Evaluation of the phenotypic performance of a Red Maasai and Dorper double backcross resource population: Natural pasture challenge with gastro-intestinal nematode parasites

Six first cross (F1) Red Maasai X Dorper rams were mated to both Red Maasai (R) and Dorper (D) ewes to produce 1342 double backcross progeny (672 3/4 D/1/4 R and 670 3/4 R/1/4 D lambs born alive). These six double backcross resource families are being used to identify quantitative trait loci controlling resistance to gastro-intestinal (GI) nematode parasites (particularly Haemonchus contortus). This paper reports the phenotypic performance of the double backcross progeny relative to the performance of straight bred Red Maasai (158 born) and Dorper (131 born) lambs evaluated at pasture from birth to 6 months of age. Resistance to GI nematode parasites was assessed in terms of faecal egg counts (FEC), while resilience (tolerance) was assessed in terms of blood packed red cell volume (PCV). The R and 3/4 R lambs were consistently more resistant (lower FEC) and resilient (higher PCV) than the D and 3/4 D lambs and these differences were significant for lambs from 3 to 6 months of age. The difference between the backcrosses for both FEC and PCV was about a half of the difference between the straightbred R and D lambs indicating additive gene action. The D lambs were consistently significantly heavier than the R lambs at all sampling times from birth to 6 months with the difference increasing from a 52% advantage at birth (3.72 versus 2.44kg, respectively) to a 86% advantage for 6-month-old lambs (23.1 versus 12.4kg, respectively). The 3/4 D lambs were also significantly heavier than the 3/4 R at all sampling times and this difference was about a half that between the D and R lambs (5.1 [plus-or-minus sign] 0.2 versus 10.7 [plus-or-minus sign] 0.5kg, respectively for 6-month-old lambs). For pre-weaning mortality there was a significant (P P < 0.05) breed effect for post-weaning mortality (3.5-6 months of age) with the D lambs (15.0%) and 3/4 D lambs (7.3%) having higher mortality rates than the 3/4 R and R lambs (0.8 and 1.4%, respectively). Mortality was attributed mainly to starvation/miss-mothering followed by pneumonia during the pre-weaning period and to haemonchosis and pneumonia from weaning to 6 months of age

Evaluation of the phenotypic performance of a Red Maasai and Dorper double backcross resource population: Indoor trickle challenge with Haemonchus contortus

Six F1 Dorper (D) x Red Maasai (R) rams were mated to both D and R ewes to produce backcross lambs. These six double backcross resource families are being analysed to identify quantitative trait loci that may be controlling resistance to gastro-intestinal (GI) nematode parasites, mainly Haemonchus contortus. After assessing the phenotypic performance of the lambs following exposure to natural infections, the surviving lambs were drenched and moved indoors for an artificial challenge with H. contortus along with straightbred D and R lambs. A total of 1317 lambs were included in the analyses consisting of 523 3/4D, 580 3/4R, 87 D and 127 R. The D lambs were significantly and consistently heavier than R lambs and 3/4D were significantly heavier than the 3/4R lambs. The difference between the backcrosses was about half of that between the straightbreds. Resistance was assessed in terms of faecal egg counts (FEC) and total worm counts (TWC) at necropsy while packed cell volume (PCV) was used to assess resilience to weekly oral doses of 2500 infective larvae of H. contortus. No significant breed differences were observed for log transformed FEC (LFEC). A significant breed difference in PCV was recorded. The backcrosses had the higher values and while no differences were observed between the straightbreds, 3/4D had significantly higher PCV than the 3/4R. Despite the absence of breed differences in FEC the R and the 3/4R had significantly fewer worms than the D and the 3/4D. The D had significantly longer worms than the R and the 3/4D had significantly longer worms than the 3/4R. Worms recovered from D had more eggs than those recovered from R. Similarly worms from 3/4D contained more eggs than those from 3/4R. Thus, on a breed basis the breed with more worms had longer worms. In contrast, when, in a small part of the experiment two doses of larvae were used to check for any breed by dose interactions, worms from the low dose (and hence fewer worms) animals were longer. We postulate that in fast growing hosts like the D, worms also have a better potential for growth and reproduction than in hosts that have less potential for growth. The overall correlation coefficient between PCV and LFEC was -0.67 while that between LFEC and LTWC was 0.72. From these results it is clear that the R do not respond to the artificial challenge the way they do to natural infection suggesting that phenotyping of R and R cross lambs for purpose of selecting those that are resistant or susceptible is best done under natural challenge

Comparison of Red Maasai and Dorper sheep for resistance to gastro-intestinal nematode parasites productivity and efficiency in a humid and a semi-arid environment in Kenya

Disease resistance/tolerance, productivity and efficiency of Red Massai and Dorper sheep evaluated in experiments at the Kenya coast (humid environment) and the Kenya highlands (Semi-arid environment). There were important breed by location interactions for LWT, mortality rates and reproduction rates. When all these parameters were combined it was found more productive and efficient than Dorper sheep in humid coastal environment. In the semi-arid environment Dorper sheep were slightly more productive than the Red Maasai, but there was no significant difference in Flock efficiency between the breeds

Comparison of Red Maasai and Dorper sheep for resistance to gastro-intestinal nematode parasites: Productivity and efficiency in a humid and a semi-arid environment in Kenya

Red Maasai [Masai] and Dorper sheep were evaluated for their resistance to gastrointestinal nematode parasites (predominantly Haemonchus contortus), productivity and efficiency in experiments undertaken at the Kenyan coast (humid environment) and the Kenya highlands (semiarid environment). It was shown that Red Maasai sheep were more resistant and tolerant to endoparasite infections than Dorper sheep in both environments. There were important breed by environment interactions for flock productivity and efficiency, with the Red Maasai sheep being about 5-fold more efficient than Dorper sheep in the humid environment, while in the semiarid environment there was no significant breed difference in efficiency

Genotype by environment interactions for productivity and resistance to gastro-intestinal nematode parasites in Red Maasai and Dorper sheep

Red Maasai and Dorper sheep were evaluated for their resistance to gastro-intestinal (GI) nematode parasites (predominantly Haemonchus contortus), productivity and productive efficiency (assessed on a metabolizable energy basis) in experiments undertaken at the Kenyan coast (sub-humid environment) and the Kenyan highlands (semi-arid environment). In both ewes and lambs there were few significant genotype by environment (G X E) interactions for either resistance (assessed by faecal egg count-FEC) or resilience (assessed by blood packed red cell volume-PCV) to GI nematodes. Red Maasai sheep were few significant G X E interactions were due to very poor performance of the Dorper compared to the Red Maasai in the sub-humid coastal environment and to the much improved performance of the Dorper in the semi-arid environment. When these component traits were combined into estimates of flock productivity and productive efficiency there were highly significant G X E interactions with the Red Maasai sheep being considerably more efficient than Dorper sheep in the sub-humid environment, while in the semi-arid environment there was a negligible breed difference in productive efficiency. The results are discussed in terms of breeding strategies for smallholder farmers and pastoralists managing sheep in low-input systems in sub-humid and semi-arid environments

Genetic variation in resistance to repeated infections with Heligmosomoides polygyrus bakeri, in inbred mouse strains selected for the mouse genome project

Since the publication of the mouse genome, attention has focused on the strains that were selected for sequencing. In this paper we report the results of experiments that characterized the response to infection with the murine gastrointestinal nematode Heligmosomoides polygyrus of eight new strains (A/J, C57BL/6, C3H, DBA/2, BALB/c, NIH, SJL and 129/J), in addition to the well-characterized CBA (poor responder) and SWR (strong responder) as our controls. We employed the repeated infection protocol (consisting of 7 superimposed doses of 125L3 each administered at weekly intervals, faecal egg counts in weeks 2, 4 and 6 and assessment of worm burdens in week 6) that was used successfully to identify quantitative trait loci for genes involved in resistance to H. polygyrus SWR, SJL and NIH mice performed indistinguishably and are confirmed as strong responder strains to H. polygyrus CBA, C3H and A/J mice all tolerated heavy infections and are assessed as poor responders. In contrast, DBA/2, 129/J and BALB/c mice performed variably between experiments, some tolerating heavy worm burdens comparable to those in poor responders, and some showing evidence of resistance, although only in one experiment with female 129/J females and one with female BALB/c was the pattern and extent of worm loss much like that in SWR mice. Because the genetic relationships between six of the strains exploited in this study are now well-understood, our results should enable analysis through single nucleotide polymorphisms and thereby provide more insight into the role of the genes that control resistance to H. polygyrus

Quantitative trait loci for resistance to Haemonchus contortus artificial challenge in Red Maasai and Dorper sheep of East Africa

A genome-wide scan was performed to detect quantitative trait loci (QTL) for resistance to the gastrointestinal nematode Haemonchus contortus in a double backcross population of Red Maasai and Dorper sheep. The mapping population comprised six sire families, with 1026 lambs in total. The lambs were artificially challenged with H. contortus at about 6.5 months of age, and nine phenotypes were measured: fecal egg count, packed cell volume decline, two weight traits and five worm traits. A subset of the population (342 lambs) was selectively genotyped for 172 microsatellite loci covering 25 of the 26 autosomes. QTL mapping was performed for models which assumed that the QTL alleles were either fixed or segregating within each breed, combined with models with only an additive QTL effect fitted or both additive and dominance QTL effects fitted. Overall, QTL significant at the 1% chromosome-wide level were identified for 22 combinations of trait and chromosome. Of particular interest are a region of chromosome 26 with putative QTL for all nine traits and a region of chromosome 2 with putative QTL for three traits. Favorable QTL alleles for disease resistance originated in both the Red Maasai and Dorper breeds, were not always fixed within breed and had significant dominance effects in some cases. We anticipate that this study, in combination with follow-up work and other relevant studies, will help elucidate the biology of disease resistance

High resolution mapping of chromosomal regions controlling resistance to gastrointestinal nematode infections in an advanced intercross line of mice

Fine mapping of quantitative trait loci (QTL) associated with resistance to the gastrointestinal parasite Heligmosomoides polygyrus was achieved on F6/F7 offspring (1076 mice) from resistant (SWR) and susceptible (CBA) mouse strains by selective genotyping (top and bottom 20% selected on total worm count in week 6). Fecal egg counts were recorded at weeks 2, 4, and 6, and the average was also analyzed. Blood packed cell volume in weeks 3 and 6 and five immunological traits (mucosal mast cell protease 1, granuloma score, IgG1 against adult worm, IgG1, and IgE to L4 antigen) were also recorded. On Chromosome 1 single-trait analyses identified a QTL with effects on eight traits located at about 24 cM on the F2 mouse genome database (MGD) linkage map, with a 95% confidence interval (CI) of 20-32 cM established from a multitrait analysis. On Chromosome 17 a QTL with effects on nine traits was located at about 18 cM on the MGD map (CI 17.9-18.4 cM). Strong candidate genes for the QTL position on Chromosome 1 include genes known to be involved in regulating immune responses and on Chromosome 17 genes within the MHC, notably the Class II molecules and tumor necrosis factor

Identification of quantitative trait loci affecting resistance to gastrointestinal parasites in a double backcross population of Red Maasai and Dorper sheep

A genome-wide scan for quantitative trait loci (QTL) affecting gastrointestinal nematode resistance in sheep was completed using a double backcross population derived from Red Maasai and Dorper ewes bred to F1 rams. This design provided an opportunity to map potentially unique genetic variation associated with a parasite-tolerant breed like Red Maasai, a breed developed to survive East African grazing conditions. Parasite indicator phenotypes (blood packed cell volume – PCV and faecal egg count – FEC) were collected on a weekly basis from 1064 lambs during a single 3-month post-weaning grazing challenge on infected pastures. The averages of last measurements for FEC (AVFEC) and PCV (AVPCV), along with decline in PCV from challenge start to end (PCVD), were used to select lambs (N = 371) for genotyping that represented the tails (10% threshold) of the phenotypic distributions. Marker genotypes for 172 microsatellite loci covering 25 of 26 autosomes (1560.7 cm) were scored and corrected by Genoprob prior to qxpak analysis that included Box–Cox transformed AVFEC and arcsine transformed PCV statistics. Significant QTL for AVFEC and AVPCV were detected on four chromosomes, and this included a novel AVFEC QTL on chromosome 6 that would have remained undetected without Box–Cox transformation methods. The most significant P-values for AVFEC, AVPCV and PCVD overlapped the same marker interval on chromosome 22, suggesting the potential for a single causative mutation, which remains unknown. In all cases, the favourable QTL allele was always contributed from Red Maasai, providing support for the idea that future marker-assisted selection for genetic improvement of production in East Africa will rely on markers in linkage disequilibrium with these QTL