CARD1935), breeding for disease resistance has made great progress in recent decades. Recently, Lyall et al. (Reference LYALL, IRVINE, SHERMAN, MCKINLEY, NÚÑEZ, PURDIE, OUTTRIM, BROWN, ROLLESTON SMITH, SANG and TILEY2011) created a flock of chickens that suppressed the transmission of AI (Avian Influenza) by transgenic methods. Enserink (Reference ENSERINK2011) commented on this research and pointed out that it was promising to breed a line of chickens that were avian flu-free. ALV (Avian Leukosis Virus), a serious oncogenic disease with no vaccine until now, may be defeated though disease resistant breeding. Salter and Crittenden (Reference SALTER and CRITTENDEN1989) obtained a flock which were resistant to the ALV subgroup A by inserting a dominant gene for resistance to ALV into the germ line of the chickens. To elucidate the mechanism of anti-ALV, Yu et al. (Reference YU, ZHANG, TIAN, BACON, ZHANG, ZHANG and SONG2008) found that a hypermethylation pattern of ALVE (Avian Leucosis Virus subgroup E) was relevant for resistance against ALV-induced tumours in chickens. Resistant disease breeding has been widely used in many other chicken diseases.
Numerous studies have reviewed resistant breeding, but few have focused on the global introduction of the process in poultry. Snowder (Reference SNOWDER2006) reviewed progress in the genetic selection for disease resistance in domestic animals. Clark and Whitelaw (Reference CLARK and WHITELAW2003) pointed out that transgenic methods would be more effective in the future in his review on transgenic livestock, and there have been many other reviews about breeding for resistance (Shim and Galvani, Reference SHIM and GALVANI2009; Swaggerty et al., Reference SWAGGERTY, PEVZNER, HE, K.J., NISBET, KAISER and KOGUT2009; Calenge et al., Reference CALENGE, KAISER, VIGNAL and BEAUMONT2010). Therefore, it is necessary to focus on a global introduction of disease resistance in poultry including the genetic basis, research methods, challenges and opportunities.
The genetic basis
genetic disease resistance
Taking into account general disease resistance and special disease resistance, the term resistance refers to an anti-disease ability or reduced susceptibility. General resistance, under the control of multi-genes and influenced by the environment, displays defensive functions and immunity to most of the pathogens. While dominated by QTL, special disease resistance has high resistance to only some particular diseases and pathogens.
disease resistant genes
Disease resistant genes are those encoding antibodies, microRNA and other materials that help the host resist the damage caused by pathogens. Up to now, lots of anti-disease genes have been discovered with the development of various molecular technologies in poultry, e.g., MHC (major histocompatibility complex) genes, the Nramp1 (Natural resistance-associated macrophage protein 1) gene, IFN (Interferon) genes, Mx (Myxovirus-resistance) genes, anti-ALV (Avian leucosis virus) genes and the Zyxin gene.
The chicken MHC (B-complex) genetic cluster, located on chromosome 16, is related to many diseases in poultry, and is composed of three classifications, i.e., class I (B-F) gene, classⅡ (B-L) gene and class Ⅳ (B-G) gene, among which the class Ⅳ (B-G) gene is unique to poultry (Bloom and Bacon, Reference BLOOM and BACON1985; Guillemot et al., Reference GUILLEMOT, BILAULT, POURQUIÉ, BÉHAR, CHAUSSÉ, ZOOROB, KREIBICH and AUFFRAY1988). Previous researchers indicated that the B-complex had a close relationship with MD (Dalgaard et al., Reference DALGAARD, BOVING, HANDBERG, JENSEN, NORUP and JUUL MADSEN2009; Jin et al., Reference JIN, WEI, WEI, ZHAO and LI2010; Lian et al., Reference LIAN, QU, ZHENG, LIU, ZHANG, CHEN, XU and YANG2010). In addition, Kim et al. (Reference KIM, LILLEHOJ, HONG, PARK, LAMONT, HAN and LILLEHOJ2008) and (2009) indicated that the mechanism of MHC genes that guarded chickens from infection by coccidiosis was boosting the hosts defensive system. Some other diseases, such as Rous sarcomas, ALV, Salmonella pullorum and fowl cholera were affected by the B-complex as well (Lamont, et al., 1987; Yoo and Sheldon, Reference YOO and SHELDON1992; Pinard-Van Der Laan et al., Reference PINARD-VAN DER LAAN, MONVOISIN, PERY, HAMET and THOMAS1998; Taylor Jr, Reference TAYLOR2004; Ma et al., Reference MA, NING, LU, HAN, WANG, MU, LI, LIAN and LI2010).
The Nramp genetic family encodes the specific protein which restrains intracellular pathogens effectively. Some mutations of Nramp genes may influence disease resistance. Hu et al. (Reference HU, BUMSTEAD, BURKE, PONCE DE LEÓN, SKAMENE, GROS and MALO1995) discovered that the mutation of Nramp 1 gene led to more susceptibility to Salmonella pullorum. Fife et al. (Reference FIFE, SALMON, HOCKING and KAISER2009) mapped the chicken salmonellosis resistance locus of the Nramp 1 gene and Tohidi et al. (Reference TOHIDI, IDRIS, PANANDAM and HAIR BEJO2011) analysed the polymorphism and allele frequency of Nramp 1 gene in Malaysian native chickens. To take full advantage of this beneficial gene, further work on the anti-disease mechanism needs to be carried out.
IFN genes impact anti-disease ability through IFN, which is a soluble secretion involved with interfering and inhibiting the duplication of viruses (Isaacs and Lindenman, Reference ISAACS and LINDENMAN1957). IFN-α and IFN-β possess antiviral immunity, while another type of IFN is antimicrobial. Up to now, scientists were enthusiastic about transferring the chicken IFN genes into plants and utilising it in recombinant DNA to prevent and treat viral diseases. Song et al. (Reference SONG, ZHAO, , WU and LI2008) indicated that it was possible to generate interferon protein in forage plants. A similar result was obtained by Wei et al. (Reference WEI, LI, LIU and ZHANG2009) who found that chicken's IFN-γ gene could be expressed in the yeast Pichia pastoris. Meng et al. (Reference MENG, YANG, XU, QIN, XU, WANG, SUN and LIU2011) indicated that recombinant chicken INF-α could inhibit H9N2 Avian Influenza Virus replication. Wang et al. (Reference WANG, SUN, TIAN, SHI, TONG, LIU, ZHI, KONG and WANG2009) found a fowlpox virus that co-expressed the chicken IBV (Infectious Bronchitis Virus) S1 gene and that the chicken IFN-γ gene could protect chickens against bronchitis. In conclusion, transgenic and recombinant DNA methods are the most promising in use of IFN genes in the future.
Mx genes, as one of the most well-studied gene families in chickens, are associated with resistance to AI and IBDV (Infectious Bursal Disease Virus). It is widely accepted that chicken Mx protein with Asn/Ser at the 631th amino acid is resistant/susceptible to AI respectively (Yin et al., Reference YIN, ZHANG, ZHANG, QIN, WANG, DU and ZHAO2010; Ewald et al., Reference EWALD, KAPCZYNSKI, LIVANT, SUAREZ, RALPH, MCLEOD and MILLER2011). Nevertheless other theories exist on this matter. Benfield et al. (Reference BENFIELD, LYALL, KOCHS and TILEY2008) pointed out that Asn631 variants of chicken Mx protein did not inhibit AI replication. Therefore, further studies are needed to make clear which viewpoint is correct and elucidate the mechanism of anti-AI ability. In addition, a new property of Mx genes has been revealed, whereby O'Neill et al. (Reference O'NEILL, LICANT and EWALD2010) found that it could inhibit IBDV in chicken embryo fibroblasts. Except for the function of anti-AI and anti-IBDV, scientists have studied more about Mx genes (Benfield et al., Reference BENFIELD, LYALL, KOCHS and TILEY2008). Despite the studies being relatively in-depth, there is still a long way to go in order to apply Mx genes to poultry production.
Anti-ALV genes play a role in resisting ALV through breaking the pathway of TVA, TVB and TVC, which is the tumour virus receptor of the ALV subgroups A, B/D/E and C, respectively. To research the mechanism of ALV-resistance, a number of studies have been carried out. Yu et al. (Reference YU, ZHANG, TIAN, BACON, ZHANG, ZHANG and SONG2008) disclosed that the hypermethylation of ALVE in the resistant line immune to the ALV was significantly higher than the susceptible line. This might be a guide to research. Zekarias et al. (Reference ZEKARIAS, TER HUURNE, LANDMAN, REBEL, POL and GRUYS2002) reported that some glycoprotein on the membrane encoded by endogenous reverse transcription viral genes, such as gp85, were able to interdict the receptors of ALV. Besides, gp85 gene of subgroup J Avian Leukosis Virus could evolve under the pressure of antibodies (Wang and Cui, Reference WANG Z.F. and CUI2006), which indicated that it is too hard to prevent ALV just through the conventional vaccine. In a word, to control the transmission of ALV in the world, further works must be done.
Although seldom reported before, the zyxin gene may be a candidate gene potentially associated with avian coccidiosis (Hong et al., Reference HONG, KIM, LILLEHOJ, LILLEHOJ and SONG2009). Considering the more and more serious infection of coccidiosis, the zyxin gene is promising to play an important role in preventing coccidiosis.
In conclusion, several resistant genes have been revealed in poultry. However, the knowledge about the relationship between resistant genes and diseases is still superficial. In addition, the accurate mechanism of resistance is not completely clear and there may be many more genes associated with disease resistance to be revealed.
Research methods
direct selection
Direct selection, as the primary method of disease resistant breeding based on the phenotype, generally involves three different scenarios, i.e., pedigree observation in a controlled environment, uniformly challenging all breeding chickens then selecting the resistant one, and attacking clone or relatives respectively. The two latter methods are not completely accurate because the exposure to the pathogen differs. Cavero et al. (Reference CAVERO, SCHMUTZ, PHILIPP and PREISINGER2009) carried out E. coli. resistant breeding by uniform challenge. The result showed that it was possible to breed for colibacillosis resistance without penalizing the improvement in the key traits. Similar work was executed by Pavlidis et al. (Reference PAVLIDIS, BALOG, STAMPS, HUGHES, HUFF and ANTHONY2007) who selected the ascites incidence divergently in chickens directly.
indirect selection
The most crucial step in indirect selection is seeking out the most reliable resistant-related indicator, including genes, pathogen products and biological or immunological responses of the host. Dennis et al. (Reference DENNIS, ZHANG and CHENG2006) chose dopamine and immunological parameters as the indicator in selecting antiviral chickens. And Ma et al. (Reference MA, NING, LU, HAN, WANG, MU, LI, LIAN and LI2010) regarded monocytes-macrophages phagocytosis as a potential marker, while Al Murrani et al. (Reference AL MURRANI, AL RAWI and RAOF2002) selected the heterophil/lymphocyte ratio. As for gene indicator, Mx genes, Nramp 1 gene, MHC genes et al. have been widely used as resistant markers (Kim et al., Reference KIM, LILLEHOJ, HONG, PARK, LAMONT, HAN and LILLEHOJ2008; Ewald et al., Reference EWALD, KAPCZYNSKI, LIVANT, SUAREZ, RALPH, MCLEOD and MILLER2011; Tohidi et al., Reference TOHIDI, IDRIS, PANANDAM and HAIR BEJO2011). Indirect selection is the most widely used approach in resistant breeding.
In order to select the right candidates effectively, traits related to the indicators must be highly heritable and the measurement should be accurate. In addition, the interactions of genetics and environment must not be significant.
transgenic method
Transgenosis is the frontal and promising approach to breed a new anti-disease strain. Three basic avenues are accepted by researchers, i.e., DNA microinjection, viral vector, stem cell pathway. The most effective and extensive avenue is viral vector. In the late 1980s, oncoretrovirus was first used in chickens as a vector (Bosselman et al., Reference BOSSELMAN, HSU, BOGGS, HU, BRUSZEWSKI, OU, KOZAR, MARTIN, GREEN and JACOBSEN1989). In recent years, a more effective vector, lentivirus, has been extensively utilized in chicken transgenic research. Lyall et al. (Reference LYALL, IRVINE, SHERMAN, MCKINLEY, NÚÑEZ, PURDIE, OUTTRIM, BROWN, ROLLESTON SMITH, SANG and TILEY2011) bred a flock of chickens suppressing the transmission of AI via lentiviral vector. Lentiviral vectors were also used for transgenic chickens by many other researchers (Lois et al., Reference LOIS, HONG, PEASE, BROWN and BALTIMORE2002; McGrew et al., Reference MCGREW, SHERMAN, ELLARD, LILLICO, GILHOOLEY, KINGSMAN, MITROPHANOUS and SANG2004; Scott and Lois, Reference SCOTT and LOIS2005; Chapman et al., Reference CHAPMAN, LAWSON, MACARTHUR, WIESE, LOECHEL, BURGOS TRINIDAD, WAKEFIELD, RAMABHADRAN, MAUCH and SCHOENWOLF2005).
Although having a great advancement, transgenic breeding for disease resistance has not been applied to the poultry industry. And the security of transgenic animals is still doubtful. Therefore, to make full advantages of transgenosis, more effective vectors should be discovered and the technology should be applied in poultry production.
genomic selection
Nowadays, GS (Genomic selection) is becoming the hot spot in the study of animal breeding with the advantages of time-saving and low-costing compared with the traditional methods. Three basic phases are involved in genomic selection. Firstly, after setting up the reference group, SNPs involved in resistant trait of each individual will be sequenced. And every SNP is conferred a specific value. Secondly, resistance-related SNPs of candidates are scanned utilizing gene chips. And GEBV (Genomic Estimated Breeding Value) is calculated. The last step is to choose the candidates with high GEBV.
However, the exorbitant price of a gene chip still limits itself in the use of chicken breeding. Gene chips also have false positive problems (Yang et al., Reference YANG, CUI, CHAZARO, CUPPLES and DEMISSIE2005; Van de Wiel and Kim, Reference VAN DE WIEL and KIM2007). There are few articles about GS being used in chickens. Fife et al. (Reference FIFE, HOWELL, SALMON, HOCKING, VAN DIEMEN, JONES, STEVENS and KAISER2010) identified major QTL for Salmonella colonization in the chicken using genome-wide SNP analysis. And Preisinger (Reference PREISINGER2010) reviewed the potential application of GS on the Lohmann Information. With the decreasing cost of gene chips and the development of new sequencing technology, e.g., second generation DNA sequencing for genome and mRNA transcript genome sequencing technology, genomic selection is expected to be used widely in poultry production in the future.
Challenges
It is widely acknowledged that disease resistant breeding in poultry has made remarkable progress, but large numbers of obstacles still exist.
Firstly, the breeding for disease resistance is costly. Different from other kinds of breeding, challenging the chickens is carried out utilizing target pathogens in the process of resistant breeding. As a result, susceptible chickens will die of infection. In addition, the expense of measurement and some other potential loss is costly, including decreased production, diagnostics and therapy.
Secondly, the correlation between disease-resistant selection and economic traits needs further studies. Some retrospective studies suggest that there is a negative genetic relationship between disease resistance and production traits. Li et al. (Reference LI, NESTOR, SAIF, ANDERSON and PATTERSON2001) found that selection for increased body weight decreased the immunity to fowl cholera and Newcastle Disease. And a similar conclusion was implied by Bayyari et al. (Reference BAYYARI, HUFF, RATH, BALOG, NEWBERRY, VILLINES, SKEELES, ANTHONY and NESTOR1997) and Ka et al. (Reference KA, KERJE, BORNOLD, LILJEGREN, SIEGEL, ANDERSSON and HALLBÖÖK2009). However, opposite results were obtained in recent studies. Van der Most et al. (Reference VAN DER MOST, DE JONG, PARMENTIER and VERHULST2011) indicated that selection for growth did indeed compromise immune function, but selection for immune function did not consistently affect growth.
Thirdly, the interaction among pathogens rendered it difficult to breed a chicken line resisting multi-pathogens. Some pathogens are antagonistic, but some are co-operative. When resisting to some special diseases, chickens may be susceptible to the antagonistic pathogens. Schou et al. (Reference SCHOU, LABOURIAU, PERMIN, CHRISTENSEN, SØRENSEN, CU, NGUYEN and JUUL MADSEN2010) concluded that despite being more resistant to A. galli and S. Enteritidis than commercial Luong Phuong chickens, Vietnamese Ri chickens were susceptible to P. multocida. Similar results appeared in the research of Nouri et al. (Reference NOURI, RAHBARIZADEH, AHMADVAND, MOOSAKHANI, SADEQZADEH, LAVASANI and VISHTEH2010). Besides, more mortality induced by secondary infection and co-infection can confuse us. Qin et al. (Reference QIN, GAO, PAN, DENG, SUN, LI, QI, GAO, LIU and WANG2010) concluded that infection of ALV-J in layers was prevalent. In addition, the co-infectious situation of ALV-J with REV, MDV and CAV (Chicken Anemia Virus) was serious. Therefore, the relationship among pathogens in resistant breeding needs more exploration.
Finally, the difficulty in identifying genetic resistant candidates accurately via phenotypes provides a further obstacle to researchers. It is not absolutely correct to consider the sick flocks as genetically susceptible and the healthy ones as genetically resistant. Chickens without clinical symptoms may be in sub-clinical infection, while the ones with clinical expression may be infected by a similar disease or be co-infected, e.g., ALV usually infects chickens with REV and MDV (Qin et al., Reference QIN, GAO, PAN, DENG, SUN, LI, QI, GAO, LIU and WANG2010). In conclusion, it isn't accurate to choose the genetic resistant chickens only via phenotypes. To improve precision, MAS must be combined with phenotypes. It will take a long time for consumers to receive some methods, especially transgenosis. Although transgenosis is a promising and rapid method in resistant breeding, people still care for the potential harm in the transgenic animals. One reason is the worry of the transgenic animals escaping to the wild and diffusing the resistant genes into the environment (FDA, Reference FDA2009). For other people they are afraid that it may be harmful to our health to eat transgenic animals. Therefore, it will take a long time to accept the genetic chicken.
In a word, the obstacles in front of resistant breeding are troublesome. And there may be more challenges in the development of resistant breeding. Consequently further research should be carried out in this field to achieve a better future for resistant breeding.
Opportunities
Despite being faced with obstacles, resistant breeding in chickens is still a promising method to solve these troubles, e.g., drug tolerance and mutation of pathogens.
With the using of novel approaches in resistant breeding, the process will be promoted sharply. Wise et al. (Reference WISE, SCHAFER, LOWENTHAL and DORAN2008) indicated that RNAi and Transgenics technologies would pave the way for further research in resistant breeding.
And with other factors, including the cost decline of gene chips and sequencing, more candidate genes and QTL will be revealed, and more tremendous progress on the breeding for disease resistance could be obtained to introduce greater social profits.
