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Schärer Group
Evolutionary Biology
Zoological Institute
University of Basel
Vesalgasse 1
CH-4051 Basel

Reproductive biology of a parasitic simultaneous hermaphrodite, Schistocephalus solidus

a living Schistocephalus solidus

The reproductive biology of the tapeworm Schistocephalus solidus was the subject of my PhD at the University of Berne (1996 to 2000). I was supervised by Claus Wedekind and Manfred Milinski, and I am very grateful for what they have taught me in these stimulating years. I mainly tried to understand how these tapeworms mate with each other and how they allocate to male and female reproduction. Below I give a short introduction to Schistocephalus solidus, an overview of the projects we did, and links to the main papers.
Introduction to Schistocephalus solidus
The pseudophyllidean tapeworm Schistocephalus solidus is a simultaneous hermaphrodite parasite that reproduces in the intestine of fish eating water birds, and eggs pass into the water with the faeces. If after hatching the free swimming first larval stage, the coracidium, is ingested by the first intermediate host, a cyclopoid copepod (e.g. Macrocyclops albidus), the second larval stage develops in the hemocoel of this host. Infectivity to the second intermediate host, the three-spined stickleback, Gasterosteus aculeatus, is reached within one to two weeks. Infection of the fish occurs upon ingestion of an infected copepod. The third larval stage, the plerocercoid, grows in the peritoneum of the fish and reaches infectivity to the final host after one to three months (Life Cycle). Here is a picture of the anatomical organization of the plerocercoid (Plerocercoid Anatomy). S. solidus is unusual among tapeworms in that the larvae at this stage are completely segmented and all but the first ten or so segments contain a full set of genitalia, which are in an advanced stage of development but immature (Many Cirri). The reproductive system primarily consists of yolk glands, testes, and ovaries containing rather inactive cells. After ingestion by the final host, the larvae mature and start to produce eggs within two days (Adult Anatomy) (Detail Adult Anatomy). In vivo, reproduction takes place within one to two weeks, after which the worms die. In the studies I present below we used an in vitro system (i.e. virtual birds) to replace the final host.
Social Situation and Strategic Egg Production
In this study, in collaboration with Claus Wedekind and Dora Strahm, we investigated the reproductive strategies played by the worms when they reproduce alone or in pairs in the in vitro system. When worms are alone, self-fertilization is the only possible mating mode, whereas worms placed in pairs have the opportunity to engage in both self- and cross-fertilization. The costs and benefits of self- vs. cross-fertilisation are important factors in determining the potential benefits that can be achieved by cooperation in reproduction with another worm. We therefore investigated if the number, volume and quality of eggs produced were affected by the social situation to which the worms were experimentally assigned. Further, we determined the energetic content of worms and investigated variation in egg size between the two social situations. (Wedekind et al. 1998)
Lifetime Reproduction
In this study, in collaboration with Claus Wedekind, we investigated in detail one result of the above study. We were interested in explaining the origin of a lower egg production in paired individuals after three days in the experiment. This difference suggested a cost of mating and we wanted to distinguish between the influence of three non-exclusive hypotheses that could lead to such a cost. A) Pairs may start reproduction later than isolated individuals, possibly due to mate choice or mate assessment. Such a delay would indicate the existence of sexual selection. B) Pairs may have a lower rate of egg production, possibly due to gamete trading, which would indicate a time cost of some form of cooperation strategies played by the worms. C) Pairs may attain a lower magnitude of egg production, possibly due to an increased male allocation. This would indicate the occurrence of sperm competition and reproductive conflict. We designed an in vitro system that allowed for measure lifetime egg production of worms under the two social situations with high temporal resolution (16 Virtual Birds) (Detail of Virtual Birds) (Sampling the Virtual Birds). (Schärer & Wedekind 1999)
Size-dependent Sex Allocation
Here, in collaboration with Mira Christen, Lars Karlsson and Claus Wedekind, we aimed at describing the sex allocation patterns that are established during the growth in the second intermediate host, the three-spined stickleback. We were particularly interested in learning to what extent sex allocation varied between individual worms, and whether there was size-dependent sex allocation, a frequently encountered condition in simultaneous hermaphrodite plants. We first developed a method based on stereology, which allows for quantitative and unbiased estimation of volumes of reproductive structures from histological sections. We then used this method to establish the relationships between individual worm size and the allocation to the different male and female reproductive structures. Finally, we looked for independent evidence for size dependent sex allocation patterns by reanalysing data from the first study. (Schärer et al. 2001)
Social Situation and Sex Allocation
In this study, in collaboration with Claus Wedekind, we used the method developed in the previous study to investigate the influence on sex allocation of the number of worms encountered by a worm in the final host (here replaced by the in vitro system). Sex allocation theory predicts an increased investment into the male function with increasing mating group size. Moreover, to give data on sperm competition, we further inferred different patterns of sperm transfer from the amount of stored autosperm (i.e. sperm that has been produced by a worm and that is ready to be used for insemination) and received allosperm (i.e. sperm that has been received through insemination). (Schärer & Wedekind 2001)
Measuring Cestode Larvae in Living Copepods
This project, in collaboration with Claus Wedekind, Mira Christen and Nathalie Treichel, was aimed at devising a method that allows for detailed measurement of growth and development of parasite larvae hatched from eggs laid by worms that had been cultured in the in vitro system. Such a method will allow studying the fitness of offspring derived from different modes of reproduction in their natural environment. The method uses a video-microscope, which is connected to a desktop computer for image analysis. It takes advantage of the fact that a) we can keep the first intermediate host, the copepod Macrocyclops albidus, in the laboratory under controlled conditions, and b) the host is nearly transparent, which allows to measure parasite size in its natural environment repeatedly and non-invasively. We determined the volumes of a wide size range of hosts and parasites and established the repeatabilities of the measurements of different parameters. (Wedekind et al. 2000)

this page was last updated on Tuesday, December 22, 2015