Worm Breeder's Gazette 13(2): 26 (February 1, 1994)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
The beginning Strongyloides is an obligate parasitic nematode. It is the parasitic nematode taxonomically closest to Caenorhabditis. Its life-cycle (see below) consists of two distinct adult generations; one female and parasitic and the other dioecious and free-living. The free-living adult generation (known as heterogonic development) can be omitted and instead larvae can develop to infective L3 sdirectly (homogonic development). Cytological studies have concluded that the parasitic generation reproduces by mitotic parthenogenesis, and the free-living generation by pseudogamy (maternal inheritance only)(1). Here I describe a genetic analysis of the life-cycle of S. ratti.
Doing genetics with Strongyloides
Clones
A rat can be infected with a single L3 ,which gives rise to a single parasitic female. Larvae produced by this worm are able to develop both directly and indirectly. We refer to such infections (and the population derived from these) as clones, but they should probably be more correctly called isofemale lines.
Controlled matings
Virgin free-living males and females can be grown by collecting early stage larvae and maintaining them individually until they are mature. Virgin males and females from different clones can be brought together so that mating can occur. The progeny can be cloned back into rats or can be analyzed directly.
Genetics of the free-living generation
Crosses have been made between individual virgin males and virgin females of different clones (the parental clones). The resulting progeny of such crosses have also been cloned (the progeny clones). Parental and progeny clones have been analyzed by minisatellite fingerprinting (Jeffreys' probe 33.15)(2). The fingerprints showed that inheritance was bi-parental, and thus that pseudogamy did not occur. The inheritance patterns are fully consistent with the occurrence of "normal" sexual reproduction(3). These results were incompatible with the earlier cytological observations.
Genetics of the parasitic generation
Parthenogenesis can occur by a number of cytological mechanisms. In some cases, progeny are identical to each other and to their mother (mitotic and some forms of meiotic parthenogenesis). In other cases the progeny differ from each other and from their mother (other forms of meiotic parthenogenesis). In view of the quite different conclusions drawn from the cytological and genetic data, I have also analyzed the progeny of single parasitic females with respect to the functional difference (mitotic or meiotic) in parthenogenetic reproduction.
Larval progeny of individual parasitic females have been analyzed for a RFLP in a PCR fragment of C. elegans actin 4 (4). Alleles of this locus segregated in a Mendelian manner in crosses of the free-living generation. Parasitic females that were heterozygous for this marker produced progeny all of which were heterozygous. Thus, it was concluded that the parasitic female was functionaIly mitotic. The genetic and cytological studies were not in conflict for this stage.
The future
Strongyloides is a parasitic nematode with a life-cycle that is amenable to genetic manipulation. Admittedly it isn't quite as easy to maintain or work with as C. elegans, but it is the most accessible of the parasitic nematodes.
I would be happy to supply further details of the methods used or the work itself, if anyone is interested.
References
1. Bolla & Roberts, 1968, J. Parasit., 54, 849-855.
2. Jeffreys, et al., 1985, Nature, 314, 67-73.
3. Viney, et al., 1993, Proc. R. Soc. Lond. B, 254, 213-219. 4. Krause, et al., 1989, J. Mol. Biol., 208, 381-392.