The parasitic nematode group

(Photos: A. Streit and A. Renz)

Group Leader: Adrian Streit

Secretary: Lorena M. Cali-Özer
Phone: +49 (0)7071 - 601 441
Fax: +49 (0)7071 - 601 498

Projects:

1) Strongyloides spp. a parasitic nematode with a facultative free-living generation

The nematode genus Strongyloides consists of parasites that live as parthenogenetic females in the small intestines of their vertebrate hosts. In addition to producing parasitic offspring, Strongyloides spp. can also form a facultative free-living generation with males and females. A generalized life cycle of Strongyloides sp. is shown in Figure 1. For a general introduction into the biology of Strongyloides sp. by Mark E. Viney and James B. Lok click here. We work mainly with S. papillosus, a common parasite of sheep and goats, which can be raised in rabbits and S. ratti, a parasite of rats.

Figure 1: Left: Life cycle of Strongyloides papillosus (from Nemetschke et al., 2010). Right: DIC micrographs of parasitic (top, photo: L. Nemetschke) and free-living (bottom) S. papillosus adults (from Streit, 2008).

Genetics

Classical genetic approaches are rarely used with metazoan endo-parasites, largely because the adult stages are usually hidden within hosts, making controlled crosses difficult. The existence of a free-living generation in Strongyloides spp. offers a remarkable opportunity for the experimental manipulation of a parasite. We would like to explore this opportunity and conduct genetic screens in Strongyloides spp. We established a genetic linkage map for S. ratti (in collaboration with Mark Viney, University of Bristol) and we are analyzing and comparing the inheritance and linkage of molecular genetic markers in S. ratti and in S. papillosus. We are particularly interested in differences between the two species, which relate to their different sex determining systems.

Sex determination and sex chromosomes

Interestingly, the sex determining mechanisms vary within the genus Strongyloides. There are species with true sex chromosomes such that individuals with two X chromosomes (plus two pairs of autosomes) are female and individuals with one X are male. Other species, for example S. papillosus have only two pairs of chromosomes, one of which is considerably larger than the other. Already more tan 30 years ago it was speculated that this is the result of a fusion of the X chromosome with one of the autosomes. In old, cytological studies some authors found no chromosomal differences between the sexes of S. papillosus. Others described that in males a portion of one chromosome is eliminated, thereby creating a hemizygous region (sex specific chromatin diminution). Recently, by combining cytological and molecular genetic approaches, we demonstrated that in S. papillosus males an internal portion of one of the two larger chromosomes is eliminated. Further we showed that the region undergoing chromatin diminution contains a high number of genes and is homologous to the X chromosome of S. ratti. The portions of the longer chromosome that is not diminished corresponds to chromosome number I of S. ratti. These findings strongly support the chromosome fusion hypothesis. Further we observed that males do not pass on their diminished chromosome to their progeny. This results in a male specific transmission ratio distortion for genetic loci closely linked with the diminished region.

People involved in this project:Alexander Eberhardt, Li Guo, Linda Nemetschke, Dorothee Harbecke, Arpita Kulkarni, Olga Zhukova

2) Population biology of the filarial nematode Onchocerca ochengi a bovine parasite closely related to the causing agent of river blindness

The filarial nematode Onchocerca ochengi is a parasite of cattle in tropical regions. It is closely related with O. volvulus that causes human onchocerciasis, commonly known as river blindness. The two species of Onchocerca share the insect intermediate host Simulium damnosum. Reproducing adult females of O. ochengi live in nodules in the skin of their host. Males, which are much smaller than the females, are present in the nodules in very small numbers, frequently only one male per nodule. The young first stage juveniles, called microfilariae, leave the nodule of their mother and wait to be taken up by the intermediate host during a blood meal.

The questions we ask include:

Do multiple males contribute genetically to the progeny of a particular female?

Do all reproductive females contribute to the pool of microfilariae in the skin of the host?

For this project we collaborate closely withAlfons Renz from the University of Tübingen and with theProgramme Onchocercoses, a Euro-African research network in Ngoundéré, Cameroon.

People involved in this project: Julia Hildebrandt

Previous projects:

Recent Publications:

Sommer, R. J. and Streit, A. (2011). Comparative Genetics and Genomics of Nematodes: Genome Structure, Development, and Lifestyle. Annual Review of Genetics 45, 1-20 (Review).

Nemetschke, L., Eberhardt, A. G., Hertzberg, H. and Streit, A. (2010). Genetics, chromatin diminution and sex chromosome evolution in the parasitic nematode genus Strongyloides. Current Biology, 20, 1687-1696.
     Dispatch to this article: Charlesworth, B. (2010). Sex Determination: A Worm Does It by Elimination. Current Biology, 20, R841-R843.

Streit, A. and Sommer, R. J. (2010). Random expression goes binary. Nature 463, 891-892 (News and Views).

Nemetschke, L., Eberhardt, A. G., Viney, M. E. and Streit, A. (2010). A genetic map of the animal-parasitic nematode Strongyloides ratti. Molecular and Biochemical Parasitology 169, 124-127.

Wegewitz, V., Schulenburg, H. and Streit, A. (2009). Do males facilitate the spread of novel phenotypes within populations of the androdioecious nematode Caenorhabditis elegans. Journal of Nematology 41, 247-254.

Minasaki, R., Puoti, A. and Streit, A. (2009). The DEAD-box protein MEL-46 is required in the germ line of the nematode Caenorhabditis elegans. BMC Developmental Biology 9, 35.

Ogawa, A., Streit, A., Antebi, A. and Sommer, R. J. (2009). A conserved endocrine mechanism controls the formation of dauer and infective larvae in nematodes. Current Biology 19, 67-71.
Summary of this article, written for a broad audience (in German). Appeared in the Newsletter of the German Society for Parasitology 1/2009

Eberhardt, A. G., Mayer, W. E., Bonfoh, B. and Streit, A. (2008). The Strongyloides (Nematoda) of sheep and the predominant Strongyloides of cattle form at least two different, genetically isolated populations. Veterinary Parasitology 157, 89-99.

Wegewitz, V., Schulenburg, H. and Streit, A. (2008). Experimental insight into the proximate causes of male persistence variation among two strains of the androdioecious Caenorhabditis elegans (Nematoda). BMC Ecology 8, 12.

Streit, A. (2008). Reproduction in Strongyloides (Nematoda): a life between sex and parthenogenesis. Parasitology 135, 285-294 (Review).

Eberhardt, A. G., Mayer, W. E. and Streit, A. (2007). The free-living generation of the nematode Strongyloides papillosus undergoes sexual reproduction. International Journal for Parasitology 37, 989-1000.

Minasaki, R. and Streit, A. (2007). MEL-47, a novel protein required for early cell divisions in the nematode Caenorhabditis elegans. Molecular Genetics and Genomics 277, 315-328.