Population genetic diversity in Spirocerca lupi

Abstract

Spirocerca lupi is a nematode that parasitises canid species across the world. Infested hosts show symptoms associated with the disease called spirocercosis. The parasite is known to cause significant damage to its final host and often leads to death. Treatment of the disease has been a challenge for veterinarians for many years since symptoms of the disease only become apparent at advanced stages of the disease, by which time, the available treatment is considered ineffective. Many studies to date have given insight into the characterisation and description of spirocercosis, however very few studies have been conducted on the molecular biology, biochemistry, genetics, epidemiology, ecology and host-parasite interactions. This study utilises molecular tools to perform genetic analyses to better understand the parasite’s population structure which will contribute to improved strategies for the treatment, prevention and control of spirocercosis. By integrating a population genetic approach with molecular marker data, it is possible to decipher the transmission dynamics of a parasite. For this purpose, microsatellite markers were developed using the FIASCO (fast isolation by AFLPs of sequences containing repeats) protocol and 454 pyrosequencing. Nine polymorphic microsatellite loci were developed to conduct population genetic analyses on S. lupi nematodes sampled across three geographical locations in South Africa, namely KwaZulu-Natal (Durban), Eastern Cape (Grahamstown) and Gauteng (Tshwane Metropole). Some of these loci proved to be effective in cross-species amplification testing. The loci were also used to provide molecular evidence that S. lupi is in fact found in jackal. Microsatellites proved to be effective markers in detecting subtle levels of population structuring between the three geographical locations, however allelic frequencies indicated that high amounts of gene flow was occurring. High levels of heterozygosity were found in individual hosts as well as between different hosts, suggesting that little or no inbreeding occurs between S. lupi nematodes within the final host. This could be due to the life cycle of the parasite. Spirocerca lupi uses a dung beetle intermediate host as well as several paratenic hosts, which allows for significant mixing of parasite genotypes before reaching the final host. In this way, the final host acquires a highly diverse genetic mixture of parasites, which influences mating patterns and results in outbreeding. This is the first study on S. lupi that uses co-dominant markers to study genetic variation and epidemiology across a wide geographical range. The implications of this study are that since S. lupi populations have high levels of genetic diversity, they have the genetic potential to adapt to changing environmental conditions as well as the ability to withstand possible treatments that are administered to final hosts. This is critical to consider when control mechanisms are implemented aimed to reduce the risk of infestation in canids. Additional research is required on the impact that different life cycle stages of the nematode have in the different hosts and its implication for effective prevention and control of spirocercosis.