Echinostoma trivolvis: mating behavior of adults raised in hamsters

Mating behaviour of Echinostoma trivolvis and E. paraensei in concurrent infections in hamsters

Young adults of Echinostoma trivolvis and E. paraensei were recovered from hamsters previously infected with metacercarial cysts. Some worms of each species were exposed for 1 h to 3-H-tyrosine to label sperm and transplanted singly to uninfected hamsters with several unlabelled worms of the same or opposite species or both species. After 5 days, recovered worms were processed for paraffin sectioning and autoradiography. The resulting slides were observed for the location of radioactive sperm in the seminal receptacles of donor (labelled) and recipient (unlabelled) worms. When E. trivolvis was the donor with the recipient E. paraensei, self-insemination took place, but only one interspecies mating occurred out of 72 possible recipient worms. When E. paraensei served as the donor, self-insemination again occurred, but no cross-insemination was observed among the 59 E. trivolvis recipient worms. When single donor worms had a choice of either species of recipient worms, no interspecies mating took place, but both self- and cross-insemination occurred in the normal, unrestricted behaviour found in single species mating studies. Rates of both self- and cross-insemination were higher in concurrent infections of both recipient species than in single species mating studies. After transplant, both species localized in their natural habitat within the small intestine, with 1/3 overlapping in the duodenum, making interspecies mating a possibility. The correlation between mating and electrophoretic studies on the genetic relationship between 37-collar-spined echinostomes is discussed.

Molluscan and vertebrate immune responses to bird schistosomes

There is a growing understanding of risks posed by human contact with the cercariae of bird schistosomes. In general, there are no fundamental biological differences between human and bird schistosomes in terms of their interactions with snail and vertebrate hosts. The penetration of host surfaces is accompanied by the release of penetration gland products and the shedding of highly antigenic surface components (miracidial ciliated plates and cercarial glycocalyx) which trigger host immune reactions. New surface structures are formed during transformation: the tegument of mother sporocysts and the tegumental double membrane of schistosomula. These surfaces apparently serve as protection against the host immune response. Certain parasite excretory-secretory products may contribute to immunosuppression or, on the other hand, stimulation of host immune reactions. Discovery of new species and their life cycles, the characterization of host-parasite interactions (including at the molecular level), the determination of parasite pathogenicity towards the host, the development of tools for differential diagnosis and the application of protective measures are all topical research streams of the future. Regularly updated information on bird schistosomes and cercarial dermatitis can be found at http://www.schistosomes.cz (web pages of Schistosome Group Prague).

Recent Advances in the Biology of Echinostoma species in the “revolutum” Group

This review examines the significant literature on the biology of Echinostoma species in the "revolutum" group. We have considered 10 species belonging to this group. There is a considerable body of literature for four of the species, i.e. Echinostoma caproni, E. trivolvis, E. paraensei and E. revolutum. For these species we have arranged coverage to include the following headings: (1) systematic and descriptive studies; (2) experimental, manipulative and ecological studies; (3) physiological and biochemical studies; (4) immunological and molecular studies. For the remaining six species, i.e. E. friedi, E. miyagawai, E. echinatum, E. parvocirris, E. luisyrei and E. jurini, the literature is not very extensive, and headings were not used. Considerable information in various areas of modern parasitology can be obtained from species in the "revolutum" complex for which the entire life cycle is maintained in the laboratory. The review includes a list of researchers and their addresses who currently maintain such life cycles.

EGG SIZE VARIABILITY IN TREMATODES: TEST OF THE BET-HEDGING HYPOTHESIS

The hypothesis according to which egg size variability in hermaphroditic parasites results from bet-hedging was investigated in a comparative analysis using trematodes as a model. We hypothesized that the species reproducing mainly by self-fertilization should produce smaller eggs than those species that regularly practice cross-fertilization. Indeed, because self-fertilization is usually associated with inbreeding depression, selection should favor individuals spreading the risk of genetically disturbed development across more but smaller eggs, instead of producing fewer eggs, each possessing a large resource supply, of which many may fail to develop because of genetic deficiencies. On the basis of earlier theoretical and empirical studies, we assumed that the ratio length of testis-length of ovary positively correlates with the mating group size and, hence, with opportunities for cross-fertilization. In accordance with the bet-hedging hypothesis, we found, across trematode species, a positive relationship between this ratio and the mean egg volume produced by adults. This result was, however, observed only for the trematodes infecting birds and not for the species infecting fishes and mammals. In addition, once the influence of trematode phylogeny was taken into account, there was no significant trend, suggesting that phylogenetic legacies played a large role in generating the previous signal. Experimental tests of the bet-hedging hypothesis will be necessary to clarify the matter.

Is there a fixed number of niches for endoparasites of fish?

Kennedy and Guégan, based on Cornell and Lawton (J Anim Ecol 1992;61:1-12), found a "curvilinear" relationship (the "fundamental form" of the relationship for parasite communities) between infracommunity and component community richness, and interpreted this as meaning that only processes acting within the infracommunities can explain the limitation in the number of parasite species in a given host. The research described here shows that an asymptotic relationship is the consequence of the differential likelihoods of parasite species to appear in an infracommunity as determined by transmission rates and intrinsic lifespans. Processes operating at the infracommunity level are not necessary to explain the curvilinear relationship. Even communities much richer than those found in European freshwater fishes cannot be assumed to be "saturated" in the sense that further species cannot be added over evolutionary time.

Mating behaviour of Echinostoma caproni and E. trivolvis in concurrent infections in hamsters

Young adults of Echinostoma caproni and E. trivolvis were recovered from hamsters. Some worms of each species were exposed to [3H]tyrosine for 1 h to label sperm, and these were transplanted singly to uninfected hamsters with various combinations of unexposed worms of either the opposite species or both species. Worms recovered 5 days later were serially sectioned, processed for autoradiography and observed for the location of radioactive sperm. Interspecies mating was detected when E. caproni was the sperm donor and E. trivolvis the recipient, but not the converse. The cross-insemination rate during interspecies mating was very low (13%) when compared to the normal rate of E. caproni intraspecies mating (52%). When single donor adults of either E. caproni or E. trivolvis had a choice of both recipient species, no interspecies mating took place, but both self- and cross-inseminated in a non-restrictive mating pattern typical of echinostome species. After transplantation, both species localized in their normal habitat within the hamster intestine. However, 25% of opposite species recoveries were found within 1 cm of each other, making interspecies mating a possibility.

Mating behaviour of Echinostoma caproni and E. paraensei in concurrent infections in mice

Ten-day-old adults of Echinostoma caproni and E. paraensei, some of which had been exposed to 3-H-tyrosine to label sperm, were transplanted to mice in various combinations. Mating behaviour was followed on autoradiograms of worms recovered after 5 days by detecting the transfer of labelled sperm from labelled worms to themselves or to unlabelled worms. When single, labelled E. caproni adults were transplanted with unlabelled E. paraensei, they self-inseminated but did not show evidence of interspecies mating. No interspecies mating but self-insemination was observed when single, labelled E. paraensei and unlabelled E. caproni were transplanted together. When the labelled species had a choice of unlabelled adults of its own species or the opposite species, it mated in similar fashion to that seen in single species infections. The labelled species acted as if the opposite species was not present and showed an unrestricted mating pattern where it would both self- and cross-inseminate. Even though after transplant the E. paraensei adults were found in the duodenum and the E. caproni adults were found in the ileum, approximately 25% of the transplanted worms of both species were found within 1 cm of each other. Thus habitat separation was not a major cause of the lack of interspecies mating. Lack of reproductive recognition is another reason to separate E. caproni and E. paraensei into distinct species.

The mating behaviour of Echinostoma paraensei grown in mice

Adults of Echinostoma paraensei grown in mice were exposed to 3-H-tyrosine and transplanted to uninfected mice singly or with unexposed worms. The presence of sperm labelled with the radioactive tyrosine in the seminal receptacles of labelled and unlabelled worms recovered after 5 days indicated the mating pattern of this species. Of nine isolated, labelled worms recovered, four had self-inseminated (44%). Of the 39 unlabelled found with eight labelled worms, only eight exhibited labelled sperm for a cross-insemination rate of 21%. Six of these eight labelled worms had self-inseminated (75%) in addition to the cross-insemination. This pattern of mating is termed unrestricted, unlike the restricted mating seen in three species of eyeflukes, which will not self-inseminate in groups. In contrast to the other species of echinostomes investigated, very little sperm was found in the seminal receptacles of the serially sectioned E. paraensei. Adults were always found in the duodenum, even when transplanted in the ileum.

The mating behaviour of Zygocotyle lunata adults grown in mice

Zygocotyle lunata adults grown in mice were treated in vitro with 3-H-adenosine to label sperm and then transplanted to the caecum of mice singly or with 6 to 8 unlabelled adults. After 6 to 10 days, worms were recovered, processed for autoradiography, and observed for radioactive sperm in their seminal receptacles. Adults would not incorporate radioactive thymidine into dividing cells. Tyrosine was utilized only by juvenile worms and then not enough to label germinal cells for mating studies. Four of 13 (31%) worms transplanted singly self-inseminated, whereas 9 of 10 (90%) labelled worms in multiple infections self-inseminated. When in groups, the labelled worms cross-inseminated with 14 of 43 (33%) unlabelled worms. This mating pattern was unrestricted in that both self- and cross-insemination took place in multiple infections, much like the mating pattern in another paramphistomatid, Megalodiscus temperatus, but unlike the restrictive pattern shown by three species of eyeflukes.

Niche restriction in parasites: proximate and ultimate causes

Hutchinson's (1957) definition of an ecological niche as a multidimensional hypervolume determined by a number of physical and biotic variables is adopted. The number of niche dimensions is very great, but as a working hypothesis it is assumed that a few are sufficient to characterize the niche of a parasite species to a high degree of accuracy. They are host species, microhabitat(s), macrohabitat(s), geographical range, sex and age of host, season, food and hyperparasites. Methods to measure niche width, in particular specificity indices, are discussed, and some examples of niche restriction are described. Proximate and ultimate causes of niche restriction are discussed, mainly using marine parasites as examples. Among proximate causes of one niche dimension, host specificity, are ecological factors restricting exposure to infection to certain host species; host-specific chemical factors that induce hatching, direct infective stages to a host and bring about settlement of a parasite; factors that lead to mortality in or on the wrong host; morphological adaptations that guarantee survival in or on the 'correct' host; and availability of suitable hosts. Many factors are likely to be responsible for microhabitat specificity, but have been little studied, except for some physiological and morphological adaptations to particular microhabitats. Macrohabitats and geographical range may be determined by the distribution of intermediate hosts and certain food items, and by a variety of chemical and physical factors. Hosts of different sexes may differ in feeding habits and the composition of the skin, and thus acquire parasites differentially. Hosts of different age may be differentially infected due to accumulation of parasites with age, loss of parasites due to developing resistance (or immunity), and different size and feeding habits. Among ultimate causes of niche restriction and segregation are avoidance of competition, predation and hyperparasites; facilitation of mating; reinforcement of reproductive barriers; and adaptations to environmental complexity. Few studies permit a decision on which factor or factors are responsible in particular cases. Interspecific competition may play a greater role in helminth communities of some host groups than of others, but it seems that, overall, its role has been exaggerated at least for marine parasites. Some 'classical' examples of microhabitat segregation explained by interspecific competition can also be explained by reinforcement of reproductive barriers. There is evidence for the importance of facilitation of mating in microhabitat restriction, and the availability of many vacant niches indicates that competition, overall, is not of great importance.