Experimental trypanosomiasis of natural hybrids between house mouse subspecies

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International Journal for Parasitology 29 (1999) 1011±1016

Research note

Experimental trypanosomiasis of natural hybrids between house mouse subspecies Jean-Marc Derothe, Claude Loubes, Marco Perriat-Sanguinet, Annie Orth, Catherine Moulia * Laboratoire GeÂnome, Populations, Interactions, UPR 9060, cc 105, Universite Montpellier II, France Received 27 January 1999; received in revised form 16 April 1999; accepted 20 April 1999

Abstract This study characterises the extent of the susceptibility to parasites (®rst demonstrated with helminths) of hybrids between Mus musculus domesticus and Mus musculus musculus. Experimental infections with Trypanosoma musculi of M. m. domesticus, M. m. musculus and their natural hybrids have been performed to compare their level of resistance/susceptibility. It appears that contrary to the results with helminths, hybrid mice present the same level of resistance/susceptibility to the trypanosome as M. m. musculus and M. m. domesticus individuals. This result is interpreted in the light of the modalities of host±parasite interactions and leads us to hypothesise on the role of parasitism in the evolution of the house mouse hybrid zone. # 1999 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Host±parasite interactions; Mus musculus; Natural hybrids; Resistance/susceptibility; Trypanosoma musculi; Experimental infections

The understanding of ecological and evolutionary implications of the hybridisation process is one of the main concerns of biology. Exploring the way hybridisation either lowers or improves the ®tness of individuals can lead us to understand the phenomenon of speciation and the current biodiversity. However, hybrid ®tness is dicult to assess. Most studies are carried out on few parameters (fertility, mortality, developmental stability...) [1±3] which provide only a partial estimation of hybrid ®tness. Moreover, in the case of animal hybridisation, parasitism is fre* Corresponding author. fax: (33) 4-67-14-46-46: e-mail: [email protected]

quently a forgotten parameter acting on hybrid ®tness [4]. Unlike most studies, the hybrid zone between the two European house mouse subspecies (Mus musculus domesticus and Mus musculus musculus) has been extensively analysed using several estimators of hybrid ®tness [3, 5±7]. In this situation, natural hybrids were shown to be more infested by helminth parasites than individuals of the two parental taxa [8, 9]. The genetic determinism of this hybrid susceptibility was demonstrated experimentally by Moulia et al. [6], using the mouse pinworm Aspiculuris tetraptera. These results suggested that the co-adapted gene systems controlling the parasite load (probably genes in¯uencing immunity) have diverged in the

0020-7519/99/$20.00 # 1999 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 0 - 7 5 1 9 ( 9 9 ) 0 0 0 6 9 - 7


J.-M. Derothe et al. / International Journal for Parasitology 29 (1999) 1011±1016

two subspecies genomes during their geographic isolation. Genetic recombination in the hybrid genomes could lead to the break-down of functional associations and to hybrid susceptibility. The parasites could then contribute to counterselect the hybrid mice and to limit the gene ¯ow between the two parental taxa [6, 8, 9]. On the other hand, a study on ¯uctuating asymmetry of this hybrid zone led to a contradictory conclusion: hybrids seem to present greater stability of development than parental taxa [10]. Moreover, the parasitological studies have only concerned helminths of the digestive tract (Cestoda, Nematoda) [6, 8, 9]. One of the main questions to deal with is to estimate the extent of this hybrid susceptibility. Indeed it may either be speci®c to one kind of parasite or concern most of the biotic aggressors of the mouse [11]. That is to say, hybrid susceptibility may result from a speci®c alteration of response to helminths or it may be the expression of a global decreasing of hybrid ®tness towards parasites. Consequently, the pressure exerted by the parasites on hybrids may be a strong or a weak component explaining the whole hybrid `break-down'. The diculty in testing these hypotheses is that there are few natural parasites of the house mouse that are not helminths and that allow experimental work in the laboratory. The present study is an attempt to characterise hybrid susceptibility using the mouse protozoan Trypanosoma musculi. In this paper, we analyse the susceptibility to this protozoan of wild mice from inside and outside the hybrid zone. We then compare these results with those previously obtained with A. tetraptera [6]. Trypanosoma musculi is an extracellular protozoan of the blood regarded as slightly pathogenic for its host [12]. It is a natural parasite of the house mouse which presents an heteroxenous life-cycle. Its intermediate host is a ¯ea [13]. In the laboratory, direct infection from mouse to mouse is achieved by i.p. injection of dividing trypanosomes. The time-course of infection in all inbred strains studied is very constant and reproducible [12]. Experimental infections of inbred mice last 20±25 days [14]. Blood parasitaemia can be divided into four stages: a pre-patent

period (3±5 days), an exponential multiplicative phase (approximately 5 days), a plateau phase (7±10 days) and an exponential immune elimination phase (5±7 days) [14]. The di€erences in resistance level among inbred strains were obtained by comparing the peak parasitaemia of the plateau phase [14, 15]. The clearance of parasites is followed by a long-lasting refractoriness to reinfection [12]. The parasite isolate used in this study is the Partinico II strain [16] provided by Professor Vincendeau (University of Bordeaux, France). We have cloned this parasite in the laboratory of Dr. Tibayrenc (UMR 9926, ORSTOM-CNRS) to minimise the e€ect of genetic variability. The parasite clone is maintained by weekly passages in male mice of the susceptible strain: C3H/ OuJIco (C3) (I€a Credo, France). Nine strains of the two sub-species M. m. domesticus and M. m. musculus and of their natural hybrids were tested in this study. The founder individuals of these strains were trapped in di€erent localities within the geographic range of the two subspecies and of the hybrid zone. The mice were provided by the `Wild Mouse Repository' (UPR 9060, CNRS) and are currently maintained by random breeding in small colonies for a few generations (from ®ve to 25) in order to maintain a genetic polymorphism. The strains used in this study were: (i) M. m. domesticus mice: BZO, DMZ and DJO (from Algeria, Morocco and Italy, respectively); (ii) M. m. musculus mice: MHT and MPB (from Hungary and Poland, respectively); and (iii) hybrid mice from the Danish and Bulgarian parts of the hybrid zone. The mean introgression index (I) indicates the degree of hybridisation of individuals of a speci®c locality. It is calculated as the mean percentage of M. m. domesticus alleles at 10 diagnostic loci (isoenzymes) between the two taxa [5]. These strains are: DDO (I = 89%) and MDH (I = 2%) from Denmark, and MBS (I = 8%) and MBT (I = 7%) from Bulgaria. Mice tested in this study were all pinworm free and infected when 1 month old. They were housed in a conventional animal facility, grouped according to sex and maintained in the same conditions during the experimental period.

J.-M. Derothe et al. / International Journal for Parasitology 29 (1999) 1011±1016

Samples of blood were collected from the tail veins of C3 mice 7- to 12-days p.i. This blood was diluted at a concentration of 5104 parasites ml ÿ 1 in a sterile sodium chloride solution (0.9%) and 0.5 ml of the ®nal dilution was injected i.p. into each tested mouse [12]. Every 3 days from the time of infection until the clearance of the parasites, the parasitaemia of each infected mouse was evaluated by snipping the tail vein and collecting a blood sample. The number of trypanosomes per millilitre of blood was determined with a haemocytometer (Thoma). The response of each mouse to T. musculi was evaluated by the maximum value of parasitaemia obtained during the course of infection (corresponding to the plateau-phase) [14, 15]. The normality and homoscedasticity of the data were tested. Even after they were Log-transformed, their variances appeared highly heterogeneous (Bartlet test, w2 = 26.965; df = 8; P = 0.0007). As homoscedasticity cannot be corrected by any classical transformation, only nonparametric tests were used. As a sex-linked di€erence in resistance to pinworms has been reported [17, 18], we ®rst tested for such a di€erence within each strain and subsequently between the nine strains. A randomsampling test [17±19] was then performed. This test was written in Turbo Pascal language. The variable D was de®ned as the di€erence between the medians of male and female samples of a par-


ticular strain. For within-strain analysis, the probability of obtaining a greater D-value by chance was computed (one-way test). For the global level test (all the strains), the probability of obtaining a greater mean D-value (calculated as the mean of the D-values of the nine strains) by chance was computed. Twenty-thousand iterations were performed for each test. Heterogeneity of response to the parasite between the strains was determined using the Kruskal±Wallis (KW) test. The Noether's test is a posthoc test for multiple comparisons between samples based on the Kruskal±Wallis mean ranks; it takes into account the type-one error [19, 20]. In all cases except MPB, males exhibited higher parasitaemia than females (Table 1). A signi®cant sex-linked di€erence was detected in three of nine strains (Table 1). Some of these signi®cant results might be due to type-one error because of the number of tests performed. Because this could not be determined, males (m) and females (f) of the concerned strains were separated in order to carry out the analysis with homogeneous samples. Moreover, when the random sampling test was applied to the whole set of data (the nine strains), there was a general tendency for males to exhibit a greater parasite load than females (P = 0.019). The KW test detected a high signi®cant heterogeneity of responses among the strains

Table 1 Comparison of parasitaemia in male and female mice experimentally infected with Trypanosoma musculi

M. m. domesticus M. m. musculus Hybrids


Number of males

Median of male parasite load(a)

Number of females

Median of female parasite loadsa



15 11 11 22 13 17 12 12 18

330.0 645.0 675.0 1020.0 510.0 150.0 630.0 135.0 765.0

15 17 16 11 13 16 17 18 7

225.0 600.0 330.0 570.0 690.0 75.0 600.0 97.5 420.0

0.0928 0.4214 0.0045 0.1183 0.9371 0.0119 0.4695 0.1673 0.0302

a Number 105 parasites ml ÿ 1 of blood. bResult of the random sampling test applied between males and females of each strain. See text for more information. Signi®cant results are indicated in bold type (a = 0.05).


J.-M. Derothe et al. / International Journal for Parasitology 29 (1999) 1011±1016

Table 2 Comparison of parasitaemia within wild derived strains of mice (see text for strain designation). Only signi®cant results of the Noether's test are given








5.67a 5.23 4.20 3.78

6.05 5.69 4.57 4.21

6.36 6.07 4.88 4.58

7.11 7.00 5.61 5.46

6.24 5.85 4.89 4.55

5.78 5.29 4.58 4.19


z: Result of the Noether's test; theoretical z-value = 3.6; a = 0.05.

(Hcorr. = 146.668; df = 11; P < 0.0001) and the Noether's tests (Table 2) discriminated among three categories of strains as far as susceptibility to the trypanosome was concerned: the resistant strains (median below 320105 parasites ml ÿ 1 of blood): DDOf, DDOm, MBT, BZO; the susceptible strains (more than 600105 parasites ml ÿ 1): MPB, DMZ, MBS, MHT, MDHm, DJOm; and the intermediate strains (from 320105 to 600105 parasites ml ÿ 1: DJOf, MDHf). These intermediate mice presented insigni®cant di€erences in levels of parasitaemia with both resistant and susceptible strains whereas resistant and susceptible strains were signi®cantly di€erent from each other (Table 2). Hybrid strains were found in all three categories. This work shows a sex-linked di€erence in parasite burden previously demonstrated in many ®eld and experimental studies on mammals [17, 18, 21, 22]; see [23] for reviews). In this study, as in most models, the males have greater parasite loads than the females. There are two main hypotheses to explain this male susceptibility: (i) eco-ethological di€erences between the two genders (feeding research, mate competition, territory defence, etc) could lead the males to be more exposed to infection than the females [24]. However, this parameter cannot be involved in this study because all the mice were kept under the same experimental conditions; (ii) the high level of testosterone of the males could impair their immunological status [25±27]. The involvement of testosterone is not clearly demonstrated yet [28], more speci®c studies on sex-linked di€erences in parasitism correlating the constitutive level of testosterone and the level of infection

have to be conducted with di€erent parasite models. However, concerning this study, the male susceptibility is an indirect proof that genetic mechanisms, modulated by the physiological status of the individuals, are involved in the response to T. musculi infection. The main result of this study is that there was no di€erence in susceptibility to T. musculi between the hybrids and the M. m. domesticus/ M. m. musculus mice. This is in contrast with previous studies which demonstrated a higher susceptibility of hybrids to the pinworm A. tetraptera when compared with the M. m. musculus/ M. m. domesticus mice [6, 9]. Three hypotheses can be proposed to explain this di€erence. The ®rst is based on one analysis of the genetic determinism of the resistance to trypanosome infection in two inbred strains of mice, the derived recombinant inbred strains and the ®rst generations of hybrids [29]. From this study, the infection appears to be controlled by a major dominant gene [29]. If the determinism of the resistance is the same in laboratory and wild-derived strains of mice, then hybrid susceptibility due to recombination of two divergent genomes is not expected. However, the data on the genetic determinism in inbred strains appear incomplete (two strains). Moreover, inbred strains are not samples from natural populations of mice. They are issued for mixing between di€erent subspecies of the complex M. musculus [30], and they have been selected in the modi®ed environment of the laboratory. So, they present speci®c characteristics they do not share with wild-derived mice, especially concerning resistance to pathogens [17].

J.-M. Derothe et al. / International Journal for Parasitology 29 (1999) 1011±1016

The second hypothesis is based on the fact that the four hybrid strains used in this study were slightly introgressed (11% at the most for DDO (I = 89%)) while two out of four previously tested with A. tetraptera showed 30 to 40% introgression [6]. If slight introgression occurred in the hybrid genomes tested in this study, one may propose that there was no breakdown of the co-adapted gene systems controlling parasitaemia in these strain genomes. The absence of di€erences of susceptibility to the murine trypanosome between hybrids and unintrogressed mice would suggest that mice regarded as `hybrids' in this study have unrecombined gene systems of resistance. However, two of these hybrid strains (MDH and DDO) were previously shown to be highly susceptible to pinworm infection, leading the authors to suggest that the functional gene associations had been broken-up in their genomes by recombination [6]. The third hypothesis to explain this di€erence is that pinworms exert enough constraints on the mouse to induce the selection of di€erent coadapted gene systems in the two sub-species genomes while T. musculi do not. Di€erences in the life traits of the two kinds of parasites may support this hypothesis. Although the geographical range of T. musculi is not well-known, it seems to be restricted to the Mediterranean area, West African shore [31] and Panama [32]; and moreover, the transmission of T. musculi depends on the occurrence of its intermediate host while pinworms have a direct cycle. The trypanosome is then not supposed to be as broadly distributed in mouse populations as the cosmopolitan pinworms. Only some Mediterranean populations of M. m. domesticus, when the intermediate host would be present, could be infected by the protozoan and then would show adaptations to limit the parasite infection. Experimental data suggest that after the ®rst infection, the mouse shows a de®nitive refractoriness to reinfection [12, 31]. The trypanosome impact on its host is then limited in time during the mouse life. On the contrary, pinworms can reinfest one host throughout its life [33] and thus may represent a real selective pressure on it [17].


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