Genetics of mating and sex determination in the parasitic nematode Haemonchus contortus

Abstract

Genetic analysis of parasitic nematodes has been a neglected area of research and the basic genetics of this important group of pathogens are poorly understood. Haemonchus contortus is one of the most economically significant livestock parasites worldwide and is a key experimental model for the strongylid nematode group that includes many important human and animal pathogens. We have undertaken a study of the genetics and the mode of mating of this parasite using microsatellite markers. Inheritance studies with autosomal markers demonstrated obligate dioecious sexual reproduction and polyandrous mating that are reported here for the first time in a parasitic helminth and provide the parasite with a mechanism of increasing genetic diversity. The karyotype of the H. contortus, MHco3(ISE) isolate was determined as 2n = 11 or 12. We have developed a panel of microsatellite markers that are tightly linked on the X chromosome and have used them to determine the sex chromosomal karyotype as XO male and XX female. Haplotype analysis using the X-chromosomal markers also demonstrated polyandry, independent of the autosomal marker analysis, and enabled a more direct estimate of the number of male parental genotypes contributing to each brood. This work provides a basis for future forward genetic analysis on H. contortus and related parasitic nematodes.

Figure 1.—

Examples of DAPI-stained mitotic metaphase spreads of early H. contortus embryos. (A) Embryo containing 11 chromosomes. (B) Embryo containing 12 chromosomes (ends of 2 of the chromosomes overlap in this specimen).

Figure 2.—

Haplotypes of the six X-linked microsatellite markers and their relative frequencies in the MHco3(ISE) isolate based on genotyping 56 adult male H. contortus worms. Numbers represent the alleles of each marker (marker names are shown across the top). *, haplotype 9 was not identified in the original 56 male worms genotyped but was identified in the progeny of MHco3(ISE) adult female 2 (see supplemental Table S4B). It is not unexpected that some rarer haplotypes may not have been represented in the 56 male worms originally genotyped.