Allopatric Combinations Research Articles

Competitive interactions between native and exotic salmonids: a combined field and laboratory demonstration

Abstract – We studied the impact of two exotic salmonid species (brook trout,Salvelinus fontinalisand rainbow trout,Oncorhynchus mykiss) on native brown trout (Salmo trutta fario) habitat, growth and survival. Habitat selection and vertical distribution between young‐of‐the‐year of the three species were examined in a stream aquarium under different sympatric and allopatric combinations. In addition, similar species combinations were introduced in a Pyrenean mountain stream (southwest France) in order to extend laboratory results to growth and apparent survival. Both laboratory and field results indicated that rainbow trout significantly affected native brown trout habitat selection and apparent survival. On the contrary, brown trout habitat, growth and apparent survival were hardly affected by brook trout. These results support the idea that rainbow trout negatively influence native brown trout, and that competition could influence the outcome of fish biological invasions in freshwater ecosystems.

Geographic and within-population structure in variable resistance to parasite species and strains in a vertebrate host

Host resistance to parasites and parasite infectivity may be subject to significant genetically determined variation within species. However, relatively little is known of how this variability is structured in natural vertebrate populations and their macroparasites. A laboratory experiment on host susceptibility–parasite infectivity variation in a wildlife host–parasite system (subspecies of the anuran X. laevis and their polystome flatworms), including 33 pairwise allopatric and sympatric host–parasite combinations (three parasite geographical isolates×11 host full-sibling families, n=600), revealed a complex pattern of infection success. Results amongst host sibships from different localities suggested that infection success was subject to a highly significant locality×parasite isolate interaction. Within localities, a highly significant sibship×isolate interaction also occurred in one of two groups of sibships examined. The existence of such interactions suggests a potential for frequency-dependent, Red Queen-like selection. Interaction between locality and isolate was partly due to higher infection levels in sympatric combinations, consistent with a general pattern of host-specific adaptation. However, some allopatric combinations produced unpredictably high infection levels, resulting in very asymmetrical cross-infectivity patterns (where the reciprocal cross-infections produced negligible infection). This phylogeographically structured host–parasite system may, therefore, sometimes generate local parasite strains with high infectivity to allopatric hosts. Secondary contact between populations could thus result in significant, and unequal, transfer of parasites.

More efficient allopatric combinations of Fasciola hepatica and Lymnaea truncatula due to modification of redial development?

Experimental infections of two susceptible French populations of Lymnaea truncatula (Courcelles and Saint Ours) with an allopatric (Fès, Morocco) or a semi-sympatric (Limoges, France) isolate of Fasciola hepatica miracidia were performed to determine the effect of allopatric miracidia on redial and cercarial production. In both populations, cercarial release was significantly greater in allopatric than in semi-sympatric snails. Compared to semi-sympatric snails, the examination of allopatric snails killed from day 14 to day 35 post-exposure demonstrated (1) a significant decrease after day 28 in the number of daughter rediae (R2a group) exiting from the first-appearing mother redia (R1a redia), and (2) the differentiation of numerous daughter rediae (R2b group) in the body of second-appearing mother rediae (R1b group). These experiments demonstrated that the exposure of L. truncatula to an allopatric isolate of miracidia disturbed the usual developmental pattern of redial generations and caused the formation of numerous R2b rediae. The authors hypothesized that the increase in cercarial release noted in allopatric snails would be assured by the rediae from the R2b group after their emergence from the body of R1b mother rediae.

Allopatric combination of Fasciola hepatica and Lymnaea truncatula is more efficient than sympatric ones

Parasites are capable of rapid evolutionary changes relative to their hosts, due to short life cycle, short generation time, and high fecundity. The direction of the evolution of parasite virulence can be studied in cross-transfer experiments, combining hosts and parasites from different localities, and comparing the outcome of established (sympatric and potentially locally adapted) and novel (allopatric) combinations of hosts and parasites. We aimed to compare the compatibility with snails hosts, the infectivity of metacercariae in rabbits and rats, and the fitness among different combinations (French—FF and Spanish—SS sympatries and allopatry—FS). The first isolate of Fasciola hepatica and its corresponding intermediate host, Lymnaea truncatula originated from Lugo’s northwestern Spain. The second isolate of parasite and snail was collected in the Limoges area in central France. The Spanish snails were more susceptible to their sympatric trematode than the French snails. The Spanish flukes were more infective to intermediate hosts (snails) than the French flukes, but subsequent definitive hosts (rats or rabbits) infections remained similar. The estimated fitness was low in sympatric infections and highly similar (from 4.7 to 5.3). The fitness similarity corresponds, however, to different variations in life-history traits that could represent different strategies among the host–parasite local combinations. The infection rate in snails, metacercarial productivity, metacercarial infectivity, and the estimated fitness were better for allopatric combination (FS). The susceptibility data showed a higher efficiency of flukes in the allopatric snail population than in their local snail population. However, our results were obtained after one generation and from a single isolate and it remains to be determined if all allopatric fluke–snail isolates may present a better fitness. Nevertheless our results indicate that introduction of liver fluke-infected cattle should be monitored carefully, as it could result in the introduction of more efficient parasites.

Onchocerca-Simulium complexes in Venezuela: can human onchocerciasis spread outside its present endemic areas?

The compatibility between sympatric and allopatric combinations of Onchocerca volvulus-anthropophilic species of Simulium was studied in the north-eastern focus of human onchocerciasis as well as in a densely populated locality of the Amazonas State in Venezuela. The objectives were to test the conjecture that local adaptation exists between the parasite and its vectors (the Onchocerca-Simulium complex hypothesis), and assess the possibility of the infection spreading from its present distributional range. For the homologous combination, O. volvulus-S. metallicum cytospecies E in Anzoátegui State (north-eastern focus), parasite yield was 45% in contrast to 1% for the heterologous, southern parasite-S. metallicum infection. This was significantly lower than the parasite yield (4-10%) expected after allowing for the effect of density-dependent limitation of infective larval output described in this paper for S. metallicum. The population of S. exiguum s.l. from southern Venezuela allowed no larval development beyond the L1 stage of either northern or southern parasites. Mechanisms for such refractoriness probably operate at the level of the thoracic muscles, not affecting microfilarial uptake or migration out of the bloodmeal. The parasite yield of southern O. volvulus in S. oyapockense s.l. flies biting man at Puerto Ayacucho (Amazonas) was about 1%, in agreement with the figures recorded for highly compatible sympatric combinations such as O. volvulus-S. ochraceum s.l. in Guatemala. No infective larval development of the northern parasite was observed in southern S. oyapockense. These results, together with considerations of typical worm burdens in the human host, presence/absence of armed cibaria in the simuliids, parasite-induced vector mortality, and fly biting rates, suggest a lower potential for onchocerciasis to spread between the northern and southern endemic areas of Venezuela than that between Amazonian hyperendemic locations and settlements outside this focus with high densities of S. oyapockense s.l.

LOCAL MALADAPTATION IN THE ANTHER-SMUT FUNGUS MICROBOTRYUM VIOLACEUM TO ITS HOST PLANT SILENE LATIFOLIA: EVIDENCE FROM A CROSS-INOCULATION EXPERIMENT.

Conventional wisdom holds that parasites evolve more rapidly than their hosts and are therefore locally adapted, that is, better at exploiting sympatric than allopatric hosts. We studied local adaptation in the insect-transmitted fungal pathogen Microbotryum violaceum and its host plant Silene latifolia. Infection success was tested in sympatric (local) and allopatric (foreign) combinations of pathogen and host from 14 natural populations from a metapopulation. Seedlings from up to 10 seed families from each population were exposed to sporidial suspensions from each of four fungal strains derived from the same population, from a near-by population (< 10 km distance), and from two populations at an intermediate (< 30 km) and remote (< 170 km) distance, respectively. We obtained significant pathogen X plant interactions in infection success (proportion of diseased plants) at both fungal population and strain level. There was an overall pattern of local maladaptation of this pathogen: average fungal infection success was significantly lower on sympatric hosts (mean proportion of diseased plants = 0.32 ± 0.03 SE) than on allopatric hosts (0.40 ± 0.02). Five of the 14 fungal populations showed no strong reduction in infection success on sympatric hosts, and three even tended to perform better on sympatric hosts. This pattern is consistent with models of time-lagged cycles predicting patterns of local adaptation in host-parasite systems to emerge only on average. Several factors may restrict the evolutionary potential of this pathogen relative to that of its host. First, a predominantly selfing breeding system may limit its ability to generate new virulence types by sexual recombination, whereas the obligately outcrossing host 5. latifolia may profit from rearrangement of resistance alleles by random mating. Second, populations often harbor only a few infected individuals, so virulence variation may be further reduced by drift. Third, migration rates among host plant populations are much higher than among pathogen populations, possibly because pollinators prefer healthy over diseased plants. Migration among partly isolated populations may therefore introduce novel host plant resistance variants more often than novel parasite virulence variants. That migration contributes to the coevolutionary dynamics in this system is supported by the geographic pattern of infectivity. Infection success increased over the first 10-km range of host-pathogen population distances, which is likely the natural range of gene exchange.

Toxoplasmosis

Hosts and parasites are in a perpetual co-evolutionary “arms race”. Due to their short generation time and large reproductive output, parasites are commonly believed to be ahead in this race, although increasing evidence exists that parasites are not always ahead in the arms race – in part owing to evolutionary lineage and recent ecological history. We assess local adaptation of hosts and parasites, and determine whether adaptation was influenced by ecological or evolutionary history, using full reciprocal cross-infections of four Gyrodactylus ectoparasite populations and their four guppy (Poecilia reticulata) host populations in Trinidad. To consider effects of evolutionary lineage and recent ecology, these four populations were collected from two different river drainages (Marianne and Aripo) and two different predation environments (high and low). The highest infection levels were obtained when parasites from the Aripo lineage infected guppies from the Marianne lineage, indicating a higher infectivity, virulence and/or reproductive success of the Aripo parasites. Aripo lineage guppies were also better able to limit Gyrodactylus population growth than guppies from the Marianne River, indicating their strong “resistance” to Gyrodactylus regardless of the source of the parasite. Predation environment had no detectable influence on host–parasite population dynamics of sympatric or allopatric combinations. The much stronger effect of evolutionary lineage (i.e., river) than recent ecological history (i.e., predation) emphasises its importance in driving co-evolutionary dynamics, and should be explored further in future studies on local host–parasite adaptation.

Sympatric and allopatric combinations of hen fleas and great tits: a test of the local adaptation hypothesis

AbstractTo test whether allopatric nest parasites differ from sympatric ones in their effect on various life history traits of their avian host, I designed a cross‐transfer experiment in which hen fleas (Ceratophyllus gallinae) were exchanged between great tit (Parus major) nests in two geographically widely separated areas. In neither of the areas did allopatric fleas influence body mass, tarsus length, wing length, duration of nestling period or mortality in great tit nestlings more severely than did sympatric fleas. Duration of incubation was also similar among females independent of experimental treatment. This lack of difference between allopatric and sympatric fleas is hypothesized to reflect the comparative harmlessness of hen fleas for their hosts.

Genetic structure and parasite compatibility of Bulinus globosus (Gastropoda: Planorbidae) from two areas of different endemicity of Schistosoma haematobium in Zimbabwe

A comparative study of the genetic structure of 8 populations of B. globosus from 2 areas of different endemicity of Schistosoma haematobium was done. Five populations were from Chiweshe (high endemicity) and three were from Plumtree (low endemicity). Genetic structure, as determined by allozyme genetics, revealed high levels of genetic variability in Chiweshe populations ( H o = 0.12–0.26) and low levels in Plumtree ( H o = 0.06–0.12). Populations from Chiweshe were genetically heterogeneous whereas in Plumtree the populations were uniform. There were significant deviations from Hardy-Weinberg proportions at different loci in Chiweshe populations. The observed excess of homozygotes was probably caused by partial selfing. The compatibility studies showed variation between allopatric and sympatric combinations of snails and parasites, and the Plumtree parasite had a significant influence on the compatibility parameters. This strain had a positive correlation ( P < 0.01) of its infection rate with allele frequency of Est-2 100 and a negative correlation ( P < 0.05) with allele Idh 100.

Parasite―host coevolution and geographic patterns of parasite infectivity and host susceptibility

Ebert (1994) has proposed the rule that parasites are, with few exceptions, more infective to sympatric hosts than to allopatric hosts. We test this rule using field data for schistosome infections of planorbid snails and find that, although sympatric parasite-host combinations do tend to be more compatible, there are exceptions where particular allopatric parasite-host populations are significantly more compatible. We develop a mathematical model of the dynamics of the parasite-host interaction where parasite infectivity and host susceptibility are defined by the matching of genotypes in a diploid system, The model predicts dynamic polymorphisms where parasite allele frequencies track host allele frequencies but with a lag. Because of this lag, it is possible for allopatric combinations to be more compatible than sympatric combinations. Any 'rule' that precludes this possibility is unlikely to prove robust.

Geographic compatibility of the freshwater snail Bulinus globosus and schistosomes from the Zimbabwe highveld

Populations of Bulinus globosus were sampled from two rivers, 60 kim apart, in nothern Zimbabwe, and offspring from each geographic strain reared in the laboratory. Schistosomes were harvested from each strain and also maintained in the laboratory. Cross-infection experiments were conducted, whereby snails of both geographic strains were exposed to parasites of both geographic strains. Compatibility was found to be related to: (i) the geographic origin of the snail, and (ii) an interaction between the geographic origin of snail and parasite, such that sympatric combinations of snails and parasites produced a greater proportion of compatible infections than allopatric combinations. Cercarial output of infected snails was found to be related to the geographic origin of the parasite, but not of that of the snail. It is suggested that, as the snails used were laboratory bred and naive to schistosome infection, differences in snail — schistosome compatibility were genetically determined, rather than an adaptive response. This study supports earlier work which suggests the existence of geographic compatibility amongst natural populations of snails and trematodes, indicative of either increased infectivity by the parasite, or decreased resistance by the snail.

THE ROLE OF COURTSHIP SONGS IN REPRODUCTIVE ISOLATION AMONG POPULATIONS OF GREEN LACEWINGS OF THE GENUS CHRYSOPERLA (NEUROPTERA: CHRYSOPIDAE).

Male and female lacewings tremulate during courtship, establishing duets that always precede copulation. Three distinct courtship songs are found in populations of the green lacewing Chrysoperla plorabunda (P1, P2 and P3 song morphs). Analysis of five features of the songs for individuals collected from Connecticut, Idaho, Oregon and California showed few differences within song morphs, but sympatric song morphs differed significantly in temporal features of the songs and their mode of presentation. Playback experiments using recorded songs were performed on females with all possible sympatric and allopatric combinations of females and recorded songs. The results showed that females strongly prefer to duet with recordings of males of their own song type and usually showed no responses to songs of other types. Thus, song differences are effective barriers to reproduction between the sympatric morphs. Our results support the hypothesis that the three song morphs are true biological species.

Does the Arctic charr ( Salvelinus alpinus) exhibit a seasonal change in seawater tolerance?

Parasites cause significant economic losses in fish farming, and knowledge of the mechanisms underlying their effects on hosts is an essential prerequisite of preventative procedures for the parasite problem in hatcheries. One such mechanism is local adaptation of parasites, which should lead to higher infectivity of parasites in sympatric host–parasite combinations compared to corresponding allopatric combinations. In this study, we investigated the infectivity of two ubiquitous species of trematode eye flukes, Diplostomum sp. and Tylodelphus clavata, in their farmed salmonid fish hosts with respect to origin of the parasite and the host. First, we exposed two anadromous and one landlocked Atlantic salmon (Salmo salar) populations to controlled infection by Diplostomum sp. and natural infection by T. clavata. The anadromous salmon populations were local to Diplostomum sp. and non-local to T. clavata. In the case of the landlocked salmon, the population was non-local to Diplostomum sp. and local to T. clavata. Second, we exposed a landlocked population of Arctic charr (Salvelinus alpinus) to Diplostomum sp. cercariae from one non-local and two local parasite populations. The results indicated that eye fluke infectivity in both Atlantic salmon and Arctic charr was 1.5–2.6 times higher in local parasite–host combinations compared to those separated by greater geographical distance. This suggests that diplostomid parasites are able to adapt to local host populations, which may have significant implications such as high abundance and prevalence of the parasites in endangered fish stocks that are maintained in hatcheries.