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. So fare, in the lab, we worked mainly with S. papillosus, a common parasite of sheep and goats, which can be raised in rabbits and with S. ratti, a parasite of rats. Recently, in order to bring our laboratory and field projects closer together, we shifting more and more to the human parasite S. stercoralis, which we maintan in Mongolian Gerbils as laboratory hosts. For comparison, we also maintain free living cultures of Parastrongyloides trichosuri, a closely related facultative parasite of Australian possums and the non parasitic close relative Rhabditophanes diutinus (previously known as Rhabditophanes. sp. KR3021, Dulovic et al. 2020).
Tool and resource development
Since Strongyloides spp. are not yet established as research models like, for example, the model nematode Caenorhabditis elegans, part of our efforts are dedicated to the developmetn of tools and resources for Strongyloides research. 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, at the time at the University of Bristol, now Universityof Liverpool, Nemetschke et al. 2010) and we established a protocol for chemical mutagenesis of S. ratti (Guo et al. 2015). Recently, we devised a protocol for RNAi gene knock down in S. ratti (Dulovic and Streit 2019). We were part of the Strongyloides genome consortium (Hunt et al. 2016) and we followed up the genomic work with transcriptomic (mRNA and small RNA) studies (Baskaran, Jaleta et al. 2017; Jaleta et al. 2017a; Holz et al. 2017). We continue to improve the S. papillosus genome assembly and we optimised the methods for single worm genotyping and whole genome sequencing (Zhou et al. 2019b).
Life cycle switches
Strongyloides spp. have intersting life history switches. First, a parthenogentic female (the parasitic female) produces progeny of two sexes (male and female) that differ in their chromosomal make up. Second, female progeny of the parasitic females swich between developing into infective third stage larvae (iL3s) and, upon entry into a host, into adult parasites and becoming free-living sexually reproducing adults. Interstingly the progeny of the free-living generation do not have these options. They are invariably female and develop into iL3s.
Sex determination and sex chromosomes:
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 40 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). 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 (Nemetschke et al. 2010). These findings, in combination with comparative studies we performed in collaboration with Warwick Grant (La Trobe University) on the sister taxon Parastrongyloides trichosuri (Kulkarni et al. 2013), as well as the whole genome sequences of multiple species of Strongyloides (Hunt et al. 2016) strongly support the chromosome fusion hypothesis .
We attempt to understand, how it is achieved that all larvae produced by the free-living generation are female. So fare we showed that in S. papillosus genetically male determining mature sperm is not formed (Nemetschke, 2010; Kulkarni, 2016a). Surprisingly, in S. ratti null-X sperm and also some very early embryos with a male karyotype appear do exist but the male embryos may be unviable (Kulkarni, 2016a). In order to further investigate how the production of male determinign sperm is avoided, we are conducting a detaild analysis of the spermatgenesis im S. papillosus and S. ratti and for comparison, in Parastrongyloides trichosuri, which does produce male progeny.
The parasitic - free-living switch:
This switch is considered to be homologous to the switch between dauer and non-dauer development in some free-living nematodes like C. elegans or Pristionchus pacificus. Using pharmacological and RNAi experiments we could show that, like in C. elegans and P. pacificus, the nuclear hormone receptor DAF-12 is a key player in this switch in Strongyloides spp. in the progeny of the parasitic and of the free-living generations (Ogawa et al. 2009; Dulovic and Streit 2019).
People involved in this project:
- Veroni Amarasinghe
- Dorothee Harbecke
- Xiaoxiao Yin
- Alex Dulovic
- Anna Dyka
- Alexander Eberhardt
- Li Guo
- Anja Holz
- Tegegn Jaleta
- Arpita Kulkarni
- Linda Nemetschke
- Siyu Zhou
- Olga Zhukova