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QTL for Oestrus ovis resistance in the Sarda x Lacaune population

Larvae of Oestrus ovis (Insecta: Diptera: Oestridae) are common parasites of nasal and sinus cavities of sheep and goats causing losses of meat, wool and milk production. Nasal discharges and sneezing are commonly found in summer and constitute the major clinical signs of in O. ovis infected sheep.

During larval development, a specific immune reaction is initiated by the host with a humoral local and systemic response and the recruitment of eosinophils and mast cells in the upper airways mucosae. Moreover, an immunization of sheep with excretory-secretory products (ESP) of larvae provided an inhibitory effect on larval growth. Reducing Oestrus ovis mature larval weight could have a considerable effect on adult fly viability and consequently on adult populations.These results suggested that at least a partial immune regulation of O. ovis larvae populations could occur within the sheep.

As previously shown by Scala A. (unpublished data), the highest values of systemic IgG were reached in July-August in ewes and decreased in November-December. By considering the chronobiology of oestrosis in Sardinia and the seasonal pattern of the IgG response, the chosen period to investigate the oestrosis resistance in BC Sarda x Lacaune ewes was mid-July.

Blood samples were collected in July 2001 (n = 869), July 2002 (n = 789) and July 2003 (n = 699) on all BC SxL ewes and sera were analysed in the National Veterinary School of Toulouse. Additional sera were taken in April 2002 to monitor the IgG decrease during the winter 2001-2002. Serum specific IgG titers were evaluated by ELISA according to Jacquiet et al., 2005. Prevalences of Oestrus ovis infections in BC SxL were very high according to the ELISA data: from 91.5% in 2001 to 82.5% in 2003. High individual variability was shown in the intensities of IgG responses: from 0 to 276% in 2001, 0 to 213% in 2002 and 0 to 183% in 2003. Nevertheless, high phenotypic correlations were shown between the different years. This is of particular importance because IgG titers returned to very low levels during winter 2001-2002. Therefore, IgG titers monitored in July 2002 correspond to an immune reaction to newly acquired infections in spring and early summer 2002.

For the whole experimental period, a significant ram (or family) effect (P < 0.001) was shown in the intensity of specific IgG response. Daughters of three sires had high IgG levels during the three years of study whereas daughters from one other sire had low levels, the others being intermediate.ELISA data (2001, 2002, and 2003) were analyzed for QTL detection. A square root transformation of data was done in order to normalize the distribution.

Two QTL detections were done: one of the whole set of data and a second one on data over 20% of antibodies only (considered as high responders in ELISA). 2001, 2002 and 2003 data were considered as single traits and were corrected for age and group of management. Two July measurements (2001 AND 2002) and three July measurements (2001 AND 2002 AND 2003) were considered as repeated traits and corrected for year and group of management (mixed procedure SAS). Interval mapping by within-sire linear regression was used and rejection thresholds were evaluated by 10, 000 within family permutations. The QTLMAP software (INRA) was used.QTL have been found on chromosomes 1, 2, 15, 17 and 21. Two of them (QTL on chromosomes 17 and 21) are highly significant (P < 0.01). Others have a P value comprised between 0.01 and 0.038. Some QTL were detected on single traits (one year data only) (chromosomes 1, 15, 17, 21) whereas QTL on chromosomes 2 was established on repeated traits (two years data, 2001 and 2002). Averages substitution effects were high especially for QTL on chromosomes 1 and 17.

The prevalence of O. ovis infections in Sardinia was high and similar to those recorded in other Mediterranean areas as Sicily or Greece. Intensities of larval infections were not checked in this study but the presence of obvious clinical signs in July is in favour of the presence of large numbers of developing stages within the ewes.

A high individual variability of specific IgG titers, a family effect on O. ovis IgG values and the high repeatability of these values from one year to another suggest that the intensity of this antibody response is, at least partially, under genetic control. This is the first time that QTL associated with Oestrus ovis specific immune response were identified. Two QTL (chromosomes 17 and 21) were strongly associated with the intensity of IgG response, nevertheless, they were found with 2003 data only. Only one QTL (chromosome 2, position 245-250 cM) was found with two sets of data (2001 single trait and 2001 AND 2002, repeated trait). Three common informative families were identified.

The chromosomes exhibiting the QTL identified in this study do not have any candidate obvious to us and this requires further work to identify the genes underlying these regions.

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Reported by

National Veterinary School of Toulouse
31 076 Toulouse