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Hybridization between differentiated lineages can have many different consequences depending on fitness variation among hybrid offspring. When introduced organisms hybridize with natives, the ensuing evolutionary dynamics may substantially complicate conservation decisions. Understanding the fitness consequences of hybridization is an important first step in predicting its evolutionary outcome and conservation impact. Here, we measured natural selection caused by differential viability of hybrid larvae in wild populations where native California Tiger Salamanders (Ambystoma californiense) and introduced Barred Tiger Salamanders (Ambystoma tigrinum mavortium) have been hybridizing for 50-60 years. We found strong evidence of hybrid vigor; mixed-ancestry genotypes had higher survival rates than genotypes containing mostly native or mostly introduced alleles. Hybrid vigor may be caused by heterozygote advantage (overdominance) or recombinant hybrid vigor (due to epistasis or complementation). These genetic mechanisms are not mutually exclusive, and we find statistical support for both overdominant and recombinant contributions to hybrid vigor in larval tiger salamanders. Because recombinant homozygous genotypes can breed true, a single highly fit genotype with a mosaic of native and introduced alleles may eventually replace the historically pure California Tiger Salamander (listed as Threatened under the U.S. Endangered Species Act). The management implications of this outcome are complex: Genetically pure populations may not persist into the future, but average fitness and population viability of admixed California Tiger Salamanders may be enhanced. The ecological consequences for other native species are unknown.

Although the ecological consequences of species invasions are well studied, the ecological impacts of genetic introgression through hybridization are less understood. This is particularly true of the impacts of hybridization on \"third party\" community members not genetically involved in hybridization. We also know little about how direct interactions between hybrid and parental individuals influence fitness. Here, we examined the ecological effects of hybridization between the native, threatened California Tiger Salamander (Ambystoma californiense) and the introduced Barred Tiger Salamander (Ambystoma tigrinum mavortium). Native x introduced hybrids are widespread in California, where they are top predators in seasonal ponds. We examined the impacts of early generation hybrids (first 2 generations of parental crosses) and contemporary hybrids derived from ponds where hybrids have been under selection in the wild for 20 generations. We found that most classes of hybrid tiger salamander larvae dramatically reduced survival of 2 native community members, the Pacific Chorus Frog (Pseudacris regilla) and the California Newt (Taricha torosa). We also found that native A. californiense larvae were negatively impacted by the presence of hybrid larvae: Native survival and size at metamorphosis were reduced and time to metamorphosis was extended. We also observed a large influence of Mendelian dominance on size, metamorphic timing and predation rate of hybrid tiger salamanders. These results suggest that both genetic and ecological factors are likely to influence the dynamics of admixture, and that tiger salamander hybridization might constitute a threat to additional pond-breeding species of concern in the region.

Pathogenic diseases and environmental contaminants are two of the leading hypotheses for global amphibian declines, yet few studies have examined the influence of contaminants on disease susceptibility. In this study, we examined effects of ecologically relevant doses of atrazine (0, 1.6, 16, and 160 micrograms/L), sodium nitrate (0, 6.8, 68 mg/L), and their interactions on susceptibility of four laboratory-bred tiger salamander families to Ambystoma tigrinum virus (ATV), a pathogen implicated in global amphibian die-offs. Salamanders were from Arizona populations where a coevolutionary history with ATV is supported, and thus cofactors rather than recent introduction may contribute to disease epizootics. Use of atrazine and nitrogenous fertilizers are ubiquitous; therefore, the impact of these cofactors on disease susceptibility is an important consideration. Atrazine and sodium nitrate significantly decreased peripheral leukocyte levels, suggesting an impact of these contaminants on the immune system. As expected from this result, atrazine significantly increased susceptibility of larvae to ATV infection. In contrast, nitrate had a marginally significant main effect and significantly decreased infection rate at the highest level. However, neither atrazine nor sodium nitrate had significant effects on viral copy number per individual. These results suggest that ecologically relevant concentrations of atrazine and nitrates have immunosuppressive effects, and atrazine may contribute to ATV epizootics, raising concerns about the influence of contaminants on diseases in general.

Polyphenisms are excellent models for studying phenotypic variation, yet few studies have focused on natural populations. Facultative paedomorphosis is a polyphenism in which salamanders either metamorphose or retain their larval morphology and eventually become paedomorphic. Paedomorphosis can result from selection for capitalizing on favorable aquatic habitats (paedomorph advantage), but could also be a default strategy under poor aquatic conditions (best of a bad lot). We tested these alternatives by quantifying how the developmental environment influences the ontogeny of wild Arizona tiger salamanders (Ambystoma tigrinum nebulosum). Most paedomorphs in our study population arose from slow-growing larvae that developed under high density and size-structured conditions (best of a bad lot), although a few fastergrowing larvae also became paedomorphic (paedomorph advantage). Males were more likely to become paedomorphs than females and did so under a greater range of body sizes than females, signifying a critical role for gender in this polyphenism. Our results emphasize that the same phenotype can be adaptive under different environmental and genetic contexts and that studies of phenotypic variation should consider multiple mechanisms of morph production.

Simultaneous estimation of survival, reproduction, and movement is essential to understanding how species maximize lifetime reproduction in environments that vary across space and time. We conducted a four-year, capture—recapture study of three populations of eastern tiger salamanders (Ambystoma tigrinum tigrinum) and used multistate mark—recapture statistical methods to estimate the manner in which movement, survival, and breeding probabilities vary under different environmental conditions across years and among populations and habitats. We inferred how individuals may mitigate risks of mortality and reproductive failure by deferring breeding or by moving among populations. Movement probabilities among populations were extremely low despite high spatiotemporal variation in reproductive success and survival, suggesting possible costs to movements among breeding ponds. Breeding probabilities varied between wet and dry years and according to whether or not breeding was attempted in the previous year. Estimates of survival in the nonbreeding, forest habitat varied among populations but were consistent across time. Survival in breeding ponds was generally high in years with average or high precipitation, except for males in an especially ephemeral pond. A drought year incurred severe survival costs in all ponds to animals that attempted breeding. Female salamanders appear to defer these episodic survival costs of breeding by choosing not to breed in years when the risk of adult mortality is high. Using stochastic simulations of survival and breeding under historical climate conditions, we found that an interaction between breeding probabilities and mortality limits the probability of multiple breeding attempts differently between the sexes and among populations.

Microsatellites and mitochondrial DNA (mtDNA) have traditionally been used in population genetics because of their variability and presumed neutrality, whereas genes of the major histocompatibility complex (MHC) are increasingly of interest because strong selective pressures shape their standing variation. Despite the potential for MHC genes, microsatellites, and mtDNA sequences to complement one another in deciphering population history and demography, the three are rarely used in tandem. Here we report on MHC, microsatellite, and mtDNA variability in a single large population of the eastern tiger salamander (Ambystoma tigrinum tigrinum). We use the mtDNA mismatch distribution and, on microsatellite data, the imbalance index and bottleneck tests to infer aspects of population history and demography. Haplotype and allelic variation was high at all loci surveyed, and heterozygosity was high at the nuclear loci. We find concordance among neutral molecular markers that suggests our study population originated from post-Pleistocene expansions of multiple, fragmented sources that shared few migrants. Differences in Ne estimates derived from haploid and diploid genetic markers are potentially attributable to secondary contact among source populations that experienced rapid mtDNA divergence and comparatively low levels of nuclear DNA divergence. We find strong evidence of natural selection acting on MHC genes and estimate long-term effective population sizes (Ne) that are very large, making small selection intensities significant evolutionary forces in this population.

Biological invasions and habitat alteration are often detrimental to native species, but their interactions are difficult to predict. Interbreeding between native and introduced species generates novel genotypes and phenotypes, and human land use alters habitat structure and chemistry. Both invasions and habitat alteration create new biological challenges and opportunities. In the intensively farmed Salinas Valley, California (U.S.A.), threatened California tiger salamanders (Ambystoma californiense) have been replaced by hybrids between California tiger salamander and introduced barred tiger salamanders (Ambystoma tigrinum mavortium). We conducted an enclosure experiment to examine the effects habitat modification and relative frequency of hybrid and native California tiger salamanders have on recruitment of salamanders and their prey, Pacific chorus frogs (Pseudacris regilla). We tested whether recruitment differed among genetic classes of tiger salamanders (hybrid or native) and pond hydroperiod (seasonal or perennial). Roughly 6 weeks into the experiment, 70% (of 378 total) of salamander larvae died in 4 out of 6 ponds. Native salamanders survived (n = 12) in these ponds only if they had metamorphosed prior to the die-offs. During die-offs, all larvae of native salamanders died, whereas 56% of hybrid larvae died. We necropsied native and hybrid salamanders, tested water quality, and queried the California Department of Pesticide Regulation database to investigate possible causes of the die-offs. Salamander die-offs, changes in the abundance of other community members (invertebrates, algae, and cyanobacteria), shifts in salamander sex ratio, and patterns of pesticide application in adjacent fields suggest that pesticide use may have contributed to die-offs. That all survivors were hybrids suggests that environmental stress may promote rapid displacement of native genotypes. Las invasiones biológicas y la alteración del hábitat a menudo son perjudiciales para las especies nativas, pero es difícil predecir sus interacciones. El entrecruzamiento de especies nativas e introducidas genera genotipos y fenotipos nuevos, y el uso de tierras por humanos altera la estructura y química del hábitat. Tanto las invasiones como la alteración del hábitat crean nuevos retos y oportunidades biológicas. En el intensivamente cultivado Valle Salinas, California (E.U.A.), la salamandra de California amenazada (Ambystoma californiense) ha sido reemplazada por híbridos de A. californiense y la salamandra introducida (Ambystoma tigrinum mavortium). Realizamos un experimento con encierros para examinar los efectos de la modificación del hábitat y de la frecuencia relativa de salamandras híbridas y nativas sobre el reclutamiento de salamandras y su presa, ranas (Pseudacris regilla). Probamos si el reclutamiento difirió entre clases genéticas de salamandras (híbridas o nativas) y el hidroperíodo del cuerpo de agua (temporal o perenne). A seis semanas de iniciado el experimento, 70% (de un total de 378) de las larvas de salamandra murieron en 4 de 6 cuerpos de agua. Las salamandras nativas sobrevivieron (n = 12) en estos cuerpos de agua solo si habían metamorfoseado antes de las declinaciones. Durante las declinaciones, murieron todas las larvas de salamandras nativas, mientras que 56% de las larvas híbridas murieron. Realizamos necropsias a las salamandras nativas e híbridas, probamos la calidad del agua y revisamos la base de datos del Departamento de Regulación de Pesticidas de California para investigar posibles causas de las declinaciones. Las declinaciones de salamandras, cambios en la abundancia de otros miembros de la comunidad (invertebrados, algas y cianobacterias), cambios en la proporción de sexos de las salamandras y patrones de aplicación de pesticidas en campos adyacentes sugieren que el uso de pesticidas pudo haber contribuido a las declinaciones. El hecho de que todos los sobrevivientes son híbridos sugiere que el estrés ambiental puede promover un rápido desplazamiento de los genotipos nativos.

An individual's physiological condition early in ontogeny often regulates natal dispersal and philopatry decisions; however, increased condition promotes dispersal in some organisms and philopatry in others. These disparate findings likely arise from interactions among an individual's early life stage physiological condition, its likelihood of surviving a dispersal event (i.e., dispersal capacity), and its motivation to leave its natal site (i.e., dispersal propensity). Due to the broad importance of reproductive site selection to population structure and dynamics, studies disentangling these various phenotype-dependent effects are critical. We evaluated the relationships between two aspects of larval physiological condition and natal philopatry in Arizona tiger salamanders (Ambystoma tigrinum nebulosum) in an isolated pond system. In this population, geographic distance between ponds is small, adult survivorship is very high, and thus the likelihood of a successful breeding event should mediate reproductive site selection as opposed to the likelihood of surviving the dispersal event. We found that natal philopatry increased with an estimate of long-term body condition in males, but there was no relationship in females. However, natal philopatry decreased with an estimate of short-term body condition. We also found that an individual's natal pond had effects on philopatry that influenced both sexes and were independent of the pond's direct effects on body condition. Together, these findings strongly support the importance of an individual's early developmental experience in the natal environment to its reproductive behaviors across its lifespan, and further highlight the value of considering how phenotype-dependent dispersal mechanisms may vary between the sexes.

Virulent parasites cannot persist in small host populations unless the parasite also has a reservoir host. We hypothesize that, in hosts with complex life histories, one stage may act as an intraspecific reservoir for another. In amphibians, for example, larvae often occur at high densities, but these densities are ephemeral and fixed in space, whereas metamorphs are long-lived and vagile but may be very sparse. Parasite persistence is unlikely in either stage alone, but transmission between stages could maintain virulent parasites in seasonally fluctuating amphibian populations. We examined this hypothesis with a lethal ranavirus, Ambystoma tigrinum virus (ATV), that causes recurrent epidemics in larval tiger salamander populations, but which has no reservoir host and degrades quickly in the environment. Although exposure to ATV is generally lethal, larvae and metamorphs maintained sublethal, transmissible infections for >5 mo. Field data corroborate the persistence of ATV between epidemics in sublethally infected metamorphs. Three-quarters of dispersing metamorphs during one epidemic were infected, and apparently healthy metamorphs returning to breed harbored ATV infections. Our results suggest that larval epidemics amplify virus prevalence and sublethally infected metamorphs (re)introduce the virus into uninfected larval populations. Intraspecific reservoirs may explain the persistence of parasites in and declines of small, isolated amphibian populations.

Recent data indicates that blotched tiger salamanders (Ambystoma tigrinum melanostictum) in northern regions of Yellowstone National Park are declining due to climate-related habitat changes. In this study, we used ancient and modern mitochondrial haplotype diversity to model the effective size of this amphibian population through recent geological time and to assess past responses to climatic changes in the region. Using subfossils collected from a cave in northern Yellowstone, we analyzed >700 base pairs of mitochondrial sequence from 16 samples ranging in age from 100 to 3300 years old and found that all shared an identical haplotype. Although mitochondrial diversity was extremely low within the living population, we still were able to detect geographic subdivision within the local area. Using serial coalescent modelling with Bayesian priors from both modern and ancient genetic data we simulated a range of probable population sizes and mutation rates through time. Our simulations suggest that regional mitochondrial diversity has remained relatively constant even through climatic fluctuations of recent millennia.