Biomphalaria tenagophila: dominant character of the resistance to Schistosoma mansoni in descendants of crossbreedings between resistant (Taim, RS) and susceptible (Joinville, SC) strains

GENERAL BIOLOGY

Biomphalaria tenagophila: dominant character of the resistance to Schistosoma mansoni in descendants of crossbreedings between resistant (Taim, RS) and susceptible (Joinville, SC) strains

Florence Mara Rosa^I; Ana Lúcia Brunialti Godard^II; Vasco Azevedo^II; Paulo Marcos Zech Coelho^III,1

^IDepartamento de Parasitologia

^IIDepartamento de Biologia Geral, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brasil

^IIILaboratório de Esquistossomose, Centro de Pesquisas René Rachou-Fiocruz, Av. Augusto de Lima 1715, 30190-002 Belo Horizonte, MG, Brasil and Santa Casa de Misericórdia de Belo Horizonte, Belo Horizonte, MG, Brasil

ABSTRACT

The aim of the present work was to study parasitological, molecular, and genetic aspects in descendants of crossbreedings between a totally resistant Biomphalaria tenagophila strain (Taim, RS) and another one highly susceptible (Joinville, SC) to Schistosoma mansoni. Descendants F₁ and F₂ were submitted to S. mansoni infection (LE strain). The susceptibility rates for individuals from Group F₁ were 0 to 0.6%, and from Group F₂ was 7.2%. The susceptible individuals from Group F₂ discharged a lower number of cercariae, when compared with the susceptible parental group, and in 2 out of 9 positive snails the cercarial elimination was discontinued. In order to identify genetic markers associated with resistance the genotype of parental snails and their offspring F₁ and F₂ were analyzed by means of the randomly amplified polymorphic DNA method. Nevertheless, it was not possible to detect any marker associated to resistance, but the results showed that in the mentioned species the resistance character is determined by two dominant genes.

Key words:Biomphalaria tenagophila - dominant resistance character - Schistosoma mansoni - crossbreedings

A number of studies has been carried out aiming at determining and understanding the susceptibility and resistance levels of the mollusk genus Biomphalaria to Schistosoma mansoni. Several researchers while studying the relationship between B. glabrata and S. mansoni were able to observe that B. glabrata from various localities presented different susceptibility levels, as well as that some variation occurred in relation to the infectivity of S. mansoni to snails (Files & Cram 1949, Kuntz 1952, Barbosa & Barreto 1960, Paraense & Correa 1963).

Newton (1952) established for the first time that susceptibility is a hereditary character in B. glabrata. Richards (1970) demonstrated that in B. glabrata the resistance character of the mollusk to S. mansoni infection, which is acquired at the maturity phase, is monogenic and dominant. The relationship between parasite and host is very complex, and S. mansoni genetic factors interact in the infection process (Richards 1975). However, the snail genotype seems to exert a crucial and determinant role in this relation (Richards et al. 1992).

Some molecular strategies have been used to identify in the snail genome the regions that are associated with the resistance phenotype (Knight et al. 2000). One of those strategies consists in carrying out crossbreeding between resistant and susceptible snails, plus identification of the resistance markers, which are chosen among descendants F₁ and F₂. Recently, two markers (1.2 Kb and l.0 Kb) were identified by means of the randomly amplified polymorphic DNA (RAPD) technique, in a resistant B. glabrata strain (Knight et al. 1999).

As can be observed, almost all the studies dealing with genetic aspects of resistance to infection were carried out using B. glabrata snails. In this way, the nature of the gene (or genes) that is (are) able to confer resistance in B. tenagophila is poorly known.

B. tenagophila is a planorbid with a wide distribution in South America (Paraense 1984), and has epidemiological importance, since it maintains the life cycle of the trematode S. mansoni in some areas in Brazil. In spite of its dominance in some areas, B. tenagophila is found in nature with low rates of infection. However, it is responsible for the majority of the autochthonous cases of schistosomiasis in the state of São Paulo (Brazil), as well as for the foci of the disease in the states of Minas Gerais and Santa Catarina (Brazil) (Paraense 1986). This last focus is maintained by a highly susceptible population (Joinville, Santa Catarina, Brazil) of mollusks to S. mansoni. There is a B. tenagophila population, indigenous to the Ecological Station at Taim, state of Rio Grande do Sul, Brazil, that systematically has been found to be resistant to different S. mansoni strains, as shown in various laboratory experiments (Santos et al. 1979). The scope of the present work was to carry out crossbreedings between this resistant strain and a highly susceptible one (Joinville), as well as to study parasitological, molecular and genetic aspects of their offspring in the presence of S. mansoni infection.

The results obtained in this study could be the basis for future strategies on the control of schistosomiasis in areas where transmission of the disease is maintained by B. tenagophila.

MATERIALS AND METHODS

Snails -B. tenagophila Taim (from the Ecological Station at Taim, state of Rio Grande do Sul, Brazil) is the resistant strain to S. mansoni infection used in this experiment. This strain has been maintained at the laboratory of the Schistosomiasis Research Unit, Federal University of Minas Gerais, Brazil, for more than 20 years, without any kind of selection favoring the resistance character, since these snails are naturally resistant to S. mansoni. The albino B. tenagophila strain from Joinville, state of Santa Catarina, Brazil, has been maintained at the same laboratory, and was kindly provided by Dr Paraense in October 2002. This strain is highly susceptible to S. mansoni infection.

Crossbreedings - Ten B. tenagophila couples were used in this study. In order to obtain crossbreeding, a susceptible B. tenagophila specimen (albino) from Joinville was placed into a plastic recipient containing 100 ml dechlorinated water, together with a resistant B. tenagophila specimen from Taim. The snails measured approximately 5 mm in diameter, and were sexually immature. The snail couples were kept together for 50 days. After that, the couples were isolately maintained for obtention of F₁ generation. In this experiment, eggs of albino and pigmented parental snails were collected for 4 weeks. It must be emphasized that the eggs obtained from pigmented parental snails were separately maintained, in order to confirm that they were generated from crossbreeding only after obtention of F₂ generation. On the other hand, when eggs were obtained from albino parental snails, soon at the first week it was possible to identify whether there was an effective crossbreeding or not. These eggs were observed by means of a stereomicroscope. When the snails were pigmented, their embryos showed ocular spots at the base of each antenna, thus indicating that F₁ individuals were generated by crossbreeding. For obtention of F₂ descendants, 10 snail couples were used. F₂ generation was obtained by crosses between F₁ individuals according to the previously described procedures.

Parasites - The S. mansoni strains used were LE (Belo Horizonte, MG) and SJ (São José dos Campos, SP), both kept under laboratory conditions for 35 and 20 years, respectively.

Snail infection - The parental snails and their offspring F₁ and F₂, aged 3 months approximately, were individually exposed to 25 miracidia, according to Pellegrino and Katz (1968). In order to assess the susceptibility of F₁ and F₂ descendants in the presence of S. mansoni, two distinct experiments were carried out at different periods.

Methods for analysis of snail infection - Aiming at verifying the emergence of S. mansoni cercariae, the snails were individually kept in small Snap-cap glass recipients, with dechlorinated water, and exposed to artificial light for 2 h. From then onwards the snails were observed weekly under the stereomicroscope up to 30-90 days after exposure to miracidia. The negative snails were squeezed between slide and coverslip in search of sporocysts. The positive snails had their respective number of cercariae counted as follows: the material containing the larvae and fixed in formol was centrifuged at 1000 rpm for 1 min, the supernatant was discarded and the cercariae resuspended in 2 ml water. After homogenization, a sample of 0.2 ml was collected, and the cercariae present in the sample were counted under stereomicroscope. The number of cercariae so obtained was multiplied by ten for assesssment of the total number shed by the snail.

DNA extraction and RAPD analysis - After the parasitological test, the snails were submitted to DNA extraction, according to the procedure described by Vidigal et al. (1994). The DNA sample of each individual was amplified using final concentrations of l5 µg. A pool with DNA from F₂ susceptible individuals was performed, due to the low DNA concentration obtained after extraction. The final volume of the reaction was 13 µl, containing the following reagents: 10 ' buffer, 2 mM Mg Cl₂, 0.2 mM of each dNTPs, 15 µg from primer, 5 U of Taq Polimerase (Cenbiot-RS), 15 µg DNA. Reaction mix was covered with 20 µl of mineral oil and placed into the thermocycler "Mini-cycler MJ Research" for PCR. Amplification conditions were as follows: 92ºC-2 min, 40 cycles of 92ºC-1 min, 35ºC-1 min, 72ºC-2 min and the last cycle of 72ºC-5 min. After reactions, the amplified products were submitted to electrophoresis in 1.5% agarose gel. In principle, 24 primers (most of them from "Operon Technologies") were utilized.

Band analysis - For each amplified DNA sample, the presence or absence of each band was observed, and DNAs from parental generation utilized as standard amplification.

RESULTS AND DISCUSSION

Crossbreeding between Taim and Joinville populations was confirmed by means of a recessive phenotypic marker (albinism character). The specimens of F₁ generation, in result of crossbreeding between those two populations, presented with pigmentation, whereas in F₂ generation there were pigmented and albino individuals. Paraense (1955) used the albino lineage aiming at observing the reproductive behavior in Biomphalaria. These studies demonstrated that the snails are able to perform self-fertilization, but in the presence of one or more individuals they prefer crossbreeding.

Individuals from F₁ and F₂ generations and parental snails were submitted to infection. In principle,the parental snails were infected with two S. mansoni strains (LE and SJ). As can be seen in Table I, B. tenagophila from Taim, RS was found to be resistant to S. mansoni (SJ and LE strains). These results were already expected, since that strain showed resistance to infection in various studies previously carried out (Santos et al. 1979). B. tenagophila from Joinville, SC presented susceptibility rates of 68.5 and 65.6% in relation to S. mansoni SJ and LE strains, respectively. These data corroborate the results obtained by Freitas et al. (1985), who demonstrated that B. tenagophila Joinville was susceptible to both S. mansoni strains (SJ and LE). It was observed that susceptible parental snails showed a mortality rate higher than the resistant parental snails. These data suggest that S. mansoni infection leads to high mortality in susceptible snails. Taking into account that B. tenagophila Joinville showed similar susceptibility to both S. mansoni strains, our experiments were carried out using only LE strain, which was readily available at our laboratory.

According to observations related to F₁ generation in the presence of infection, we can notice that this generation was almost 100% resistance to the LE strain of S. mansoni (Table II). In the first experiment, out of 170 infected F₁ individuals, 150 survived. From these surviving individuals only one was positive and shedded a mean of 15 cercariae/snail. Thus, the surviving F₁ individuals showed a susceptibility rate of 0.6% to S. mansoni (LE strain). The mortality rate of this group was 11.7. In order to confirm these results, a second experiment was carried out with 50 infected specimens from F₁ generation. From this total, 44 individuals survived, but none of them shed cercariae. The mortality rate of this group was 12%. Previous experiments carrying out crossbreedings between B. tenagophila Taim, RS and susceptible specimens from Belo Horizonte, MG, produced F₁ descendants with 4.1% positivity rate to SJ strain (Santos et al. 1979). Freitas et al. (1985) were able to obtain F₁ generation (with 0 and 4.5% susceptibility to LE strain) as a result of crossbreeding between B. tenagophila Joinville and B. tenagophila Taim.

F₂ generation individuals were also submitted to infection with S. mansoni (LE strain) (Table III). Due to homogeneity of the samples resulted from two experiments that we carried out, the data were placed together into a group, therefore increasing the total samples. One hundred twenty five out of 160 infected individuals survived. Among the surviving individuals, only 9 were able to shed cercariae. The susceptibility rate of this group was 7.2%, and the mortality rate was 21.88%. It was observed that the mean cercarial production among susceptible F₂ individuals was lower than that obtained in the parental group (Joinville) (Figure), and 2 out of 9 individuals from group F₂ had the cercarial production discontinued. A comparison between the two groups, taking into account the mean of shedded cercariae, showed high statistically significant differences (p < 0.01), according to the Student's t test.

In the intermediate host, the phenotype of resistance to S. mansoni is very complex. Richards (1970) demonstrated that in B. glabrata the resistance character of the mollusk to S. mansoni infection, which is acquired at the maturity phase, is monogenic and dominant. However, there are some specimens of B. glabrata that present with a peculiar behavior in relation to the parasite. In this case, the resistance character seems to be determined by various genes (Jones et al. 2001). The aim of our work was to study B. tenagophila, and up to now we did not know how was the transmission process of the resistance character in this species. F₁ individuals were found to be almost 100% resistant to infection, thereby showing that the gene (or genes) involved in resistance has (have) a dominant character. Nevertheless, what we cannot affirm in this study is whether resistance is associated or not to an unique gene, since a typical mendelian segregation could not be seen among F₂ individuals. However, we cannot discard the possibility that, among the dead F₂ snails, some of them could be susceptible (mortality rate of 21.8%). Considering such hypothesis, it could be that the number of susceptible snails was greater than the one that was obtained, and in this way the resistance character determined by an unique gene would be confirmed, since we could obtain the expected mendelian proportion (25% of susceptible individuals).

According to the results obtained, the most plausible hypothesis is that resistance in this species is determined by two dominant genes. Taking into account that the parental lineages are endogamic and, therefore, homozygote for the majority of their loci, we can consider that the resistant Taim lineage is double dominant homozygote for the genes which control resistance, whereas the susceptible Joinville lineage is double recessive homozygote for the same loci. Thus, in F₁ generation all the individuals are double heterozygote and, therefore, resistant. After analysing the data depicted in Table III, and after testing such hypothesis, we could see that the results obtained were those expected for this model. According to mendelian segregation related to two loci, the expected number of susceptible individuals in F₂ generation was, approximately, 8 (1/6 from the total of 125 F₂ individuals). Taking into account the susceptibility rate, which can be considered incomplete penetration (approximately 65%), we would expect to detect 5 susceptible F₂ individuals, but we detected 7 specimens that remained positive. Although the results obtained corroborated the suggested model (susceptible character conditioned by a double recessive homozygosis with an incomplete penetration), we observed that the calculated c² value has a probability relatively low (p = 0.07). This result suggests a possible interaction of other modulator (genetic and/or environmental) factors. These modulator factors may be partially acting in susceptible F₂ individuals since, as it was previously observed, these individuals presented a decrease in cercarial production. Thus, we stress the need for further studies in order to confirm or not whether resistance in B. tenagophila is really associated to two dominant genes.

Another scope of this work was to identify molecular markers, which could be associated to resistance. To this end we used the RAPD technique, and 24 primers were tested, but no molecular marker could be detected. Recently, our staff was able to detect a molecular marker, which is typical of Taim lineage and presents a mendelian behavior (Rosa et al. 2004). This marker, although not associated to resistance, is represented by the presence of a band with 350 base pairs of the ITS region in the ribosomal mitocondrial DNA detected by means of PCR-RFPL technique, using the Ddel I enzyme. This marker will be very important in monitoring a possible biological control. Our idea would be changing the genetic inheritance of the local susceptible strain by the genetic inheritance of the introduced resistant strain. In this case, we consider that further studies are necessary to elucidate our proposition.

ACKNOWLEDGEMENTS

To Dr W Lobato Paraense for providing the albino strain of Biomphalaria tenagophila used in this work.

Received 5 April 2004

Accepted 3 January 2004

Partial financial support: CNPq-Pronex, Fapemig

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Publication Dates

• Publication in this collection
  12 Apr 2005
• Date of issue
  Feb 2005

History

• Accepted
  03 Jan 2004
• Received
  05 Apr 2004