Heterosis and inbreeding depression in bottlenecked populations: a test in the hermaphroditic freshwater snail Lymnaea stagnalis

Small population size is expected to induce heterosis, due to the random fixation and accumulation of mildly deleterious mutations, whereas within‐population inbreeding depression should decrease due to increased homozygosity. Population bottlenecks, although less effective, may have similar consequences. We tested this hypothesis in the self‐fertile freshwater snail Lymnaea stagnalis, by subjecting experimental populations to a single bottleneck of varied magnitude. Although patterns were not strong, heterosis was significant in the most severely bottlenecked populations, under stressful conditions. This was mainly due to hatching rate, suggesting that early acting and highly deleterious alleles were involved. Although L. stagnalis is a preferential outcrosser, inbreeding depression was very low and showed no clear relationship with bottleneck size. In the less reduced populations, inbreeding depression for hatching success increased under high inbreeding. This may be consistent with the occurence of synergistic epistasis between fitness loci, which may contribute to favour outcrossing in L. stagnalis.     

Environment-dependent inbreeding depression in a hermaphroditic freshwater snail

Inbreeding depression was simultaneously studied under contrasted environments, laboratory and natural conditions, using individuals originating from 14 families of the freshwater snail Physa acuta. Both survival and growth of juveniles showed inbreeding depression under laboratory conditions. The same fitness components were monitored with mature snails either kept under laboratory conditions or released at a natural site and analysed using capture-mark-recapture models. Genetic composition of both samples was similar. Inbreeding depression on survival was highest in the laboratory while strong outbreeding depression was revealed in the field. Thus inbreeding depression may not be always higher under natural conditions, at the opposite of what is commonly assumed. We suggest that inbreeding depression is dependent on metabolic requirements imposed by the environment. Other evidences showing that inbreeding depression is environment-dependent are reviewed. We conclude that genetic models should include both genetic and environmental variance in inbreeding depression for studying mating system evolution.     

Copulation in the hermaphroditic snail Lymnaea stagnalis: a review

Summary

Male copulatory behavior of the hermaphroditic snail Lymnaea stagnalis is a complex one: the appetitive behavior consists of a number of elements which do not always appear in the same sequence and have variable durations. Backfills of the penis nerve revealed the neurons that send projections to the male copulatory apparatus. Immunocytochemical experiments have demonstrated that these neurons contain at least ten different messenger molecules. Based on in situ hybridization and chemical purification data, it is suspected that this number will probably be doubled. How the different neurons and the molecules they contain might be involved in generation of the different elements of male copulatory behavior is discussed.     

The mixed mating system of Impatiens capensis and infection by a foliar rust pathogen: patterns of resistance and fitness consequences

Outcrossing by hosts may offer protection from natural enemies adapted to parental genotypes by creating diverse progeny that differ from their parents through genetic recombination. However, past experimental work addressing the relationship between mating system and disease in offspring has given conflicting results, suggesting that outcrossing might also cause the dissolution of resistant genotypes. To determine if selfed progeny are more susceptible to disease caused by the heteroecious rust, Puccinia recondita, or if selfing preserves existing resistant genotypes, we used a factorial design to compare levels of infection of selfed and outcrossed progeny of Impatiens capensis, a woodland annual with a mixed mating system. We compared the level of host infection when exposed to three pathogen sources in the field: the sympatric rust population, and two allopatric rust populations. Outcrossed progeny exposed to sympatric rust had higher infection scores than selfed progeny exposed to the same rust, suggesting that outcrossing breaks up resistant genotypes. In addition, there was a trend for the rust to be more infective on sympatric rather than allopatric hosts. We also examined whether rust infection differentially alters the fitness of selfed and outcrossed progeny. Outcrossed plants that escaped infection had higher fitness, as measured by fruit production, than selfed plants, but there was no difference in fitness between infected selfed and infected outcrossed plants. Thus, outcrossing was advantageous in the absence of disease, but there was no fitness difference between selfed and outcrossed progeny in the presence of disease. In sum, our results indicate that interactions with pathogens can eliminate or reverse the advantage of outcrossing.     

Severe outbreeding and inbreeding depression maintain mating system differentiation in Epipactis (Orchidaceae)

In hermaphroditic plants, theory for mating system evolution predicts that populations will evolve to either complete autonomous selfing (AS) or complete outcrossing, depending on the balance between automatic selection favouring self‐fertilization and costs resulting from inbreeding depression (ID). Theory also predicts that selection for selfing can occur rapidly and is driven by purging of genetic load and the loss of ID. Therefore, selfing species are predicted to have low levels of ID or even to suffer from outbreeding depression (OD), whereas predominantly outcrossing species are expected to have high levels of ID. To test these predictions, we related the capacity of AS to the magnitude of early‐acting inbreeding or OD in both allogamous and autogamous species of the orchid genus Epipactis. For each species, the level of AS was assessed under controlled greenhouse conditions, whereas hand‐pollinations were performed to quantify early costs of inbreeding or OD acting at the level of fruit and seed production. In the autogamous species, the capacity of AS was high (> 0.72), whereas in the allogamous species AS was virtually absent (< 0.10). Consistent with our hypothesis, allogamous Epipactis species had significantly higher total ID (average: 0.46) than autogamous species, which showed severe costs of OD (average: ?0.45). Overall, our findings indicate that strong early‐acting ID represents an important mechanism that contributes to allogamy in Epipactis, whereas OD may maintain selfing in species that have evolved to complete selfing.     

Consequences of mating and predation risk for longevity in a freshwater snail: abstinence makes the heart beat longer

Senescence is not a static property of an individual or population, but rather it is a dynamic process that may be influenced by environmental conditions. This can occur in at least two ways: in the long‐term across multiple generations, and in the short‐term via phenotypic plasticity. The former has attracted a lot of attention, both theoretically and empirically; the latter has lagged behind. To determine whether two important environmental variables (predation risk and mate availability) affect the pattern of actuarial senescence (i.e. the increase in mortality with age), we reared 30 full‐sib families of the simultaneously hermaphroditic freshwater snail Physa acuta under four different environmental conditions and tracked individuals until death. Individuals were reared in a 2×2 factorial experiment that manipulated the nonlethal presence of chemical cues from predatory crayfish (presence/absence) and the opportunity to mate with an unrelated partner (mated/not mated). Snails that receive a partner reproduce by outcrossing, whereas those that remain in isolation can reproduce by self‐fertilization. We compared the cumulative survival curves to test for an effect of predation risk and mating. The hazard ratio (HR) for the predation risk comparison was 1.042 indicating no significant difference between the curves. However, the HR for the mating comparison was 4.021, reflecting a significant reduction in survival probability for mated snails relative to isolated snails. As such, mating resulted in a much shorter lifespan, an outcome that we interpret in terms of shifting resource allocation.     

Range limits and parasite prevalence in a freshwater snail

Geographical range limits are thought to be set by species' physiological or ecological adaptation to abiotic factors, but the importance of biotic factors such as parasitism in determining range limits has not been well explored. In this study the prevalence of trematode parasitism in populations of a freshwater gastropod snail, Lymnaea stagnalis, increased sharply as this species approached its western UK range limit. The likelihood of trematode infection increased with snail size, but high prevalence at the range edge was not a result of interpopulation variation in snail size. Changes in population growth rates resulting from high rates of parasitism at the range edge could contribute to range limitation. The mechanism driving high rates of parasitism at the range edge is not clear, but changes in abiotic factors towards the range limit may influence snail life history and immune response to trematode infection, indirectly altering the prevalence of parasites in marginal host populations.     

Functional pleiotropy and mating system evolution in plants: frequency-independent mating

Mutations that alter the morphology of floral displays (e.g., flower size) or plant development can change multiple functions simultaneously, such as pollen export and selfing rate. Given the effect of these various traits on fitness, pleiotropy may alter the evolution of both mating systems and floral displays, two characters with high diversity among angiosperms. The influence of viability selection on mating system evolution has not been studied theoretically. We model plant mating system evolution when a single locus simultaneously affects the selfing rate, pollen export, and viability. We assume frequency-independent mating, so our model characterizes prior selfing. Pleiotropy between increased viability and selfing rate reduces opportunities for the evolution of pure outcrossing, can favor complete selfing despite high inbreeding depression, and notably, can cause the evolution of mixed mating despite very high inbreeding depression. These results highlight the importance of pleiotropy for mating system evolution and suggest that selection by nonpollinating agents may help explain mixed mating, particularly in species with very high inbreeding depression.     

The effect of mating system on growth of Arabidopsis lyrata in response to inoculation with the biotrophic parasite Albugo candida

The effects of variation in host reproductive systems on response to pathogens are not well understood. We inoculated individuals from outcrossing and inbreeding populations of North American Arabidopsis lyrata with Albugo candida (white blister rust) to test the effect of mating system and heterozygosity on disease response. We observed three host infection phenotypes, classified as fully resistant, partially resistant and fully susceptible. Overall, inbreeding populations had more susceptible and fewer partially resistant individuals than outcrossing populations, but the highest proportion of resistant individuals was found in two of the inbreeding populations. Mating system did not affect relative growth rate of inoculated plants, but there were strong effects of population and infection phenotype. We conclude that mating system per se does not determine the resistance of natural A. lyrata populations to infection by Albugo, but that the increased variability in responses among inbreeding populations may be due to reduced effective population size.     

Inbreeding depression and selfing rates in a self-compatible,hermaphroditic species,Schiedea membranacea (Caryophyllaceae)

Inbreeding depression and selfing rates were investigated in Schiedea membranacea (Caryophyllaceae), a hermaphroditic species endemic to the Hawaiian Islands. Most theoretical models predict high inbreeding depression in outcrossing hermaphroditic species and low inbreeding depression in inbreeding species. Although high outcrossing rates and high levels of inbreeding depression are characteristic of many species of Schiedea, self- fertilization is common among relatives of hermaphroditic S. membranacea, and high selfing rates and low levels of inbreeding depression were predicted in this species. Sixteen individuals grown in the greenhouse were used to produce selfed and outcrossed progeny. Inbreeding depression, which was evident throughout the stages measured (percentage viable seeds per capsule, mean seed mass, percentage seed germination, percentage seedling survival, and biomass after 8 mo), averaged 0.70. Inbreeding depression among maternal families varied significantly for all measured traits and ranged from −0.12 to 0.97. Using isozyme analysis, the multilocus selfing rate varied from 0.13 to 0.38 over 4 yr. Contrary to the initial prediction of high selfing and low inbreeding depression based on phylogenetic relationships within Schiedea, low selfing rates and high levels of inbreeding depression were found in S. membranacea. These results indicate that outcrossing is stable in this species and maintained by high levels of inbreeding depression.     

Association behaviour of the self-fertilizing Kryptolebias marmoratus (Poey): the influence of microhabitat use on the potential for a complex mating system

Preliminary observations were conducted to identify conspicuous body postures and movements of males and hermaphrodites in the mangrove killifish Kryptolebias marmoratus . These behaviours were used quantitatively to examine the social interactions for experimental pairings of K. marmoratus of different sexual states ( i.e. simultaneous hermaphrodite and male) in an aquarium with an open-water area and simulated crab burrows. This allowed observation of behaviours that could not be observed in the field. Kryptolebias marmoratus , regardless of sexual state and experimental treatment, spent 40% of the time in the burrow. Hermaphrodites exhibited a preference for associating with males rather than other hermaphrodites. The observed complexity of displayed behaviours and interactions between paired conspecifics indicate that K. marmoratus has a rich repertoire of social behaviour not predicted for a strictly selfing species. Also, land crab burrows play an important role in their social interactions.     

Genetic variability and drift load in populations of an aquatic snail

Abstract Population genetic theory predicts that in small populations, random genetic drift will fix and accumulate slightly deleterious mutations, resulting in reduced reproductive output. This genetic load due to random drift (i.e., drift load) can increase the extinction risk of small populations. We studied the relationship between genetic variability (indicator of past population size) and reproductive output in eight isolated, natural populations of the hermaphroditic snail Lymnaea stagnalis . In a common laboratory environment, snails from populations with the lowest genetic variability mature slower and have lower fecundity than snails from genetically more variable populations. This result suggests that past small population size has resulted in increased drift load, as predicted. The relationship between genetic variability and reproductive output is independent of the amount of nonrandom mating within populations. However, reproductive output and the current density of snails in the populations were not correlated. Instead, data from the natural populations suggest that trematode parasites may determine, at least in part, population densities of the snails.     

Experimental differences in mortality patterns between European minnows, Phoxinus phoxinus, infected with sympatric or allopatric trematodes, Diplostomum phoxini

A factorial experiment was conducted to examine if the digenetic trematode parasite Diplostomum phoxini influences minnow growth and survival negatively and if different parasite populations differ in their effects on hosts. Juvenile full-sibling minnows from a lake located at the northern edge of the Swiss Alps were infected experimentally with D. phoxini from either their own or another lake. When exposed to sympatric parasites, the minnows survived a low and a high infection dose more or less equally, but with allopatric parasites mortality increased with infection dose. Parasites did not reduce host growth and minnows exposed to a low infection dose grew quicker than either non-infected ones or ones exposed to a high infection dose. Thus, the results show different patterns of pathogenicity between two parasite populations and suggest that (1) the observed differences are at least partially genetic and that (2) the co-evolved, sympatric host-parasite association has reached a degree of low pathogenicity. Differences between the findings presented here and those of a previously published study are discussed.     

Phenology of recruitment and infection patterns of Ascocotyle pachycystis, a digenean parasite in the sheepshead minnow, Cyprinodon variegatus

In northwest Florida, Cyprinodon variegatus are parasitized by Ascocotyle pachycystis, a digenean parasite that forms metacercarial cysts in the lumen of the bulbus arteriosus. Field experiments revealed that fish accumulated parasites at an uneven rate within the highly seasonal trematode recruitment period. Older (= larger) fish had higher rates of parasite recruitment and higher parasite prevalences and densities (numbers of metacercariae per individual fish) than did younger (= smaller) fish. Nearly all adults were parasitized (prevalence range 70-100%), and parasite densities ranged from zero to 6800 per fish. Parasite distributions were clumped (= aggregated) in fish of all age classes but were less heterogeneous in early juveniles and adults than they were in late juveniles. Parasites affected the population dynamics of sheepshead minnows by causing reduced winter survivorship, as evidenced by an increase in the average size of fish and a decrease in the average infection intensity over the winter. This revised version was published online in July 2006 with corrections to the Cover Date.     

Environmental diel variation, parasite loads, and local population structuring of a mixed-mating mangrove fish

Genetic variation within populations depends on population size, spatial structuring, and environmental variation, but is also influenced by mating system. Mangroves are some of the most productive and threatened ecosystems on earth and harbor a large proportion of species with mixed-mating (self-fertilization and outcrossing). Understanding population structuring in mixed-mating species is critical for conserving and managing these complex ecosystems. Kryptolebias marmoratus is a unique mixed-mating vertebrate inhabiting mangrove swamps under highly variable tidal regimes and environmental conditions. We hypothesized that geographical isolation and ecological pressures influence outcrossing rates and genetic diversity, and ultimately determine the local population structuring of K. marmoratus. By comparing genetic variation at 32 microsatellites, diel fluctuations of environmental parameters, and parasite loads among four locations with different degrees of isolation, we found significant differences in genetic diversity and genotypic composition but little evidence of isolation by distance. Locations also differed in environmental diel fluctuation and parasite composition. Our results suggest that mating system, influenced by environmental instability and parasites, underpins local population structuring of K. marmoratus. More generally, we discuss how the conservation of selfing species inhabiting mangroves and other biodiversity hotspots may benefit from knowledge of mating strategies and population structuring at small spatial scales.     

Inbreeding depression is high in a self‐incompatible perennial herb population but absent in a self‐compatible population showing mixed mating

High inbreeding depression is thought to be one of the major factors preventing evolutionary transitions in hermaphroditic plants from self‐incompatibility (SI) and outcrossing toward self‐compatibility (SC) and selfing. However, when selfing does evolve, inbreeding depression can be quickly purged, allowing the evolution of complete self‐fertilization. In contrast, populations that show intermediate selfing rates (a mixed‐mating system) typically show levels of inbreeding depression similar to those in outcrossing species, suggesting that selection against inbreeding might be responsible for preventing the transition toward complete self‐fertilization. By implication, crosses among populations should reveal patterns of heterosis for mixed‐mating populations that are similar to those expected for outcrossing populations. Using hand‐pollination crosses, we compared levels of inbreeding depression and heterosis between populations of Linaria cavanillesii (Plantaginaceae), a perennial herb showing contrasting mating systems. The SI population showed high inbreeding depression, whereas the SC population displaying mixed mating showed no inbreeding depression. In contrast, we found that heterosis based on between‐population crosses was similar for SI and SC populations. Our results are consistent with the rapid purging of inbreeding depression in the derived SC population, despite the persistence of mixed mating. However, the maintenance of outcrossing after a transition to SC is inconsistent with the prediction that populations that have purged their inbreeding depression should evolve toward complete selfing, suggesting that the transition to SC in L. cavanillesii has been recent. SC in L. cavanillesii thus exemplifies a situation in which the mating system is likely not at an equilibrium with inbreeding depression.     

Variation in parasite resistance of Arctic charr,Salvelinus alpinus,between and within sympatric morphs

Genetic variation in resistance against parasite infections is a predominant feature in host–parasite systems. However, mechanisms maintaining genetic polymorphism in resistance in natural host populations are generally poorly known. We explored whether differences in natural infection pressure between resource‐based morphs of Arctic charr (Salvelinus alpinus) have resulted in differentiation in resistance profiles. We experimentally exposed offspring of two morphs from Lake Þingvallavatn (Iceland), the pelagic planktivorous charr (“murta”) and the large benthivorous charr (“kuðungableikja”), to their common parasite, eye fluke Diplostomum baeri, infecting the eye humor. We found that there were no differences in resistance between the morphs, but clear differences among families within each morph. Moreover, we found suggestive evidence of resistance of offspring within families being positively correlated with the parasite load of the father, but not with that of the mother. Our results suggest that the inherited basis of parasite resistance in this system is likely to be related to variation among host individuals within each morph rather than ecological factors driving divergent resistance profiles at morph level. Overall, this may have implications for evolution of resistance through processes such as sexual selection.     

Natural range expansion and human-assisted introduction leave different genetic signatures in a hermaphroditic freshwater snail

Colonization events like range expansion or biological invasions can be associated with population bottlenecks. Small population size may lead to loss of genetic diversity due to random genetic drift, to loss of heterozygosity due to increased inbreeding and should leave a signature on the genetic polymorphism and genetic structure of populations. The mating system might additionally influence the outcome of such a process. Here, we compare invasive and native populations of the hermaphroditic freshwater snail Lymnaea stagnalis. In the native range we included populations that were ice-free during the last glaciation period and populations that were glaciated and are located at the edge of the species’ native distribution range. The microsatellite data show substantial loss of genetic variation in the introduced range and no signs of high propagule pressure or admixture. The expressed polymorphism was so low that mating system analysis was not possible. In the native region, all populations display strong levels of differentiation (global F _ST: 0.341) independent of colonization history and exhibit no significant pattern of inbreeding. However, the populations in more recently colonized habitats show diminished genetic diversity. Overall, these results illustrate how dramatic the reduction in genetic diversity can be for hermaphroditic animals and that gene flow in the native range can be surprisingly low despite short distances.     

Genetic differentiation,dispersal and mating system in the schistosome-transmitting freshwater snail Biomphalaria glabrata

Biomphalaria glabrata is the main intermediate host of Schistosoma mansoni in America and one of the most intensely studied species of freshwater snail, yet very little is known about its population biology. Here, we used seven highly polymorphic microsatellite loci to analyse genetic diversity in populations from three regions (Lesser Antilles, Venezuela and southern Brazil). Considerable genetic variation was detected, with an average (s.d.) H(0) = 0.32 (0.24). More diversity per population was found in the Valencia lake basin in Central Venezuela, which suggests an influence of dispersal (via inter-population connectivity) on the restoring of genetic diversity after the demographic bottlenecks recurrently experienced by populations. A marked population structure was detected and there seems to be a relationship between mean differentiation and genetic diversity within regions. There is also a significant isolation-by-distance pattern. The Lesser Antilles populations appear clearly differentiated from the rest, which suggests a single colonisation event followed by local radiation within these islands or multiple colonisation events from the same source area. Our results indicate that B. glabrata essentially cross-fertilises, with little variation in selfing rates among populations. However, significant deficits in heterozygotes and linkage disequilibria were detected in two Venezuelan populations suggesting a mixture of at least two different genetic entities, probably with differences in their respective mating systems.