Genetic Variation In Resistance Research Articles

The role of genetic variability of the host on the resistance to Neospora caninum infection in cattle.

Neospora caninum is one of the most frequently diagnosed abortifacient pathogens in cattle. There is abundant genomic information about the parasite itself, but very little is known about the genetic variability of resistance in the most common intermediate host. The aim of this review was to compile all the available information about the genetic variability associated with the resistance to N. caninum both between and within cattle breeds. We systematically searched for published studies that investigated the influence of genetics of the host on the prevalence of N. caninum and risk of abortion. Beyond the potential confounding effects of feeding systems, management and animal density, some lines of evidence suggest that Holstein, the most popular breed for milk production, has a comparatively higher risk of abortion due to infections by N. caninum, whereas some beef breeds from Continental Europe seem to be more resistant. It is still not clear if different genetic mechanisms of resistance are involved in the two known routes of infection: postnatal ingestion of oocysts or transplacental transmission from the infected dam to the fetus. Genomic information associated with susceptibility to infection and risk of abortion in different cattle breeds is still scarce. The information reported here could be useful to identify new research alternatives and to define novel strategies to deal with this major problem of animal production.

Variation in Isolate Virulence and Accession Resistance Associated with Diaporthe aspalathi, D. caulivora, and D. longicolla in Soybean

Important stem and grain diseases of soybean ( Glycine max [L.] Merr.) caused by Diaporthe aspalathi, D. caulivora, and D. longicolla reduce yield in the United States. Sources of resistance to these pathogens have previously been reported; however, there is limited information regarding their resistance when exposed to geographically distinct isolates of the same species. In this study, four accessions from the USDA Soybean Germplasm Collection with reported resistance to D. aspalathi, D. caulivora, or D. longicolla were evaluated using geographically representative isolates within each species from the United States. For each fungus, a greenhouse experiment was conducted as a completely randomized design with a factorial arrangement (isolate × accession). Plants were inoculated at the second to third trifoliate growth stage using toothpicks infested with isolates of each Diaporthe species. Pathogenicity was assessed 21 days postinoculation as 0 = no lesion, 0.5 = lesion length > 1 cm, and 1 = dead plant. A significant isolate-by-accession interaction ( P < 0.05) was observed to affect pathogenicity as analyzed using nonparametric statistics (relative treatment effects [RTEs]), indicating that accessions responded differently to the isolates. Correlation analyses suggested that the RTEs on ‘Tracy-M’, ‘Dowling’, and ‘Crockett’ were weakly to moderately correlated with those of the D. aspalathi-susceptible ‘Bragg’, as well as for ‘PI567473B’ and ‘Century’ ( D. caulivora), and ‘PI417507’ ( D. longicolla), with the RTEs on ‘Hawkeye’ ( P > 0.05) indicating possible genetic variation for resistance within these accessions. Our results provide information related to the resistance of previously identified accessions to develop commercial cultivars with resistance to important pathogens within the genus Diaporthe. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

Phyllosphere microbial diversity and specific taxa mediate within-cultivar resistance to Phytophthora palmivora in cacao.

The oomycete pathogen Phytophthora palmivora, which causes black pod rot (BPR) on cacao (Theobroma cacao L.), is responsible for devastating yield losses worldwide. Genetic variation in resistance to Phytophthora spp. is well documented among cacao cultivars, but variation has also been observed in the incidence of BPR even among trees of the same cultivar. In light of evidence that the naturally occurring phyllosphere microbiome can influence foliar disease resistance in other host-pathogen systems, it was hypothesized that differences in the phyllosphere microbiome between two field accessions of the cultivar Gainesville II 164 could be responsible for their contrasting resistance to P. palmivora. Bacterial alpha diversity was higher but fungal alpha diversity was lower in the more resistant accession MITC-331, and the accessions harbored phyllosphere microbiomes with distinct community compositions. Six bacterial and 82 fungal amplicon sequence variants (ASVs) differed in relative abundance between MITC-333 and MITC-331, including bacterial putative biocontrol agents and a high proportion of fungal pathogens, and nine fungal ASVs were correlated with increased lesion development. The roles of contrasting light availability and host mineral nutrition, particularly potassium, are also discussed. Results of this preliminary study can be used to guide research into microbiome-informed integrated pest management strategies effective against Phytophthora spp. in cacao. IMPORTANCE Up to 40% of the world's cacao is lost each year to diseases, the most devastating of which is black pod rot, caused by Phytophthora palmivora. Though disease resistance is often attributed to cacao genotypes (i.e., disease-resistant rootstocks), this study highlights the role of the microbiome in contributing to differences in resistance even among accessions of the same cacao cultivar. Future studies of plant-pathogen interactions may need to account for variation in the host microbiome, and optimizing the cacao phyllosphere microbiome could be a promising new direction for P. palmivora resistance research.

Intraspecific genetic variation in host vigour, viral load and disease tolerance during Drosophila C virus infection.

Genetic variation for resistance and disease tolerance has been described in a range of species. In Drosophila melanogaster, genetic variation in mortality following systemic Drosophila C virus (DCV) infection is driven by large-effect polymorphisms in the restriction factor pastrel (pst). However, it is unclear if pst contributes to disease tolerance. We investigated systemic DCV challenges spanning nine orders of magnitude, in males and females of 10 Drosophila Genetic Reference Panel lines carrying either a susceptible (S) or resistant (R) pst allele. We find among-line variation in fly survival, viral load and disease tolerance measured both as the ability to maintain survival (mortality tolerance) and reproduction (fecundity tolerance). We further uncover novel effects of pst on host vigour, as flies carrying the R allele exhibited higher survival and fecundity even in the absence of infection. Finally, we found significant genetic variation in the expression of the JAK-STAT ligand upd3 and the epigenetic regulator of JAK-STAT G9a. However, while G9a has been previously shown to mediate tolerance of DCV infection, we found no correlation between the expression of either upd3 or G9a on fly tolerance or resistance. Our work highlights the importance of both resistance and tolerance in viral defence.

Defensive patterns of chestnut genotypes (Castanea spp.) against the gall wasp, Dryocosmus kuriphilus

Plants draw from various genetically controlled defenses to protect against herbivores and pathogens. The efficacy of alternative defenses can vary with the biology and phylogeny of the enemy.Dryocosmus kuriphilus, an invasive gall maker native to China, has become one of the main pests of chestnut trees around the world. We studied genetic variation in resistance and susceptibility toD. kuriphiluswithin a replicated blocked planting of 12 chestnut genotypes established in Galicia, Spain in 2004. We found very high genetic variation in susceptibility toD. kuriphilus. We evaluated if the variation was due to host selection by the wasp, differential efficacy of inducible defenses in the trees, or variability in susceptibility of the plant to manipulation by the gallmaker. We assessed host selection by counting number of eggs laid by females in tree buds and comparing preferences with phytochemistry. We also measured inducible changes in phytochemistry within and around galls, gall physical characteristics, parasitism, and insect fitness. The effective defense mechanisms in resistant genotypes involved (1) hypersensitive reactions to eggs or neonates within buds and (2) early precise abscission of nascent galls from expanding leaves. Surprisingly, the genetic resistance toD. kuriphilusof the chestnut genotypes we tested was not related to phenols, terpenes, and primary nutrition.

Additive genetic variation in resistance of Nile tilapia (Oreochromis niloticus) to Francisella orientalis and its genetic (co)variation to both harvest weight and resistance to Streptococcus agalactiae Ib

An understanding of the genetic relationship between traits of economic importance for a widely cultured species such as Nile tilapia (Oreochromis niloticus) is important, in particular the relationship between disease resistance and growth. Francisellosis results in economic losses for the industry, and we confirmed that Francisellosis resistance is under additive genetic control in our population. Analysing four generations, significant additive genetic variation was found for survival to F. orientalis challenge (P < 0.001; Log-likelihood-ratio test) with an estimated heritability of h2 = 0.31 ± 0.05 across all generations. The random effect common to full-sibs was also significant, but of lower magnitude (c2 = 0.075 ± 0.02) (P < 0.001; Log-likelihood-ratio test). Results of assortative mating groups confirmed that genetic gain could be obtained for survival to F. orientalis challenge since offspring of fish selected with high estimated breeding values exhibited higher survival to F. orientalis challenge. The genetic correlation between harvest weight and survival to F. orientalis challenge was very low (rg = −0.02 ± 0.13) and not significantly different from zero (Log-likelihood-ratio test p > 0.05). The genetic correlation between survival to F. orientalis challenge and survival to S. agalactiae Ib challenge was positive but not significantly different from zero (rg = 0.14 ± 0.11; Log-likelihood-ratio test p > 0.05). The genetic correlation between harvest weight and survival to S. agalactiae challenge was negative but also not significantly different (rg = − 0.19 ± 0.10; Log-likelihood-ratio test p > 0.05). Lack of significant favourable genetic relationships between economically important traits favours multi-trait selection since selection for growth alone does not improve survival to F. orientalis challenge or survival to S. agalactiae challenge.

Can disease resistance evolve independently at different ages? Genetic variation in age-dependent resistance to disease in three wild plant species.

Juveniles are typically less resistant (more susceptible) to infectious disease than adults, and this difference in susceptibility can help fuel the spread of pathogens in age‐structured populations. However, evolutionary explanations for this variation in resistance across age remain to be tested.One hypothesis is that natural selection has optimized resistance to peak at ages where disease exposure is greatest. A central assumption of this hypothesis is that hosts have the capacity to evolve resistance independently at different ages. This would mean that host populations have (a) standing genetic variation in resistance at both juvenile and adult stages, and (b) that this variation is not strongly correlated between age classes so that selection acting at one age does not produce a correlated response at the other age.Here we evaluated the capacity of three wild plant species (Silene latifolia, S. vulgaris and Dianthus pavonius) to evolve resistance to their anther‐smut pathogens (Microbotryum fungi), independently at different ages. The pathogen is pollinator transmitted, and thus exposure risk is considered to be highest at the adult flowering stage.Within each species we grew families to different ages, inoculated individuals with anther smut, and evaluated the effects of age, family and their interaction on infection.In two of the plant species, S. latifolia and D. pavonius, resistance to smut at the juvenile stage was not correlated with resistance to smut at the adult stage. In all three species, we show there are significant age × family interaction effects, indicating that age specificity of resistance varies among the plant families. Synthesis. These results indicate that different mechanisms likely underlie resistance at juvenile and adult stages and support the hypothesis that resistance can evolve independently in response to differing selection pressures as hosts age. Taken together our results provide new insight into the structure of genetic variation in age‐dependent resistance in three well‐studied wild host–pathogen systems.

Tomato Defense against Whiteflies under Drought Stress: Non-Additive Effects and Cultivar-Specific Responses.

Two of the main causes of losses in tomato production are the greenhouse whitefly, Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), and drought, which is becoming a central problem in agriculture due to global climate change. The separate effects of whitefly infestation and drought have been amply studied in many crop systems. However, less is known about their combined effects. To evaluate whether drought stress (DS) affects plant defense against whiteflies, we assessed the joint effects of whitefly infestation and DS on plant vegetative and reproductive performance in four tomato cultivars, and assessed the effects of DS on plant resistance and tolerance (compensatory ability) to whiteflies in a greenhouse experiment. Generally, we found negative effects of DS and whiteflies on plant performance, but the combined effects of DS and herbivory were not worse than those of either stress alone. In fact, plant performance under the combined effect of both stresses was usually similar to that in the presence of whiteflies without DS. Plants growing under DS had greater trichome density. However, plant resistance—as measured by whitefly population growth—decreased under DS in two cultivars and was unaffected in the other two. Compensatory ability decreased under DS in all but one cultivar. These cultivar-specific responses suggest genetic variation in resistance and tolerance to whiteflies and could be associated with differences in drought tolerance among cultivars. Our findings underscore the difficulty in predicting the combined effects of DS and herbivory and point to the need for a better understanding of the mechanisms underlying plant responses to both stresses at the molecular, cellular, and organismal levels.

Estimation of genetic parameters and resistance to Meloidogyne spp. in melon genotypes

Root-knot nematodes (Meloidogyne spp.) are one of the major pests of agricultural crops worldwide. In Brazil, these nematodes are widely distributed in melon fields, and there are no reports of resistant melon cultivars. This study aimed to estimate genetic variance components for the selection of melon genotypes resistant to Meloidogyne incognita, M. javanica, and M. enterolobii. Two experiments were carried out in a completely randomized design, the first with 14 replications and the second with 10 replications, in a factorial design. Thirteen melon genotypes were tested. In both experiments, the total number of eggs and juveniles (TNEJ) was estimated and used to determine the reproduction factor (RF) and reproduction index (RI) of nematodes. Melon genotypes showed wide genetic variability in resistance to the three nematode species, resulting in high estimates of heritability (92%) and accuracy (96%). Such findings indicate the possibility of genetic gains with selection. Melon AC-09, PI157082, and ‘Ouro’ were found to be sources of resistance to M. incognita, M. javanica, and M. enterolobii, respectively.

Seeking Alfalfa Resistance to a Rhizophagous Pest, the Clover Root Curculio (Sitona hispidulus F.).

Simple SummaryClover root curculio (CRC) is a root feeding pest of alfalfa and clover that reduces stand life and yield. With the cancellation of soil-active insecticides in alfalfa, CRC populations and associated root damage have increased. Current CRC management practices are limited in their ability to suppress larval feeding belowground. Here, we evaluated alfalfa populations for resistance to CRC larval feeding and development belowground, and adult leaf consumption and oviposition aboveground. Divergent selection in two alfalfa cultivars in field nurseries revealed that there is genetic variability in resistance to CRC larval feeding and that significant gains in resistance from selection can occur in as few as two or three cycles of selection. While larval development was similar across the alfalfa populations tested in the lab, one alfalfa population (NY1713) displayed an overall increase in nodulation resulting in significantly lower proportions of nodules being consumed by larvae. These results provide possible candidates and soil-less method for the development and evaluation of alfalfa cultivars that may reduce the impacts of CRC root feeding and that offer an additional option for CRC management. Since the cancellation of broad-spectrum soil-active insecticides in alfalfa (Medicago sativa L.) production, clover root curculio (Sitona hispidulus F.) (CRC) larval root damage has increased. Current CRC management practices are limited in their ability to suppress larval feeding belowground. First, we field screened developmental alfalfa populations for CRC damage. Subsequently, we developed a soil-less arena to observe nodule feeding and development (head capsule width) of larvae in the lab. This method was used to evaluate five alfalfa populations (two CRC-susceptible (control) and three CRC-resistant populations) against larvae. Further, one CRC-resistant population paired with its genetically similar susceptible population were tested against adult leaf consumption and oviposition in the greenhouse. Field screening revealed that the alfalfa populations selected for little or no larval root feeding damage were more resistant to CRC larval feeding than their corresponding unselected cultivars and significantly more resistant than populations selected for susceptibility. The development of a soil-less arena provided a useful method for evaluation of root-larva interactions. Although larval development was similar across susceptible and resistant alfalfa populations, one CRC-resistant population (NY1713) displayed overall increased nodulation and, thus, had a significantly lower proportion of nodules consumed by larvae. Adult feeding and oviposition aboveground were similar across all populations tested. These results provide possible candidates and screening method for the development and evaluation of alfalfa cultivars that may reduce the impacts of larval feeding and that offer an additional option for CRC management.

Variation in parasite resistance of Arctic charr, Salvelinus alpinus, between and within sympatric morphs.

Genetic variation in resistance against parasite infections is a predominant feature in host–parasite systems. However, mechanisms maintaining genetic polymorphism in resistance in natural host populations are generally poorly known. We explored whether differences in natural infection pressure between resource‐based morphs of Arctic charr (Salvelinus alpinus) have resulted in differentiation in resistance profiles. We experimentally exposed offspring of two morphs from Lake Þingvallavatn (Iceland), the pelagic planktivorous charr (“murta”) and the large benthivorous charr (“kuðungableikja”), to their common parasite, eye fluke Diplostomum baeri, infecting the eye humor. We found that there were no differences in resistance between the morphs, but clear differences among families within each morph. Moreover, we found suggestive evidence of resistance of offspring within families being positively correlated with the parasite load of the father, but not with that of the mother. Our results suggest that the inherited basis of parasite resistance in this system is likely to be related to variation among host individuals within each morph rather than ecological factors driving divergent resistance profiles at morph level. Overall, this may have implications for evolution of resistance through processes such as sexual selection.

Assessment of Maternal Effects and Genetic Variability in Resistance to Verticillium dahliae in Olive Progenies

The use of genetic resistance is likely the most efficient, economically convenient and environmentally friendly control method for plant diseases, as well as a fundamental piece in an integrated management strategy. This is particularly important for woody crops affected by diseases in which mainly horizontal resistance mechanisms are operative, such as Verticillium wilt, caused by Verticillium dahliae. In this study, we analyzed the variability in resistance to Verticillium wilt of olive trees in progenies from five crosses: ‘Picual’ × ‘Frantoio’, ‘Arbosana’ × ‘Koroneiki’, ‘Sikitita’ × ‘Arbosana’, ‘Arbosana’ × ‘Frantoio’ and ‘Arbosana’ × ‘Arbequina’ and their respective reciprocal crosses. Additionally, seedlings of ‘Picual’ and ‘Frantoio’ in open pollination were used as controls. In October 2016 and 2018, the fruits were harvested, and seeds germinated. Six-week-old seedlings were inoculated by dipping their bare roots in a conidial suspension of V. dahliae, and disease progress in terms of symptom severity and mortality was evaluated weekly. Additionally, seedling growth was evaluated every two weeks. At the end of the experiment, no significant differences were found for any of the assessed parameters when reciprocal crosses were compared. These results suggest that there is no maternal or paternal effect in regard to the heritability of resistance. In addition, this study identifies the best crosses for obtaining the highest number of resistant genotypes, highlighting the importance of the selection of specific cultivars to optimize the breeding process.

Resistance Correlations Influence Infection by Foreign Pathogens.

AbstractReciprocal selection promotes the specificity of host-pathogen associations and resistance polymorphisms in response to disease. However, plants and animals also vary in response to pathogen species not previously encountered in nature, with potential effects on new disease emergence. Using anther smut disease, we show that resistance (measured as infection rates) to foreign pathogens can be correlated with standing variation in resistance to an endemic pathogen. In Silene vulgaris, genetic variation in resistance to its endemic anther smut pathogen correlated positively with resistance variation to an anther smut pathogen from another host, but the relationship was negative between anther smut and a necrotrophic pathogen. We present models describing the genetic basis for assessing resistance relationships between endemic and foreign pathogens and for quantifying infection probabilities on foreign pathogen introduction. We show that even when the foreign pathogen has a lower average infection ability than the endemic pathogen, infection outcomes are determined by the sign and strength of the regression of the host's genetic variation in infection rates by a foreign pathogen on variation in infection rates by an endemic pathogen as well as by resistance allele frequencies. Given that preinvasion equilibria of resistance are determined by factors including resistance costs, we show that protection against foreign pathogens afforded by positively correlated resistances can be lessened or even result in elevated infection risk at the population level, depending on local dynamics. Therefore, a pathogen's emergence potential could be influenced not only by its average infection rate but also by resistance variation resulting from prior selection imposed by endemic diseases.

Field Screening of Sorghum [Sorghum bicolor (L.) Moench] Inbred Lines for Resistance to Striga [Striga hermonthica (Del.)] in East Hararghe, Ethiopia

Drought and the obligate Striga root parasite are two of the most important constraints to sorghum production in the northern and north eastern parts of Ethiopia. This study assessed the field reaction of selected advanced lines under Striga hermonthica infestation condition. Twenty-two sorghum genotypes with two resistant checks (Gobiye and SRN-39) and one susceptible check (Teshale) were evaluated under Striga hot spot area at Fadis Agricultural Research Center, Boko research station (Ethiopia) in the 2016 season using a 5x5 triple lattice design. During the field experiment, 11 parameters were measured among which Striga count emerged on each genotype was the indicator of resistance genotypes. Genotypes differed significantly in all measured parameters in their reactions to Striga. Genotypes 2006 MW 6044, ETSC 300003, ETSC 300081, 05 MW 6019, and ETSC 300080 showed Striga resistance in the field. Thus, these genotypes were the most promising sources of resistance to S. hermonthica. From field experiments, it could be suggested that genetic variability for resistance and tolerance is available in a range of genotypes, which could be used for future breeding and production in Striga infested areas. Keywords: genotypes, germination stimulant, parasitic weed, resistant, screening techniques DOI: 10.7176/FSQM/105-01 Publication date: February 28 th 2021

Genetic variation in resistance and high fecundity impede viral biocontrol of invasive fish

Common carp Cyprinus carpio is one of the top global invasive vertebrates and can cause significant ecological damage. The Australian Government's National Carp Control Program (NCCP) proposes to release Koi herpesvirus (KHV) to eradicate feral carp in one of the largest ecological interventions ever attempted. Ecological and human health risks have been highlighted regarding the release of a highly pathogenic viral biocontrol for an aquatic species. The efficacy of KHV has also been questioned, and it has not been demonstrated to produce lasting population reductions. We developed an individual-based model (IBM) to examine the ecological and evolutionary response of a carp population after KHV release. This simulated the interaction between fish life history, viral epidemiology, host genetic resistance and population demography to critically evaluate the impact of KHV release under optimal conditions and a ‘best-case scenario’ for disease transmission. KHV will rarely result in prolonged reductions or population extinctions. Crucially, realistic scenarios result in a rapidly rebounding population of resistant individuals. Additional measures aimed to reduce carp population recovery rate (e.g. with genetic engineering) require rapid efficacy to significantly reduce carp numbers alongside KHV. Fish fecundity has an overwhelming influence on viral efficacy as a biocontrol agent when combined with genetic resistance within a population. A high probability of population extinction is only met when carp fecundity is reduced to 1% of biological observations. Synthesis and applications. We use an individual-based model to evaluate the efficacy of Koi herpesvirus biocontrol in Common Carp, and find that high host fecundity combined with genetic resistance results in rapid population rebound after initial large fish kills. Biocontrol approaches relying on natural selection lose efficacy over successive generations as resistance genes increase in frequency. Given the intense logistical effort and risks to ecosystems and human health associated with large fish kills after viral release, we suggest that sustained manual removal, alongside ecological restoration to favour recovery of native species, provides a risk-free approach to reducing populations.

Genetic variation for resistance to the specific fly pathogen Entomophthora muscae

We found substantial variation in resistance to the fly-specific pathogen Entomophthora muscae 'Berkeley' (Entomophthoromycota), in 20 lines from the Drosophila melanogaster Genetic Reference Panel (DGRP). Resistance to E. muscae is positively (r = 0.55) correlated with resistance to the broad host range ascomycete entomopathogen Metarhizium anisopliae (Ma549), indicative of generalist (non-specific) defenses. Most of the lines showing above average resistance to Ma549 showed cross-resistance to E. muscae. However, lines that succumbed quickly to Ma549 exhibited the full range of resistance to E. muscae. This suggests fly populations differ in E. muscae-specific resistance mechanisms as well as generic defences effective against both Ma549 and E. muscae. We looked for trade-offs that could account for inter-line variation, but increases (decreases) in disease resistance to E. muscae are not consistently associated with increases (decreases) of resistance to oxidative stress, starvation stress and sleep indices. That these pathogens are dynamic agents of selection on hosts is reflected in this genetic variation for resistance in lines derived from wild populations.

Genome-Wide Association Mapping Identifies Novel Loci for Quantitative Resistance to Blackleg Disease in Canola.

Blackleg disease, caused by the fungal pathogen Leptosphaeria maculans, continues to be a major concern for sustainable production of canola (Brassica napus L.) in many parts of the world. The deployment of effective quantitative resistance (QR) is recognized as a durable strategy in providing natural defense to pathogens. Herein, we uncover loci for resistance to blackleg in a genetically diverse panel of canola accessions by exploiting historic recombination events which occurred during domestication and selective breeding by genome-wide association analysis (GWAS). We found extensive variation in resistance to blackleg at the adult plant stage, including for upper canopy infection. Using the linkage disequilibrium and genetic relationship estimates from 12,414 high quality SNPs, GWAS identified 59 statistically significant and “suggestive” SNPs on 17 chromosomes of B. napus genome that underlie variation in resistance to blackleg, evaluated under field and shade-house conditions. Each of the SNP association accounted for up to 25.1% of additive genetic variance in resistance among diverse panel of accessions. To understand the homology of QR genomic regions with Arabidopsis thaliana genome, we searched the synteny between QR regions with 22 ancestral blocks of Brassicaceae. Comparative analyses revealed that 25 SNP associations for QR were localized in nine ancestral blocks, as a result of genomic rearrangements. We further showed that phenological traits such as flowering time, plant height, and maturity confound the genetic variation in resistance. Altogether, these findings provided new insights on the complex genetic control of the blackleg resistance and further expanded our understanding of its genetic architecture.

Genetic parameters for resistance to Tilapia Lake Virus (TiLV) in Nile tilapia (Oreochromis niloticus)

Tilapia Lake Virus (TiLV) is one of the primary disease concerns for tilapia farming, with mass mortality events and biosecurity restrictions threating aquaculture in several continents. Selective breeding for improved host resistance to TiLV may help to mitigate this problematic disease, but the extent of genetic variation in resistance is not yet known. The objective of the current study was to estimate genetic parameters for host resistance to TiLV in a Nile tilapia breeding population of the Genetically Improved Farmed Tilapia (GIFT) strain. Using data from 1821 pedigreed fish (from 124 full-sibling families) collected during and after a pond ‘field’ outbreak, resistance was defined using both binary survival (BS) and days to death (TD) traits. Animal and sire-dam linear mixed models were fitted for BS and TD, and BS was also evaluated with using two sire-dam threshold models with either probit (Pro-SD) or logit-link (Log-SD) functions. Cumulative mortality was 39.6% at the end of the outbreak, with family survival rates ranging from 0 to 100%. Moderate to high heritability values were estimated for resistance to TiLV using all models. Significant heritabilities were estimated on the binary scale (0.40 for both animal and sire-dam models) which equates to 0.63 on the underlying liability scale. Using threshold models, heritabilities of 0.56 and 0.48 were estimated for Pro-SD and Log-SD, respectively. Correlation among the full-sib families EBVs predicted by the different models ranged from 0.912 to 0.999, suggesting a low re-ranking of the families and a high consistency of the results obtained using the different models. In addition, significant and moderate heritability of 0.41 (0.06) was estimated for harvest weight (HW), and the genetic correlation between this trait and resistance to TILV was not statistically different from zero. These results demonstrate that host resistance to TiLV is highly heritable in a Nile tilapia breeding population with GIFT origin. Therefore, selective breeding to increase resistance and reduce mortalities due to TiLV is a feasible and promising approach.

Resistance to Streptococcus iniae and its genetic associations with traits of economic importance in Asian seabass (Lates calcarifer).

Streptococcus iniae is one of the most serious aquatic pathogens, causing significant economic losses in marine and freshwater species, including Asian seabass (Lates calcarifer). Controlling this gram-positive bacterial pathogen has been an issue in aquaculture systems, due to the combined effects of aquaculture intensification and climatic impacts. To date, there have not been any genetic parameter estimates for S.iniae resistance in Asian seabass. The main aim of this study was to examine genetic variation in S.iniae resistance and its genetic correlations with growth and cannibalism in Asian seabass families produced from a breeding programme for high growth in 2016 and 2017. The study included a total of 5,835 individual fish that were offspring of 41 sires and 60 dams (31 half-sib and 34 full-sib families). The experimental fish were challenged by intraperitoneal injection with a volume containing 105 CFU (colony-forming unit)/fish. Resistance to S.iniae was measured as survival rate at 6hr, 3, 5, 7, 10 and 15days post-challenge test. There were significant variations in S.iniae resistance among families at different observation periods (ranging from 24.4% to 80%). Restricted maximum-likelihood method and mixed model analysis were applied to estimate heritability for S.iniae resistance. The heritability for S.iniae resistance ranged from 7% to 18% across different statistical models used. The common full-sib effects accounted for 0.1%-2% of the total variation in resistance to S.iniae. Genetic correlations of the S.iniae resistance at 6hr and 3days with later post-challenge test periods were low to moderate. However, these estimates for S.iniae resistance between successive measurement times (5, 7, 10 and 15days) were high and close to 1. The genetic correlations of resistance with body weights at 180, 270 and 360days post-hatch were not significant as well with cannibalism. It is concluded that there is substantial additive genetic variation in resistance to S.iniae, suggesting there is potential for genetic improvement of Asian seabass for resistance to S.iniae through selective breeding.

Genetic variation in resistance of the tiger pufferfish Takifugu rubripes to a host-specific monogenean parasite Heterobothrium okamotoi

The tiger pufferfish Takifugu rubripes is one of the most valuable aquaculture species in Japan. In this study, we investigated the genetic variation in resistance to heterobothriosis caused by a host-specific monogenean parasite, Heterobothrium okamotoi, in the tiger pufferfish, and examined the possibility of selective breeding for enhancing resistance traits. A total of 299 tiger pufferfish produced from ten males and one female caught from the wild were experimentally exposed to oncomiracidia of H. okamotoi. The number of parasites on the gills at 3 weeks post-infection ranged from 0 to 68, with great variation observed both within and between families. The estimated heritability for parasite density was small, but significantly different from zero (H2 = 0.098; 95% confidence interval: 0.025–0.201). Although we may have sampled a limited fraction of the genetic variance as the number of families was small, the results suggest that there are genetic components that affect heterobothriosis resistance in the species. A positive phenotypic correlation was observed between parasite count and standard length (r = 0.66). Further investigation of the genetic correlation between the two traits using larger families is needed to assess the impacts of selective breeding on heterobothriosis resistance in this species.