Nematode development

Group Leader: Adrian Streit

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

Members of the group:

Li Guo
Dorothee Harbecke
Julia Hildebrandt
Arpita Kulkarni
Linda Nemetschke
Olga Zhukova

Alumni:

Alexander Eberhardt
Stephan Knierer
Ryuji Minasaki
Viktoria Wegewitz

Projects:

1) Strongyloides papillosus 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, which can be raised in rabbits and S. ratti, a parasite of rats.

Figure 1: Left: Life cycle of Strongyloides papillosus. Right: Scanning electron micrographs of the front parts of a free-living female (top) and an infective larva (bottom).

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. In other species, no chromosomal difference between the sexes has been found. These species have only two pairs of chromosomes, one of which is considerably larger than the other. It has been speculated that this is the result of a fusion of the X chromosome with one of the autosomes. For S. papillosus there are contradictory reports. While some authors found no chromosomal differences between the sexes, others described that in males a portion of one chromosome is eliminated, thereby creating a hemizygous region (sex specific chromatin diminution).

We have confirmed that in our isolates of S. papillosus embryos with a diminuted karyotype exist among the progeny of the parasitic generation. We are now combining molecular and genetic approaches to address the following questions:

-Are there genes in the eliminated region, and if yes, which ones?

-Does this region correspond to the X chromosome in other species of Strongyloides?

-How is it controlled that the entire progeny of the free-living generation receives a complete set of chromosomes.

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

2) Spatiotemporal control of the Hox gene ceh-13 in the nematode Caenorhabditis elegans

The Hox gene ceh-13 encodes a transcription factor that is orthologous to the mammalian HOX1 and the Drosophila Labial proteins. In contrast to other C. elegans Hox genes, ceh-13 is required for viability. Animals that lack ceh-13 function arrest as embryos or early larvae with severe, predominantly anterior, body morphology defects. ceh-13 is expressed in well defined cells at various time points throughout development. Not only loss of function but also ectopic expression of ceh-13 has deleterious effects on embryogenesis. Therefore, appropriate spatiotemporal control of ceh-13 is crucial for development. In our lab we concentrate on the characterization of factors that, directly or indirectly, regulate the ceh-13 transcription in the early embryo.

Figure 2: Early C. elegans embryo expressing a ceh-13::gfp reporter gene, differential interference contrast, fluorescence and merged.

We have identified an enhancer fragment (enh740) that is sufficient to drive a GFP reporter gene in a pattern indistinguishable from ceh-13 in the early embryo (Figure 1). To further define important sequence elements in enh740 promoter and to find the factors that act through them we use a combination of classical genetics, mutational analysis, phylogenetic foot printing, yeast-one-hybrid screens and candidate gene approaches.

People involved in this project: Stephan Knierer, Ryuji Minasaki, Viktoria Wegewitz.

3) If females can reproduce by themselves, why are there males?

In both nematode species we are working with, there exists a "choice" between self-reproduction and out-crossing. In S. papillosus parasitic females can either produce parasitic offspring that reproduce parthenogenetically or they can give rise to a sexually reproducing free-living generations.

Figure 3: Decline of the proportion of males in laboratory cultures of different strains of C. elegans over time.

In C. elegans, the two sexes are hermaphrodites and males. Hermaphrodites can either reproduce by virtue of self-fertilization or they can mate with a male and use the male derived sperm to fertilize their eggs. While virtually the entire progeny that is produced by self-fertilization is hermaphroditic, half of the cross-progeny is male. We and many others have noticed that males disappear from laboratory cultures of some wild isolates of C. elegans while they persist in others (Fig. 3). In collaboration with Hinrich Schulenburg at the University of Kiel, we address the questions what might cause these differences in male maintenance and under what circumstances out-crossing or self-fertilization might be favored.

Wegewitz, V., Schulenburg, H. and Streit, A. (2010). Do males facilitate the spread of novel phenotypes within populations of the androdioecious nematode Caenorhabditis elegans. Journal of Nematology, in press.

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.

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.

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.