Jarrett R. Johnson

Rapid spread of invasive genes into a threatened native species.

When introduced or cultivated plants or animals hybridize with their native relatives, the spread of invasive genes into native populations might have biological, aesthetic, and legal implications. Models suggest that the rate of displacement of native by invasive alleles can be rapid and inevitable if they are favored by natural selection. We document the spread of a few introduced genes 90 km into a threatened native species (the California Tiger Salamander) in 60 years. Meanwhile, a majority of genetic markers (65 of 68) show little evidence of spread beyond the region where introductions occurred. Using computer simulations, we found that such a pattern is unlikely to emerge by chance among selectively neutral markers. Therefore, our results imply that natural selection has favored both the movement and fixation of these exceptional invasive alleles. The legal status of introgressed populations (native populations that are slightly genetically modified) is unresolved by the US Endangered Species Act. Our results illustrate that genetic and ecological factors need to be carefully weighed when considering different criteria for protection, because different rules could result in dramatically different geographic areas and numbers of individuals being protected.

Genome-enabled development of DNA markers for ecology, evolution and conservation.

Molecular markers have become a fundamental piece of modern biology’s toolkit. In the last decade, new genomic resources from model organisms and advances in DNA sequencing technology have altered the way that these tools are developed, alleviating the marker limitation that researchers previously faced and opening new areas of research for studies of non‐model organisms. This availability of markers is directly responsible for advances in several areas of research, including fine‐scaled estimation of population structure and demography, the inference of species phylogenies, and the examination of detailed selective pressures in non‐model organisms. This review summarizes methods for the development of large numbers of DNA markers in non‐model organisms, the challenges encountered when utilizing different methods, and new research applications resulting from these advances.

Rapid fixation of non-native alleles revealed by genome-wide SNP analysis of hybrid tiger salamanders

BackgroundHybrid zones represent valuable opportunities to observe evolution in systems that are unusually dynamic and where the potential for the origin of novelty and rapid adaptation co-occur with the potential for dysfunction. Recently initiated hybrid zones are particularly exciting evolutionary experiments because ongoing natural selection on novel genetic combinations can be studied in ecological time. Moreover, when hybrid zones involve native and introduced species, complex genetic patterns present important challenges for conservation policy. To assess variation of admixture dynamics, we scored a large panel of markers in five wild hybrid populations formed when Barred Tiger Salamanders were introduced into the range of California Tiger Salamanders.ResultsAt three of 64 markers, introduced alleles have largely displaced native alleles within the hybrid populations. Another marker (GNAT1) showed consistent heterozygote deficits in the wild, and this marker was associated with embryonic mortality in laboratory F2s. Other deviations from equilibrium expectations were idiosyncratic among breeding ponds, consistent with highly stochastic demographic effects.ConclusionWhile most markers retain native and introduced alleles in expected proportions, strong selection appears to be eliminating native alleles at a smaller set of loci. Such rapid fixation of alleles is detectable only in recently formed hybrid zones, though it might be representative of dynamics that frequently occur in nature. These results underscore the variable and mosaic nature of hybrid genomes and illustrate the potency of recombination and selection in promoting variable, and often unpredictable genetic outcomes. Introgression of a few, strongly selected introduced alleles should not necessarily affect the conservation status of California Tiger Salamanders, but suggests that genetically pure populations of this endangered species will be difficult to maintain.

Invasive hybrid tiger salamander genotypes impact native amphibians

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.

Breeding and Recruitment Phenology of Amphibians in Missouri Oak-Hickory Forests

ABSTRACT Knowing the phenologies of species in a region helps guide management and conservation activities in breeding ponds and surrounding terrestrial habitats. We examined the phenology of pond-breeding amphibians in central Missouri oak-hickory forests. Two ponds were monitored for 4 y from 2000–2003 and five other ponds were monitored for 1 y during 2004 using drift fences with pitfall traps. We found 15 species of pond-breeding amphibians, nine of which we captured in sufficient abundance to evaluate breeding phenology. Among the nine species, breeding migrations occur from Feb. to Nov., while subsequent metamorph emigration occurred primarily from May to Oct.. Our ponds were nearly permanent, resulting in salamander-dominated communities in these oak-hickory forests. Pond-use was partitioned by species that differed in their timing of reproduction and post-metamorphic emergence. For example, breeding in the fall gives larval ringed salamanders a potential size advantage over the spring-breeding spotted salamander larvae. However, the fall breeding strategy requires ponds with long hydroperiods that hold water continuously from Aug. through May. Green frogs and central newts also required long hydroperiods for their larval stage (>160 d). American toads, however, are adapted to exploit ponds with shorter, less predictable hydroperiods as they only required ponds to hold water for as little as 60 d. Management for multiple species of pond-breeding amphibians in a landscape benefits from the inclusion of wetlands with a variety of hydroperiods.

Retention of low-fitness genotypes over six decades of admixture between native and introduced tiger salamanders

BackgroundIntroductions of non-native tiger salamanders into the range of California tiger salamanders have provided a rare opportunity to study the early stages of secondary contact and hybridization. We produced first- and second-generation hybrid salamanders in the lab and measured viability among these early-generation hybrid crosses to determine the strength of the initial barrier to gene exchange. We also created contemporary-generation hybrids in the lab and evaluated the extent to which selection has affected fitness over approximately 20 generations of admixture. Additionally, we examined the inheritance of quantitative phenotypic variation to better understand how evolution has progressed since secondary contact.ResultsWe found significant variation in the fitness of hybrids, with non-native backcrosses experiencing the highest survival and F2 hybrids the lowest. Contemporary-generation hybrids had similar survival to that of F2 families, contrary to our expectation that 20 generations of selection in the wild would eliminate unfit genotypes and increase survival. Hybrid survival clearly exhibited effects of epistasis, whereas size and growth showed mostly additive genetic variance, and time to metamorphosis showed substantial dominance.ConclusionsBased on first- and second- generation cross types, our results suggest that the initial barrier to gene flow between these two species was relatively weak, and subsequent evolution has been generally slow. The persistence of low-viability recombinant hybrid genotypes in some contemporary populations illustrates that while hybridization can provide a potent source of genetic variation upon which natural selection can act, the sorting of fit from unfit gene combinations might be inefficient in highly admixed populations. Spatio-temporal fluctuation in selection or complex genetics has perhaps stalled adaptive evolution in this system despite selection for admixed genotypes within generations.

Diet of the Gray Treefrog (Hyla Versicolor) in Relation to Foraging Site Location

Abstract Despite growing concern over habitat destruction, little is known regarding the activities of pond-breeding amphibians in the terrestrial environment. Yet, because most pond-breeding amphibian species spend the majority of their time in terrestrial habitats, it is important to understand what role terrestrial habitat plays in their life history. We examined the stomach contents of the Gray Treefrog (Hyla versicolor) in central Missouri using a stomach-flushing technique. Treefrogs were stomach-flushed; stomach contents were dried and weighed; and prey items were counted and identified for frogs caught in both artificial arboreal refugia and at breeding ponds. The majority of prey consisted of ants (41.2%) and beetles (29.6%). Both males and females caught in artificial refugia contained greater stomach content mass than those caught at breeding ponds. There was a positive correlation between mass of stomach contents and distance from breeding ponds, with the average number of beetles per stomach increasing with distance from ponds. There was also greater stomach content mass in frogs found in artificial refugia on white oaks than red oaks or sugar maples, but there was no relationship between tree diameter and stomach content mass. These results demonstrate the importance of protecting terrestrial habitat to maintain foraging areas for treefrogs.

SEASONAL TERRESTRIAL MICROHABITAT USE BY GRAY TREEFROGS (HYLA VERSICOLOR) IN MISSOURI OAK-HICKORY FORESTS

It is important to identify and understand the critical habitat components of organisms inhabiting landscapes that are increasingly altered by human activities to adequately predict the effects of habitat alteration on natural populations. Our study identifies terrestrial habitats that are important to the gray treefrog (Hyla versicolor) during the nonbreeding season using radiotracking and mark-recapture/release of individuals captured in artificial arboreal refugia. High humidity and rainfall were associated with decreased captures in artificial refugia, while high ambient air temperatures were correlated with increases in the number of treefrog captures. Refugia placed in small trees recorded more small individuals than those in larger trees, and refugia in white oaks (Quercus alba) contained females more frequently than expected at random. We found that space-use estimates encompassing foraging and overwintering locations of gray treefrogs were relatively small and typically included only a few adjacent trees during the nonbreeding season. We discuss the physical and biological aspects of habitat patches that may be important in determining the persistence of gray treefrog populations.

Microsatellite analysis supports mitochondrial phylogeography of the hellbender (Cryptobranchus alleganiensis).

We investigated genetic diversity of the hellbender (Cryptobranchus alleganiensis) throughout its range in the eastern US using nuclear markers and compared our results to a previously published mitochondrial analysis. A variety of nuclear markers, including protein-coding gene introns and microsatellites were tested but only microsatellites were variable enough for population level analysis. Microsatellite loci showed moderate among population sharing of alleles, in contrast to the reciprocal monophyly exhibited by mitochondrial DNA. However, analyses using F-statistics and Bayesian clustering algorithms showed considerable population subdivision and clustered hellbender populations into the same major groups as the mtDNA. The microsatellites combined with the mtDNA data suggest that gene flow is severely restricted or non-existent among eight major groups, and potentially among populations (rivers) within groups. The combined mtDNA and microsatellite data suggest that the currently recognized hellbender subspecies are paraphyletic. We suggest that the eight independent groups identified in our study should be managed as such, rather than basing conservation decisions on the two named subspecies of hellbender.

Level of UV-B radiation influences the effects of glyphosate-based herbicide on the spotted salamander

Glyphosate-based herbicides are the number one pesticide in the United States and are used commonly around the world. Understanding the affects of glyphosate-based herbicides on non-target wildlife, for example amphibians, is critical for evaluation of regulations pertaining to the use of such herbicides. Additionally, it is important to understand how variation in biotic and abiotic environmental conditions, such as UV-B light regime, could potentially affect how glyphosate-based herbicides interact with non-target species. This study used artificial pond mesocosms to identify the effects of generic glyphosate-based herbicide (GLY-4 Plus) on mortality, cellular immune response, body size, and morphological plasticity of larvae of the spotted salamander (Ambystoma maculatum) under conditions that reflect moderate (UVM) and low (UVL) UV-B light regimes. Survival within a given UV-B level was unaffected by herbicide presence or absence. However, when herbicide was present, survival varied between UV-B levels with higher survival in UVM conditions. Herbicide presence in the UVM treatments also decreased body size and reduced cellular immune response. In the UVL treatments, the presence of herbicide increased body size and affected tail morphology. Finally, in the absence of herbicide, body size and cellular immune response were higher in UVM treatments compared to UVL treatments. Thus, the effects of herbicide on salamander fitness were dependent on UV-B level. As anthropogenic habitat modifications continue to alter landscapes that contain amphibian breeding ponds, salamanders may increasingly find themselves in locations with reduced canopy cover and increased levels of UV light. Our findings suggest that the probability of surviving exposure to the glyphosate-based herbicide used in this study may be elevated in more open canopy ponds, but the effects on other components of fitness may be varied and unexpected.