Preliminary work in a previous ACIAR project suggested that Indonesian Thin Tail (ITT) sheep carry an inherent resistance to F. gigantica, which may be controlled by a single gene - a unique situation in terms of known resistances to large parasites. It is also possible that the same gene is responsible for the partial resistance to H. contortus. This would suggest a broad effect of the gene, because H. contortus is a nematode worm whereas F. gigantica is a flatworm.
Identification of the gene would allow scientists to look for similar genes in cattle and buffalo, because there is a high degree of homology between the genomes of these animals and sheep. Genetic markers could then be used to allow selective breeding of strains of these animals that would be resistant to internal parasitic worms. There are even potential spinoffs in terms of understanding the interplay between parasite and host in several serious human parasitic diseases.
ITT sheep are naturally highly resistant to the liver fluke Fasciola gigantica, a common sheep parasite. Recent work has suggested that the breed also has good resistance to Haemonchus contortus, a nematode worm that is another destructive parasite of sheep. The aim of this project is to unravel the immunological basis of this unusual resistance and, in particular, to identify the gene or genes responsible for the resistance to F. gigantica.
Genetic crossing between resistant ITT sheep and susceptible Merino sheep will be carried out to produce a backcross flock from which resistant individuals will be selected. DNA from these animals will be analysed and candidate resistance genes identified. By the end of the project, markers (DNA sequences) consistently found in animals showing resistance will be used to suggest an approximate location for the gene.
At the same time, immunological studies of ITT sheep will find out how exactly the immune systems of the sheep eliminate the parasites. By selectively 'knocking out' arms of the immune system it should be possible to identify which part is responsible for conferring resistance. The immune responses of resistant and tolerant sheep will then be compared to see if the differences are related to the features known to be responsible for parasite elimination. The immunological techniques required were developed in ACIAR project 9049 and are established in Australia and Indonesia.
Internal parasites Haemonchus contortus and tropical liver fluke Fasciola gigantica adversely affect ruminants in Asia and Africa. Infection of animals with these parasites leads to a decrease in productivity. This project aimed to determine the genetic basis of the resistance of ITT sheep to F. gigantica and H. contortus.
The project developed leading edge techniques that were subsequently transferred to Indonesia. Most of the scientific milestones were met, but further analysis of the F2 populations is under way to better understand the genetic basis of F. gigantica resistance.
A major scientific outcome has been evidence of a gene strongly linked to the expression of H. contortus resistance and three possible gene loci for resistance to F. gigantica. A by-product of the project has been the identification of gene loci important to wool growth and carcass traits such as growth and composition. Currently, large amounts of research funding in Australia are spent on these production topics without producing the results that this project has already achieved. This information is considered of potential value to both the Indonesian and Australian livestock industries.
Links:
[1] http://www.aciar.gov.au/country/Indonesia
[2] http://www.aciar.gov.au/programarea/Animal Health