Selection For Disease Resistance Research Articles

Amphibians with infectious disease increase their reproductive effort: evidence for the terminal investment hypothesis.

Mounting an immune response to fight disease is costly for an organism and can reduce investment in another life-history trait, such as reproduction. The terminal investment hypothesis predicts that an organism will increase reproductive effort when threatened by disease. The reproductive fitness of amphibians infected with the deadly fungal pathogen Batrachochytrium dendrobatidis (Bd) is largely unknown. In this study, we explored gametogenesis in two endangered and susceptible frog species, Pseudophryne corroboree and Litoria verreauxii alpina. Gametogenesis, both oogenesis and spermatogenesis, increased when animals were experimentally infected with Bd. In P. corroboree, infected males have thicker germinal epithelium, and a larger proportion of spermatocytes. In L. v. alpina, infected males had more spermatic cell bundles in total, and a larger proportion of spermatozoa bundles. In female L. v. alpina, ovaries and oviducts were larger in infected animals, and there were more cells present within the ovaries. Terminal investment has consequences for the evolution of disease resistance in declining species. If infected animals are increasing reproductive efforts and producing more offspring before succumbing to disease, it is possible that population-level selection for disease resistance will be minimized.

Characterization of two novel gadd45a genes in hybrid tilapia and their responses to the infection of Streptococcus agalactiae.

Diseases are one of the major challenges in tilapia aquaculture. Identification of DNA markers associated with disease resistance may facilitate the acceleration of the selection for disease resistance. Gadd45a (growth arrest and DNA damage 45 A), a stress-inducible gene in humans and mice, has not been studied in fish. We characterized the two prologues of Gadd45a genes in hybrid tilapia. Gadd45a1 and Gadd45a2 shared an identical gene structure and showed an amino acid sequence identity of 73.8%. Their expressions were detected in all 10 tissues examined, with the kidney and gill having high transcriptional expressions. The expression levels of Gadd45a1 were significantly lower than those of Gadd45a2 in all examined tissues. After a challenge with a bacterial pathogen Streptococcus agalactiae, the expressions of the two genes were up-regulated significantly in the spleen, kidney, liver and intestine. These findings suggest that the two Gadd45a genes play an important role in the resistance to S.agalactiae in tilapia. We identified 10 SNPs in the two genes. The SNP markers in the two Gadd45a genes could be used to examine whether they are associated with resistance to S.agalactiae.

Use of Near Infrared Reflectance Spectroscopy (NIRS) for the rapid compositional analysis of di-, tri-, and tetraploid eastern oysters (Crassostrea virginica)

The oyster aquaculture industry in the USA, especially the Chesapeake Bay, has shown rapid growth over the last decade. Breeding has played a large role in this growth in the Chesapeake Bay, with emphasis on the selection for disease resistance, growth and the development of triploid Crassostrea virginica. With major gains realized in both of these traits, we are attempting to add more, such as, compositional traits. A specific interest for the Aquaculture Genetics and Breeding Technology Center is to derive compositional data for use with our selective breeding program. Near Infrared Reflectance Spectroscopy (NIRS) estimates of moisture and glycogen may lead to the definition of these new traits for selection to accompany those traditional measures of survival, meat yield, and growth rate. This study describes the development of NIRS models to estimate moisture and glycogen in the eastern oyster C. virginica.NIR spectra of 138 homogenized oyster meats were modeled against physicochemically measured moisture and glycogen data. The robustness of resulting models was assessed by an independent validation that showed a high coefficient of determination for validation (R2val=0.95) and high residual predictive deviation (RPD≥4.1) for both parameters, making them suitable for quantification. In a second step, these models have also been validated to obtain compositional data on different levels of ploidy in C. virginica (diploid, triploid, and tetraploid). These parameters will be applied to our family based selection program where heritabilities will be calculated. Statement of relevanceFor the first time we have developed NIRS models to determine compositional analysis in C. virginica. NIRS estimates of moisture and glycogen may lead to the definition of new traits for selection to accompany other traditional measures (survival, meat yield, and growth rate). NIRS measurements will be applied to our family based selection program where heritabilities will be calculated.

A Rapid and Efficient Method for In Vitro Screening of Taro for Leaf Blight Disease Caused by Phytophthora colocasiae

AbstractTaro leaf blight caused by Phytophthora colocasiae presents the single biggest constraint for taro cultivation globally. To accelerate breeding and selection for disease resistance to leaf blight, it is important to develop bioassays which could differentiate resistant and susceptible cultivars efficiently. In this study, thirty taro accessions and four released cultivars were evaluated for resistance to leaf blight using a modified floating leaf disc assay. A novel method for mass production of P. colocasiae zoospores was developed and used as inoculum for the assay. There were significant (P < 0.05) differences among accessions in their response to P. colocasiae infection in the detached leaf assay. The accessions could be efficiently classified into various resistance groups based on a 0–4 score. Also, the assay results were consistent with the field evaluation scores of taro accessions. Thus, this study reports the development of a rapid, simple and repeatable assay that can be used to screen large numbers of taro cultivars for resistance to P. colocasiae.

Mutation breeding as an effective tool for papaya improvement in South Africa

Previous plant improvement efforts at the Institute for Tropical and Sub Tropical crops of the Agricultural Research Council (ARC-ITSC) concentrated on conventional breeding of papaya which subsequently led to the release of 'Bh65' and 'Selection 42' to the local industry. Gamma irradiation has proved to be an effective method to improve cultivars in several crops and in recent years, the seed of promising papaya selections were irradiated in order to develop dwarfed and/or disease-resistant selections. Seeds were irradiated at different dosages ranging from 0 to 120 Gy. Results showed that gamma irradiation has an effect on seed germination. The higher dosages of 100 and 120 Gy were lethal and almost no seeds germinated. Furthermore the different irradiation dosages showed a correlation between higher irradiation dosage and lower plant height and stem circumference on cultivars 'Bh65', 'Maradol' and 'Sunrise Solo' whereas the cultivars 'OShew Mee', 'Tainung' and 'V3' were less affected by radiation. The higher dosage of 80 Gy had the most profound effects, reduced plant height and stem circumference. Radiation did not affect yield negatively except for 'Shew Mee' and 'V3' where some differences were observed. The higher dosages especially 80 Gy resulted in smaller fruit. No effect on disease resistance for black spot (Asperisporium spp.) was observed in the irradiated treatments as the leaves on all the plants were infected.

Resistance to infectious diseases is a heritable trait in rabbits.

Selection for disease resistance is a powerful way to improve the health status of herds and to reduce the use of antibiotics. The objectives of this study were to estimate 1) the genetic parameters for simple visually assessed disease syndromes and for a composite trait of resistance to infectious disease including all syndromes and 2) their genetic correlations with production traits in a rabbit population. Disease symptoms were recorded in the selection herds of 2 commercial paternal rabbit lines during weighing at the end of the test (63 and 70 d of age, respectively). Causes of mortality occurring before these dates were also recorded. Seven disease traits were analyzed: 3 elementary traits visually assessed by technicians on farm (diarrhea, various digestive syndromes, and respiratory syndromes), 2 composite traits (all digestive syndromes and all infectious syndromes), and 2 mortality traits (digestive mortality and infectious mortality). Each animal was assigned only 1 disease trait, corresponding to the main syndrome ( = 153,400). Four production traits were also recorded: live weight the day before the end of test on most animals ( = 137,860) and cold carcass weight, carcass yield, and perirenal fat percentage of the carcass on a subset of slaughtered animals ( = 13,765). Records on both lines were analyzed simultaneously using bivariate linear animal models after validation of consistency with threshold models applied to logit-transformed traits. The heritabilities were low for disease traits, from 0.01 ± 0.002 for various digestive syndromes to 0.04 ± 0.004 for infectious mortality, and moderate to high for production traits. The genetic correlations between digestive syndromes were high and positive, whereas digestive and respiratory syndromes were slightly negatively correlated. The genetic correlations between the composite infectious disease trait and digestive or respiratory syndromes were moderate. Genetic correlations between disease and production traits were favorable. Our results indicate that it is possible to select rabbits using visually assessed disease syndromes without the need for a trade-off between health and production traits. Using a composite criterion that includes all infectious syndromes is easy to implement and heritable and is, therefore, a promising way to improve the general disease resistance in livestock species.

Novel Pathways Revealed in Bursa of Fabricius Transcriptome in Response to Extraintestinal Pathogenic Escherichia coli (ExPEC) Infection.

Extraintestinal pathogenic Escherichia coli (ExPEC) has major negative impacts on human and animal health. Recent research suggests food-borne links between human and animal ExPEC diseases with particular concern for poultry contaminated with avian pathogenic E. coli (APEC), the avian ExPEC. APEC is also a very important animal pathogen, causing colibacillosis, one of the world’s most widespread bacterial diseases of poultry. Previous studies showed marked atrophy and lymphocytes depletion in the bursa during APEC infection. Thus, a more comprehensive understanding of the avian bursa response to APEC infection will facilitate genetic selection for disease resistance. Four-week-old commercial male broiler chickens were infected with APEC O1 or given saline as a control. Bursas were collected at 1 and 5 days post-infection (dpi). Based on lesion scores of liver, pericardium and air sacs, infected birds were classified as having mild or severe pathology, representing resistant and susceptible phenotypes, respectively. Twenty-two individual bursa RNA libraries were sequenced, each yielding an average of 27 million single-end, 100-bp reads. There were 2469 novel genes in the total of 16,603 detected. Large numbers of significantly differentially expressed (DE) genes were detected when comparing susceptible and resistant birds at 5 dpi, susceptible and non-infected birds at 5 dpi, and susceptible birds at 5 dpi and 1 dpi. The DE genes were associated with signal transduction, the immune response, cell growth and cell death pathways. These data provide considerable insight into potential mechanisms of resistance to ExPEC infection, thus paving the way to develop strategies for ExPEC prevention and treatment, as well as enhancing innate resistance by genetic selection in animals.

Effects of pathogen exposure on life-history variation in the gypsymoth (Lymantria dispar).

Investment in host defences against pathogens may lead to trade-offs with host fecundity. When such trade-offs arise from genetic correlations, rates of phenotypic change by natural selection may be affected. However, genetic correlations between host survival and fecundity are rarely quantified. To understand trade-offs between immune responses to baculovirus exposure and fecundity in the gypsy moth (Lymantria dispar), we estimated genetic correlations between survival probability and traits related to fecundity, such as pupal weight. In addition, we tested whether different virus isolates have different effects on male and female pupal weight. To estimate genetic correlations, we exposed individuals of known relatedness to a single baculovirus isolate. To then evaluate the effect of virus isolate on pupal weight, we exposed a single gypsy moth strain to 16 baculovirus isolates. We found a negative genetic correlation between survival and pupal weight. In addition, virus exposure caused late-pupating females to be identical in weight to males, whereas unexposed females were 2-3 times as large as unexposed males. Finally, we found that female pupal weight is a quadratic function of host mortality across virus isolates, which is likely due to trade-offs and compensatory growth processes acting at high and low mortality levels, respectively. Overall, our results suggest that fecundity costs may strongly affect the response to selection for disease resistance. In nature, baculoviruses contribute to the regulation of gypsy moth outbreaks, as pathogens often do in forest-defoliating insects. We therefore argue that trade-offs between host life-history traits may help explain outbreak dynamics.

Association of Mycoflora with Coffee (Coffea arabica L.) Beans at Limmu Coffee Plantation, Southwestern Ethiopia

A B S T R A C T Coffee (Coffea arabica L.) bean quality can be affected by a number of factors of which storage fungi are one of the major ones. In Ethiopia coffee is a number one export commodity supporting the national economy but there was little information about the association of mycoflora with coffee beans. Therefore, the objective of this study was to assess the associations of mycoflora with coffee seeds and their effect on coffee infection at Limmu Coffee Plantation. The comparison was done using two coffee berry disease resistant selections (74112 and 74110) with and without parchment and with and without surface treatment of coffee beans with 5% sodium hypochlorite and storing under two storage conditions (local cold house and corrugated iron warehouse). Thus, the experiment was laid down as 2×2×2×2 factorial experiment with four replications. The study showed the association of four fungal species (Fusarium spp., Aspergillus spp., Penicillium spp. and Mucor spp.) and some unidentified species in both blotting and agar plating techniques. In general, significantly higher infection percentage was found in coffee seeds without parchment and surface disinfection irrespective of the storage type and coffee selection. Thus, keeping coffee beans with parchment and disinfecting by disinfectants may reduce the association and prevalence of mycoflora on coffee and minimize postharvest problems.

Characterization of the duodenase-1 gene and its associations with resistance to Streptococuus agalactiae in hybrid tilapia (Oreochromis spp.).

Tilapia is a group of cultured teleost fishes whose production is threatened by some diseases. Identification of DNA markers associated with disease resistance in candidate genes may facilitate to accelerate the selection of disease resistance. The gene encoding a duodenase, which can trigger immune response, has not been studied in fish. We characterized the cDNA of duodenase-1 gene of hybrid tilapia. Its ORF is 759 bp, encoding a serine protease of 252 amino acids. This gene consisted of five exons and four introns. Its expression was detected in all 10 tissues examined, and it was highly expressed in the intestine and kidney. After a challenge with the bacterial pathogen, Streptococcus agalactiae, its expression was up-regulated significantly in the intestine, liver and spleen. We identified seven SNPs in the gene and found that four of them were significantly associated with the resistance to S. agalactiae (P < 0.05). The CGTCC haplotype, CAGTC/CGGTC and CGTCC/CGTCC diplotype were significantly associated with the resistance to S. agalactiae (P = 0.00, 0.04 and < 0.0001, respectively). In addition, one SNP was associated significantly with growth traits (P < 0.05). These findings suggest that the duodenase-1 gene plays an important role in the resistance to S. agalactiae in tilapia. The SNP markers in the duodenase-1 gene associated with resistance to the bacterial pathogen, may facilitate the selection of tilapia resistant to the bacterial disease.

The identification of loci for immune traits in chickens using a genome-wide association study.

The genetic improvement of disease resistance in poultry continues to be a challenge. To identify candidate genes and loci responsible for these traits, genome-wide association studies using the chicken 60k high density single nucleotide polymorphism (SNP) array for six immune traits, total serum immunoglobulin Y (IgY) level, numbers of, and the ratio of heterophils and lymphocytes, and antibody responses against Avian Influenza Virus (AIV) and Sheep Red Blood Cell (SRBC), were performed. RT-qPCR was used to quantify the relative expression of the identified candidate genes. Nine significantly associated SNPs (P < 2.81E-06) and 30 SNPs reaching the suggestively significant level (P < 5.62E-05) were identified. Five of the 10 SNPs that were suggestively associated with the antibody response to SRBC were located within or close to previously reported QTL regions. Fifteen SNPs reached a suggestive significance level for AIV antibody titer and seven were found on the sex chromosome Z. Seven suggestive markers involving five different SNPs were identified for the numbers of heterophils and lymphocytes, and the heterophil/lymphocyte ratio. Nine significant SNPs, all on chromosome 16, were significantly associated with serum total IgY concentration, and the five most significant were located within a narrow region spanning 6.4kb to 253.4kb (P = 1.20E-14 to 5.33E-08). After testing expression of five candidate genes (IL4I1, CD1b, GNB2L1, TRIM27 and ZNF692) located in this region, changes in IL4I1, CD1b transcripts were consistent with the concentrations of IgY, while abundances of TRIM27 and ZNF692 showed reciprocal changes to those of IgY concentrations. This study has revealed 39 SNPs associated with six immune traits (total serum IgY level, numbers of, and the ratio of heterophils and lymphocytes, and antibody responses against AIV and SRBC) in Beijing-You chickens. The narrow region spanning 247kb on chromosome 16 is an important QTL for serum total IgY concentration. Five candidate genes related to IgY level validated here are novel and may play critical roles in the modulation of immune responses. Potentially useful candidate SNPs for marker-assisted selection for disease resistance are identified. It is highly likely that these candidate genes play roles in various aspects of the immune response in chickens.

Genes and environmental factors that influence disease resistance to microbes in the female reproductive tract of dairy cattle.

Microbes commonly infect the female reproductive tract of cattle, causing infertility, abortion and post partum uterine diseases. When organisms reach the uterus, the resistance to disease depends on the balance between the classic triad of the virulence of the microbes, the host defence systems and the environment. The present review considers each aspect of this triad, using postpartum uterine disease as an exemplar for understanding disease resistance. The bacteria that cause postpartum uterine disease are adapted to the endometrium, and their microbial toxins cause tissue damage and inflammation. However, non-specific defence systems counter ascending infections of the female reproductive tract, and inflammatory responses in the endometrium are driven by innate immunity. Disease resistance to bacterial infection involves many genes involved in the maintenance or restoration of tissue homeostasis in the endometrium, including antimicrobial peptides, complement, cytokines, chemokines and Toll-like receptors. The most important environmental factors facilitating the development of postpartum uterine disease are related to trauma of the reproductive tract and to the metabolic stress of lactation in dairy cows. Long-term solutions for uterine disease will include genetic selection for disease resistance and optimising the care of the animal before, during and after parturition.

Markers for selection of disease resistance in fish: a review

Aquaculture is the fastest growing food producing sector in the world. However, diseases result in a major loss in production. Among the several management techniques, selective breeding for resistant trait is one of the most reliable methods and provides long-term control over disease problems. Field survival or challenge survival records provide initial information of resistant stocks. However, selection programmes demand a proper marker which can identify the resistant stock easily with much accuracy. Several immunological indicators associated with resistance to a particular disease have been reported. These include positively correlated markers such as level of natural and specific antibody, respiratory burst activity of phagocytes and level of acute-phase ceruloplasmin protein and negatively correlated markers such as serum lysozyme, haemolytic and haemagglutination activities. However, for few fish species and pathogen-specific markers viz., myeloperoxidase activity, cortisol and glucose levels, macrophage aggregation and the level of plasma proteins, a confined conclusion of association has not been established. Besides immunological markers, molecular markers are in use because of their reliable screening methods. Genome can be scanned for molecular markers which provide the full linkage map of a species or stock. Association of these molecular markers with resistant traits gives rise to quantitative trait loci (QTL) for resistance to a particular pathogen. Screening of resistant stock can be carried out using these markers. Among the several marker systems, amplified fragment length polymorphism, microsatellites and single-nucleotide polymorphism play a major role in the process of selection for disease resistance. With the use of these marker techniques, we can avoid challenge testing where a huge number of animals were being killed. Multiple trait selection can also be possible with multiple QTL identification for resistance to two or more diseases simultaneously.

Breeding and Domestication of Eastern Oyster (Crassostrea virginica) Lines for Culture in the Mid-Atlantic, Usa: Line Development and Mass Selection for Disease Resistance

A selective breeding program for Crassostrea virginica was established in 1997 as part of an initiative in Virginia to address declining oyster harvests caused by the two oyster pathogens Haplosporidium nelsoni (MSX) and Perkinsus marinus (Dermo). Housed in the Aquaculture Genetics and Breeding Technology Center (ABC), the objective of the program was to develop and disseminate disease-resistant lines that would enable an oyster culture industry. Today, culture of disease-resistant cultivars accounts for more than 90% of oyster production in the state, where 28.1 million half-shell oysters and 2 billion eyed larvae were sold in 2012. Results of our line development program as of 2006 are reported. Eight ABC lines from 3 genetic groups—East Coast (EC), Louisiana (LA), and hybrids between the 2 (HY)—and 1 wild control line, were produced and tested. These 9 groups were deployed in 4 replicates across 4 Virginia sites characterized by low (Kinsale (KIN)), medium (York River (YRK) and Lynnhaven (LYN)), and high (Wachapreague (WAC)) salinity regimes. Groups were sampled routinely for survival, growth, and disease diagnosis between November 2004 and December 2006. At KIN, where salinity was low and below the threshold for MSX and Dermo, survival was 41%-46% greater than survival at the other 3 sites by the end of the trial. Where the diseases were present (LYN, YRK, and WAC), ABC lines in general had greater survival than the control, but this varied by genetic group. The EC groups had 52%- 82% greater survival, the HY groups had as much as 40% greater survival, and the LA groups performed worse than the control. Poor performance of the LA groups was a result of their susceptibility to MSX, and the majority of them died before the end of the study. The genetic effects varied with site, and the rank of the lines was inconsistent, such that the best line in one site was, in some cases, one of the worst in another. Genotype-by-environment interaction is clearly driven by disease and salinity. Growth was also influenced by site, genetic group, and an interaction between them. Compared with the wild control, ABC lines were 31%, 20%, 42%, and 24% heavier at the end of the trial in the KIN, YRK, LYN, and WAC sites, respectively. However, unlike survival, the best performers were those from the LA and HY groups. Again, line rankings changed across sites. For this reason, a salinity-specific breeding strategy to develop lines that perform optimally within a salinity range has been adopted.

Relatedness severely impacts accuracy of marker-assisted selection for disease resistance in hybrid wheat

The accuracy of genomic selection depends on the relatedness between the members of the set in which marker effects are estimated based on evaluation data and the types for which performance is predicted. Here, we investigate the impact of relatedness on the performance of marker-assisted selection for fungal disease resistance in hybrid wheat. A large and diverse mapping population of 1739 elite European winter wheat inbred lines and hybrids was evaluated for powdery mildew, leaf rust and stripe rust resistance in multi-location field trials and fingerprinted with 9 k and 90 k SNP arrays. Comparison of the accuracies of prediction achieved with data sets from the two marker arrays revealed a crucial role for a sufficiently high marker density in genome-wide association mapping. Cross-validation studies using test sets with varying degrees of relationship to the corresponding estimation sets revealed that close relatedness leads to a substantial increase in the proportion of total genotypic variance explained by the identified QTL and consequently to an overoptimistic judgment of the precision of marker-assisted selection.

The Pontin series of recombinant alien translocations in bread wheat: single translocations integrating combinations of Bdv2, Lr19 and Sr25 disease-resistance genes from Thinopyrum intermedium and Th. ponticum

Two bread wheat lines each with a translocation on chromosome 7DL from either Thinopyrum intermedium (TC5 and TC14) or Thinopyrum ponticum (T4m), were hybridized in a ph1b mutant background to enhance recombination between the two translocated chromosomal segments. The frequency of recombinants was high in lines derived from the larger and similar-sized translocations (TC5/T4m), but much lower when derived from different-sized translocations (TC14/T4m). Recombinant translocations contained combinations of resistance genes Bdv2, Lr19 and Sr25 conferring resistance to Barley yellow dwarf virus (BYDV), leaf rust and stem rust, respectively. Their genetic composition was identified using bioassays and molecular markers specific for the two progenitor Thinopyrum species. This set of 7DL Th. ponticum/intermedium recombinant translocations was termed the Pontin series. In addition to Thinopyrum markers, the size of the translocation was estimated with the aid of wheat markers mapped on each of the 7DL deletion bins. Bioassays for BYDV, leaf rust and stem rust were performed under greenhouse and field conditions. Once separated from ph1b background, the Pontin recombinant translocations were stable and showed normal inheritance in successive backcrosses. The reported Pontin translocations integrate important resistance genes in a single linkage block which will allow simultaneous selection of disease resistance. Combinations of Bdv2+Lr19 or Lr19+Sr25 in both long and short translocations, are available to date. The smaller Pontins, comprising only 20% of the distal portion of 7DL, will be most attractive to breeders.

Application of molecular markers to wheat breeding in Canada

AbstractMarker‐assisted breeding provides an opportunity for wheat breeders to introgress/pyramid genes of interest into breeding lines and to identify genes and/or quantitative trait loci in germplasm to be used as parents. Molecular markers were deployed to assist selection for disease resistance, agronomic and quality traits in several wheat cultivars released for commercial cultivation in Canada. Marker‐assisted breeding is routinely used in most wheat breeding programmes for rust resistance (leaf, stem and stripe rust), orange wheat blossom midge resistance, high grain protein concentration, Fusarium head blight and common bunt resistance. Markers are being used selectively within breeding programmes to target traits that relate to market class or regional adaptation. For example, marker‐assisted breeding for low lipoxygenase activity and low grain cadmium is being performed in durum breeding programmes and for enhancing stem solidness in programmes targeting resistance to the wheat stem sawfly. Markers are also being utilized for ergot resistance in durum wheat. Increased gluten strength is being selected with a marker for the overexpression of the Bx7 high‐molecular‐weight glutenin subunit. Marker‐assisted breeding is also being used to pyramid resistance genes against a group of stem rust races related to TTKS (Ug99), a disease that poses a serious threat to global wheat production. Development of tightly linked diagnostic markers and high‐throughput genotyping with SNP markers will result in more effective molecular wheat breeding in the near future and will open the door to genomic selection.

Breeding strategies for tick resistance in tropical cattle: a sustainable approach for tick control.

About 80% of world cattle population is under the risk of ticks and tick borne diseases (TTBDs). Losses caused by bovine tick burdens in tropical countries have a tremendous economic impact on production systems. Chemical control of disease has been found to be ineffective and also involving large cost. To reduce our reliance on these chemical products, it is necessary to embark on programs that include habitat management, genetic selection of hosts, and development of a strain capable of inducing host resistance to ticks. Selection for disease resistance provide alternate method for sustainable control of TTBDs. Domestic livestock manifests tick-resistance by skin thickness, coat type, coat color, hair density and skin secretions etc. Zebu cattle have, on average, greater tick resistance than either European cattle or African cattle. Heritability for tick burden in cattle has been shown to range about 0.30, which is sufficient to result in the success of some programs of selection for tick resistance in cattle. To select animals at younger age, to reduce generation interval and to increase genetic gain, marker assisted selection is an important tool. There are also various MHC molecules which are associated with resistance to TTBDs.

DEVELOPMENT OF SNP MARKERS FOR MARKER-ASSISTED SELECTION IN PEAR

Single-nucleotide polymorphisms (SNPs) are desirable DNA markers for marker-assisted selection because of their high abundance, codominant inheritance, locus specificity and potential for automated high-throughput analysis. The average number of SNPs in introns has been reported to be several times higher than that in exons in many organisms. Potential intron polymorphism (PIP) markers, designed from expressed sequence tag (EST) sequences (exon regions), could amplify potential flanking intron regions without requiring genome sequence information. In this study, we used PIP markers designed from apple (Malus × domestica) ESTs to identify intron regions and to detect SNPs in pear (Pyrus spp.). We evaluated 170 PIP markers for Japanese pear P. pyrifolia cultivar 'Housui' and the European pear P. communis cultivar 'Bartlett'. Of the 170 PIP markers, 142 markers amplified fragments in both cultivars. The fragment size was larger than the size predicted from the apple EST sequences, suggesting the existence of introns in the amplified fragments. We sequenced the amplified fragments and detected candidate bi-allelic SNPs in intron regions of both cultivars. The genome sequences obtained from pear were aligned to the apple EST sequences, revealing that the sequences included donor and acceptor sites, and contained deduced exons and flanking introns in pear. SNP analysis showed that 55 and 54 markers were mapped on the genetic linkage maps of 'Bartlett' and 'Housui', respectively. The PIP markers designed from apple ESTs were applicable in pear and could be used to detect SNPs in intron regions and to develop SNP markers in pear. The PIP markers will facilitate marker-assisted selection for disease resistance and other important traits in pear breeding programs.

Immunogenomics for identification of disease resistance genes in pigs: a review focusing on Gram-negative bacilli

Over the past years, infectious disease has caused enormous economic loss in pig industry. Among the pathogens, gram negative bacteria not only cause inflammation, but also cause different diseases and make the pigs more susceptible to virus infection. Vaccination, medication and elimination of sick pigs are major strategies of controlling disease. Genetic methods, such as selection of disease resistance in the pig, have not been widely used. Recently, the completion of the porcine whole genome sequencing has provided powerful tools to identify the genome regions that harboring genes controlling disease or immunity. Immunogenomics, which combines DNA variations, transcriptome, immune response, and QTL mapping data to illustrate the interactions between pathogen and host immune system, will be an effective genomics tool for identification of disease resistance genes in pigs. These genes will be potential targets for disease resistance in breeding programs. This paper reviewed the progress of disease resistance study in the pig focusing on Gram-negative bacilli. Major porcine Gram-negative bacilli and diseases, suggested candidate genes/pathways against porcine Gram-negative bacilli, and distributions of QTLs for immune capacity on pig chromosomes were summarized. Some tools for immunogenomics research were described. We conclude that integration of sequencing, whole genome associations, functional genomics studies, and immune response information is necessary to illustrate molecular mechanisms and key genes in disease resistance.