Schistosoma intercalatum: an endangered species in Cameroon?

Despite the extremely wide distribution of both intermediate (snail) and definitive hosts, the species Schistosoma intercalatum has a highly restricted distribution in parts of West Africa (Cameroon, Equatorial Guinea, Gabon, Nigeria, Democratic Republic of Congo and S?o Tomé). Recent studies showed that the epidemiology of this species is very dynamic, including invasions and extinctions. This article reviews the epidemiology of S. intercalatum in Cameroon, where the species is endangered, and analyses its overall status in Africa. The possible reasons for the limited distribution of S. intercalatum are discussed.     

Mating interactions between Schistosoma haematobium and S. mansoni

Schistosoma haematobium and S. mansoni are two medically important schistosomes, commonly occurring sympatrically in Africa and so potentially able to infect the same human host. Experiments were designed to study the mating behaviour of these two species in mixed infections in hamsters. Analysis of the data obtained showed that both heterospecific and homospecific pairs readily form. No significant difference was seen between the two species in their ability in forming pairs, however, S. mansoni showed a greater homospecific mate preference. Analysis of the data using the Mantel-Haenszel test suggests that mating competition does occur between S. haematobium and S. mansoni, the former being the more dominant species. Both species appeared to be able to change mate, with S. haematobium showing a greater ability in taking S. mansoni females away from S. mansoni males when introduced into a pre-established S. mansoni infection highlighting the competitiveness of S. haematobium. The significance of the results is discussed in relation to the epidemiological consequences occurring in Senegal, and other areas where both species are sympatric.     

Observations on Schistosoma intercalatum in south-east Gabon

Observations were made in the field and laboratory to determine the strain characteristics of Schistosoma intercalatum in south-east Gabon. For an isolate from Franceville, data are given for egg shape, behaviour of cercariae, seven enzyme systems separated by isoelectric focusing, and intermediate host specificity. Isolates from Cameroun (Edea) and Zaire (Kisangani) were included in a comparative study of the enzymes; Franceville and Edea isolates resembled each other but differed from the Zaire isolate in hexokinase and phosphoglucomutase. The Franceville isolate was polymorphic in phosphoglucomutase and glucosephosphate isomerase. The sum of characters indicates that S. intercalatum as known from south-east Gabon belongs to the strain found in Cameroun and western Gabon, rather than to the strain known from Zaire. More information is needed on strain distribution, particularly for an area including western Zaire and the Republic of the Congo, which appears to separate the two known strains.     

A comparison of the characteristics of two strains of Schistosoma intercalatum Fisher, 1934 in mice

Schistosoma intercalatum from Cameroon and Zaire each have their specific snail intermediate host. Hybridization experiments between S. intercalatum from Cameroon and Zaire have resulted in viable miracidia for only two generations. If the two strains are of the same species, cross breeding should be possible for several generations. This paper presents a comparison of the characteristics of the two strains of S. intercalatum in experimental infections in mice. Worm return, male-female worm ratio, growth rate, number of testes in male adult worms are compared. The quantitative distribution of eggs in the tissues of the mice is recorded. A comparison of the size and shape of the parasite eggs is described as are the changes in the weight of the liver and spleen of the infected mice. It is considered that the differences observed between the two strains of S. intercalatum do not necessitate a division of the two strains into two separate species.     

Inter-specific parasite competition: mixed infections of Schistosoma mansoni and S. rodhaini in the definitive host

Competition between parasite species has been predicted to be an important force shaping parasite and host ecology and evolution, although empirical data are often lacking. Using the Mus musculus-Schistosoma mansoni and Schistosoma rodhaini host-parasite systems we characterized mate choice and inter-specific competition between these two schistosome species. Simultaneous infections revealed species-specific mate preferences for both species as well as suggesting mating competition, with male S. rodhaini appearing dominant over male S. mansoni. S. rodhaini homologous pairs were also shown to have increased reproduction per paired female in the presence of a competitor in simultaneous infections. Overall total reproductive success was, however, similar between the two species under conditions of direct competition due to the greater initial infectivity of S. mansoni in comparison to S. rodhaini. Inter-specific competition was also implicated as increased parasite virulence to the host. The potential effects of such interactions on parasite and host ecology and evolution in nature are discussed.     

Studies of the relationship between Schistosoma and their intermediate hosts. III. The genus Biomphalaria and Schistosoma mansoni from Egypt, Kenya, Sudan, Uganda, West Indies (St. Lucia) and Zaire (two different strains: Katanga and Kinshasa)

The compatibility between strains of Schistosoma mansoni from Egypt, Kenya, Sudan, Uganda, the West Indies, and Zaire (two strains which came from Katanga and from Kinshasa), and various species and strains of Biomphalaria, i.e. Biomphalaria pfeifferi, B. alexandrina, B. glabrata and B. camerunensis was investigated. Data as mortality, rate of infection of the surviving snails, duration of infection, cercarial production per day per positive snail, etc., were observed. The main emphasis was placed on determining the total cercarial production per 100 exposed snails for each snail population. It was possible to infect all the tested populations of B pfeifferi with the various strains of S. mansoni, but the observation as e.g. TCP/100 exposed snails varied greatly according to the population of snail and the strain of S. mansoni. The results for the remaining species of Biomphalaria varied greatly, depending on the combination, e.g. B. alexandrina was only susceptible to the local S. mansoni from Egypt. The highest TCP/100 exposed snails was more than 1 million for the strains of S. mansoni from Egypt, Kenya and the West Indies in B. alexandrina, B. pfeifferi and B. glabrata, respectively. The next group, with a TCP/100 exposed snails on 7--800 000 consists of S. mansoni from Sudan, Uganda and Zaire (Katanga) all in B. pfeifferi. The last tested strain of S. mansoni, Zaire (Kinshasa) yielded a cercarial production on 500 000 per 100 exposed snails in B. pfeifferi and B. camerunensis. The shortest prepatent period, 19 days, was observed for S. mansoni from Kinshasa, Zaire, in B. camerunensis, and the longest prepatent period, 25 days, was found for strains from Egypt and from the West Indies in B. alexandrina and B. glabrata, respectively. In general, a very long duration of infection, lasting up to 200 days, was observed.     

A revision of the Lower Guinean Hepsetus species (Characiformes; Hepsetidae) with the description of Hepsetus kingsleyae sp. nov

An α‐taxonomic revision of the African pike, Hepsetus odoe, from Lower Guinea is provided. The results show that three different species occur in Lower Guinea instead of one. Hepsetus akawo, recently described from West Africa, is present in the northern part of Lower Guinea; Hepsetus lineata, the most widespread species within Lower Guinea, is known from the Sanaga (Cameroon) in the north to the Shiloango (Democratic Republic of the Congo) in the south and Hepsetus kingsleyae sp. nov. is endemic to the Ogowe Basin. The new species H. kingsleyae is described and H. lineata, which is elevated here to the species level, is redescribed. Hepsetus lineata can easily be recognized by its prominent horizontal line pattern on the flanks and differs further from H. akawo and H. kingsleyae in the number of lateral‐line scales and the number of gill rakers. Hepsetus kingsleyae differs from H. lineata and H. akawo by its narrow head, elongated snout and narrow, knife‐shaped body. All three species are also distinguishable from H. odoe and the recently revalidated H. cuvieri. A few exceptional specimens could not be allocated to one of the three species and may represent hybrids because of their mixed diagnostic characters or their intermediate values.     

Intraspecific competition and mating between fungal strains of the anther smut Microbotryum violaceum from the host plants Silene latifolia and S. dioica

Abstract We studied intraspecific competition and assortative mating between strains of the anther smut fungus Microbotryum violaceum from two of its host species, Silene latifolia and S. dioica . Specifically, we investigated whether strains from allopatric host populations have higher competitive ability on their native host species and show positive assortative mating. In general, strains isolated from S. latifolia outcompeted strains isolated from S. dioica on both host species, but in female hosts, heterotypic dikaryons (i.e., dikaryons composed of a haploid strain originating from S. latifolia and a haploid strain originating from S. dioica ) were most successful in competition. Furthermore, the latency period was significantly shorter for heterokaryons that contained at least one strain originating from S. latifolia , compared to heterokaryons that only contained strains originating from S. dioica . The frequencies of conjugations between strains originating from S. latifolia were much higher than conjugation frequencies between strains originating from S. dioica . A significant positive correlation was detected between the relative success of strains in competition and in conjugation, suggesting that success of a strain in competition might be partly determined by its swiftness of mating. In addition, reciprocal differences within heterotypic crosses revealed a significant effect of fungal mating type, with mating type a₁ being the main determinant of mating pace. The observed differences in infection success, conjugation rate, and latency period in favor of strains from S. latifolia relative to strains from S. dioica on both host species are discussed in an evolutionary context of opportunities for the maintenance of differentiation between different formae speciales upon secondary contact.     

Carbohydrate metabolism in adult schistosomes of different strains and species

In this comparative study the carbohydrate metabolism of Schistosoma japonicum, S. haematobium, S. intercalatum, S. bovis and three strains of S. mansoni was investigated. No large differences were found in the protein and glycogen contents of all species involved. In all species investigated, lactate was the main end product of carbohydrate breakdown. However, all parasites degraded part of the glucose to CO2 via the Krebs cycle. No difference was observed in the contribution of this aerobic process to energy production in the three strains of S. mansoni investigated. The differences in Krebs-cycle activity between the five schistosome species investigated were very small. Therefore, this study refutes the idea that significant differences exist in the carbohydrate metabolism of various schistosome species or strains.     

Reproductive interference via display signals: the challenge of multiple receivers

Sexually selected traits important in both mate and competitor recognition provide an opportunity to understand the tradeoffs associated with reproductive and competitive interference. When co-occurring species compete over similar resources, selection may promote signal similarity to facilitate competitive interactions in opposition to selection for signal divergence to maintain assortative mating. Bird song provides a classic example of contrasting selection on signal design, because songs function both in mate discrimination and in territorial advertisement. Similarity in songs aids competitor recognition both within and across species, and song convergence or mixing is widespread in the songbirds. Two related mechanisms can maintain mate recognition in the face of song convergence. First, multiple recognition signals, both across and within signaling modalities, provide a basis for mate and competitor discrimination using different sets of cues. Second, stricter female song preferences may allow interspecific male–male competitive communication without compromising female mate discrimination. I suggest that increased understanding of the neurobiology underlying song recognition will provide insight into the relative importance and prevalence of these different mechanisms along a continuum of species divergence.     

Analysis and comparison of cercarial emergence rhythms of Schistosoma haematobium, S. intercalatum, S. bovis, and their hybrid progeny

In identical experimental conditions, the three schistosomes of the terminal spined egg group (S. haematobium, S. intercalatum and S. bovis) showed significant differences in their cercarial shedding patterns. The cercariae of the F1 hybrids, obtained by experimental crosses between these three species, showed the same circadian emergence rhythm (with one peak) as the cercariae of the parental species. However, the mean shedding time of these hybrid parasites (12:14 +/- 1 h 34 min for S. haematobium x S. intercalatum; 09:58 +/- 1 h 24 min for S. haematobium x S. bovis; 08:57 +/- 1 h 19 min for S. bovis x S. intercalatum) was always in advance to the one of their parental species (13:51 +/- 2 h 04 min for S. haematobium; 13:59 +/- 1 h 55 min for S. intercalatum; 10:09 +/- 2 h 04 min for S. bovis). These results are compared with those obtained from crosses between schistosomes with lateral spined eggs, and from crosses between intraspecific chronobiological variants of S. mansoni. They corroborate the genetic determinism of the cercarial emergence of schistosomes. Moreover, such significant differences between the hybrids and their parents in the times of cercarial emission may be of value in the epidemiological research and characterization of naturally occurring hybrids.     

Assortative Mating and Lack of Temporality Between Corn and Rice Strains of Spodoptera frugiperda (Lepidoptera,Noctuidae) from Central Colombia

Spodoptera frugiperda is a Neotropical moth that has diverged into “corn” and “rice” strains. In the US, prezygotic isolation studies have shown that both populations mate assortatively. In addition, recent studies have demonstrated that mating by the same strain individuals is enhanced by an allochronic shift in mating activity and male pheromones are important during courtship. In Colombia, studies on mate choice have never been performed previously, although earlier analyses made in populations from Central Colombia showed no significant differences in time of first mating and copula duration between the strains. Here, we performed multiple choice experiments using a tetrad design composed of individuals of the corn and the rice strains. We found that corn strain females rarely mate with rice strain males, but rice strain females mate with both strains of males. In addition, no temporal isolation was found. A ML (maximum likelihood) approach was used to discriminate between mating propensity and mate choice behaviors in S. frugiperda strains. This approach showed that mating propensity of corn strain males is three times greater than rice strain males. In contrast, in females, the propensity of mating was slightly higher for the corn strain. Finally, the isolation index between the corn and the rice strains from Colombia produced a value of I?=?0.33. Our results suggest that prezygotic isolation at the behavioral and temporal levels differ between the US and Colombia. Moreover, in the US temporal isolation appears to have an important role in behavioral isolation but in Colombia temporality is not necessary to reduce encounters between S. frugiperda strains.     

ITS2 ribosomal RNA indicates Schistosoma hippopotami is a distinct species

The internal transcribed spacer region of the ribosomal RNA, ITS2, was sequenced from a singlé specimen of S. hippopotami collected from a pulmonary artery of the hippopotamus, Hippopotamus amphibius in South Africa. The nucleotide sequence was aligned with those of S. mansoni, S. rodhaini, S. haematobium, S. intercalatum, S. curassoni. S bovis and S. japonicum. Both maximum parsimony and genetic distance analyses were performed on these data sets. Using S. japonicum as outgroup to the African schistosomes, a single most-parsimonious tree was obtained of length 64 steps with a consistency index of 1. S. hippopotami was the sister-group to the remaining African species. This species has lateral-spined eggs and its basal position in the tree suggests that this condition is primitive and that terminal-spined eggs developed secondarily. Molecular data clearly show that S. hippopotami cannot be considered synonymous with S. mansoni. Assuming the hippopotamus is the normal host of S. hippopotami, phylogenetic analysis is consistent with an ancient association between schistosomes and ungulates.     

Evidence for a discrete evolutionary lineage within Equatorial Guinea suggests that the tsetse fly Glossina palpalis palpalis exists as a species complex

Tsetse flies of the palpalis group are major vectors of Human African Trypanosomiasis in Africa. Accurate knowledge of species identity is essential for vector control. Here, we combine ribosomal internal transcribed spacer 1 ( ITS1 ), mitochondrial Cytochrome Oxidase 1 ( COI ) and microsatellites to determine the population structure and phylogenetic relations of Glossina p. palpalis in Equatorial Guinea. CO1 sequence data suggest that G. p. palpalis in Equatorial Guinea is a distinct subspecies from previously described G. p. palpalis in West Africa and Democratic Republic of Congo. Glossina p. palpalis in Equatorial Guinea and DRC share a common ancestor which diverged from West African G. p. palpalis around 1.9 Ma. Previous ITS1 length polymorphism data suggested the possible presence of hybrids in Equatorial Guinea. However, ITS1 showed incomplete lineage sorting compared with clearly defined COI groups, and data from 12 unlinked microsatellites provided no evidence of hybridization. Microsatellite data indicated moderate but significant differentiation between the populations analysed (Rio Campo, Mbini and Kogo). Moreover, unlike previous studies of G. p. palpalis , there was no evidence for heterozygote deficiency, presence of migrants or cryptic population structure. Variance effective population size at Rio Campo was estimated at 501–731 assuming eight generations per year. This study of the population genetics of G. p. palpalis in central Africa provides the first estimate of genetic differentiation between geographically separated G. p. palpalis populations.     

Schistosomiasis at Loum, Cameroun; Schistosoma haematobium, S. intercalatum and their natural hybrid.

A survey of 500 schoolchildren in Loum in 1968 revealed an overall infection rate of 54.2% with Schistosoma intercalatum and this was the only species of schistosome encountered. In 1972 a number of children were found to be passing schistosome eggs in their urine and these eggs ranged in shape and size from the forms characteristic for S. haematobium to those of S. intercalatum. Preliminary laboratory studies demonstrated that hybridisation between the two species was occurring. Subsequent field surveys showed that the snail hosts for the two parasites (B. rohlfsi for S. haematobium and B. forskali for S. intercalatum) were both present in the river Mbette and its tributaries in Loum and the distribution of the two snail species coincided closely with the distribution of the schistosomes in the human population. Detailed study of a small group of children passing hybrid eggs in their urine revealed that few of them were passing eggs in their faeces and that those eggs which were found in faeces were not viable. Analysis of schistosome egg-shape by plotting cumulative size-frequency data on probability paper demonstrated that the graph obtained from a natural hybrid series was different from that given by a known mixture of the two separate species. The hybrid series included a number of exceptionally large eggs resembling those of S. bovis but isolation of these eggs and subsequent laboratory passage of the parasites showed that they were part of the series and were not evidence of the presence of a third species. Hybridisation experiments in the laboratory showed that the cross S. haematobium male X S. intercalatum femal is fully viable but that the reverse mating is not successful, thus accounting for the failure of the faecal eggs recovered from children with hybrid infections. Histological results from laboratory passaged hybrids suggest that the Ziehl-positive staining reaction of the egg-shells of S. intercalatum may be a recessive character. The observations reported here indicate that S. haematobium has only recently become established in Loum and that it is, through introgressive hybridisation, replacing the indigenous S. intercalatum. A suggested explanation for the change in the parasite fauna is offered and this depends upon ecological changes resulting from forest clearance and agricultural development providing improved conditions for the spread of B. rohlfsi, the snail host for S. haematobium. It is suggested that, in contrast to recent reports on the spread of S. intercalatum, this species is in fact retreating and being replaced by S. haematobium in areas where forest clearance is taking place. In conclusion it is suggested that introgressive hybridisation of this kind may have been responsible for the evolution of certain characteristic local strains of African schistosomes.     

Interspecific variation within the 'hypervariable' region of the 18S ribosomal RNA gene among species of Schistosoma Weinland, 1858 (Digenea)

The 'hypervariable' region of the 18S ribosomal DNA gene, comprising 344 base pairs, was sequenced for five species of the genus Schistosoma (S. intercalatum, S. margrebowiei, S. mattheei, S. malayensis and S. rodhaini), representing three of the schistosome species groups. Heterobilharzia americana was also included as an outgroup. The sequences were compared with previously published sequences (S. haematobium, S. japonicum, S. mansoni and S. spindale). The analysis confirmed that the Asian schistosomes were distantly related to the other groups of schistosomes and showed that the amount of variation within the S. japonicum group (1.49%) was greater than that detected within the S. mansoni and S. haematobium groups (0% in each case).     

Conservation of CD4+ T cell-dependent developmental mechanisms in the blood fluke pathogens of humans

Schistosoma blood flukes are trematode parasites with a cosmopolitan distribution that infect over 200 million people globally. We previously showed that Schistosoma mansoni growth and development in the mammalian host is dependent on signals from host CD4+ T cells. To gain insight into the mechanisms that underlie this dependence, we sought to determine the evolutionary origins and limits of this aspect of the host-pathogen relationship. By infecting RAG-1-/- mice with a range of different schistosome species and strains, we tested several hypotheses concerning the time during Schistosoma evolution at which this dependence arose, and whether this dependence is specific to Schistosoma or is also found in other blood flukes. Our data indicate that the developmental dependence on CD4+ T cells previously described for S. mansoni is conserved in the evolutionarily basal species Schistosoma japonicum, suggesting this developmental adaptation arose early in Schistosoma evolution. We also demonstrate that the development of the more evolutionarily derived species Schistosoma haematobium and Schistosoma intercalatum are dependent on adaptive immune signals. Together, these data suggest that the blood fluke parasites of humans utilise common mechanisms to infect their hosts and to co-opt immune signals in the coordination of parasite development. Thus, exploitation of host-schistosome interactions to impair or prevent parasite development may represent a novel approach to combating all of the schistosome pathogens of humans.     

Intra- and Intersexual Conflicts and Cooperation in the Evolution of Mating Strategies: Lessons Learnt From Ungulates

The traditional narrow focus on male mate competition in studies of mammalian mating strategies has been successful in explaining many conspicuous outcomes of sexual selection. However, focusing on ungulates, I here review increasing evidence that a comprehensive understanding of sexually selected adaptations requires consideration of a broader range of sex roles, particularly those relating to female behaviour. I show how mate competition and mate choice in both sexes can interact and often lead to intra- and intersexual conflicts and sometimes also cooperation. By causing phenotypic selection, these behavioural interactions can shape evolution although the outcome depends on genetic constraints. I conclude that to advance the understanding of mammalian mating systems we must broaden our conceptual framework from being largely focused on male competitive strategies to encompass coevolutionary dynamics between the mating strategies of both sexes. Future challenges include clarifying (1) the ecological basis for the inter- and intraspecific diversity in sex roles, (2) the condition-dependence of female mating strategies, and (3) the ultimate causes of mate competition and promiscuity in females.     

Reproductive interference, priority effects and the maintenance of parapatry in Skistodiaptomus copepods

Reproductive interactions among species, such as attempts to mate with heterospecifics, can have profound impacts on patterns of geographic range and co-occurrence. For example, several theoretical models demonstrate that reproductive interference-the negative influence of heterospecifics on the abilities of conspecifics to successfully mate-can lead to parapatry, even across homogeneous landscapes, when the parapatric species are initially allopatric. However, the potential consequences of reproductive interactions on patterns of range limits and co-occurrence have been largely ignored by ecologists. Here, I use a combination of laboratory mating experiments and a genetic survey for two parapatric species of Skistodiaptomus copepods to evaluate the potential importance of interspecific reproductive interactions on the maintenance of parapatry. The genetic survey demonstrates phylogenetic exclusivity of these species, suggesting that gene flow has not occurred between them. Moreover, laboratory crosses between both species demonstrate that their parapatric boundary is not maintained as a hybrid zone because the two species cannot form viable hybrids. However, Skistodiaptomus oregonensis and S. pygmaeus males mate indiscriminately and interfere with the ability of heterospecific females to successfully reproduce. I suggest that reproductive interference results in priority effects that strongly influence the maintenance of this parapatric boundary and stress that reproductive traits are important and often-overlooked in ecological models for species range limits.     

Mate choice assays and mating propensity differences in natural yeast populations

In sexual microbes, mating occurs by fusion of individual cells. This complete fitness investment suggests that cell behaviour could potentially mediate prezygotic isolation between microbial species, a topic about which very little is known. To investigate this possibility, we conducted individual cell mate choice trials and mass-culture mating propensity assays with isolates from sympatric natural populations of the closely related yeasts Saccharomyces cerevisiae and Saccharomyces paradoxus. Although we found no evidence for active species recognition in mate choice, we observed a marked difference in mating propensity between these two species. We briefly discuss the possibility that this mating propensity difference may contribute to reproductive isolation between S. cerevisiae and S. paradoxus in nature.