Max R. Lambert

Adaptive evolution in urban ecosystems

Urban ecologists have demonstrated that cities are functioning ecosystems. It follows then that species living in these contexts should participate in and experience the same suite of biological processes, including evolution, that have occupied scientists for centuries in more “natural” contexts. In fact, urban ecosystems with myriad novel contexts, pressures, and species rosters provide unprecedentedly potent evolutionary stimuli. Here, we present the case for studying adaptive evolution in urban settings. We then review and synthesize techniques into a coherent approach for studying adaptive evolution in urban settings that combines observations of phenotypic divergence, measurements of fitness benefits of novel genetically based phenotypes, and experimental manipulations of potential drivers of adaptation. We believe that studying evolution in urban contexts can provide insights into fundamental evolutionary biology questions on rate, direction, and repeatability of evolution, and may inform species and ecosystem service conservation efforts.

Suburbanization, estrogen contamination, and sex ratio in wild amphibian populations

Significance We focus on a critical issue, the influence of human-derived contaminants on wildlife populations. Endocrine disrupting chemicals that act through hormonal pathways are capable of having large influences even when concentrations are relatively low. While there is evidence that such endocrine disruption can result from the application of agricultural pesticides and through exposure to wastewater effluent, we have identified a diversity of endocrine disrupting chemicals within suburban neighborhoods. Sampling populations of a local frog species, we found a strong association between the degree of landscape development and frog offspring sex ratio. Our study points to rarely studied contamination sources, like vegetation landscaping and impervious surface runoff, that may be associated with endocrine disruption environments around suburban homes. Research on endocrine disruption in frog populations, such as shifts in sex ratios and feminization of males, has predominantly focused on agricultural pesticides. Recent evidence suggests that suburban landscapes harbor amphibian populations exhibiting similar levels of endocrine disruption; however the endocrine disrupting chemical (EDC) sources are unknown. Here, we show that sex ratios of metamorphosing frogs become increasingly female-dominated along a suburbanization gradient. We further show that suburban ponds are frequently contaminated by the classical estrogen estrone and a variety of EDCs produced by plants (phytoestrogens), and that the diversity of organic EDCs is correlated with the extent of developed land use and cultivated lawn and gardens around a pond. Our work also raises the possibility that trace-element contamination associated with human land use around suburban ponds may be contributing to the estrogenic load within suburban freshwaters and constitutes another source of estrogenic exposure for wildlife. These data suggest novel, unexplored pathways of EDC contamination in human-altered environments. In particular, we propose that vegetation changes associated with suburban neighborhoods (e.g., from forests to lawns and ornamental plants) increase the distribution of phytoestrogens in surface waters. The result of frog sex ratios varying as a function of human land use implicates a role for environmental modulation of sexual differentiation in amphibians, which are assumed to only have genetic sex determination. Overall, we show that endocrine disruption is widespread in suburban frog populations and that the causes are likely diverse.

Sex-linked markers in the North American green frog ( Rana clamitans ) developed using DArTseq provide early insight into sex chromosome evolution

BackgroundThe extent to which sex reversal is associated with transitions in sex determining systems (XX-XY, ZZ-ZW, etc.) or abnormal sexual differentiation is predominantly unexplored in amphibians. This is in large part because most amphibian taxa have homomorphic sex chromosomes, which has traditionally made it challenging to identify discordance between phenotypic and genetic sex in amphibians, despite all amphibians having a genetic component to sex determination. Recent advances in molecular techniques such as genome complexity reduction and high throughput sequencing present a valuable avenue for furthering our understanding of sex determination in amphibians and other taxa with homomorphic sex chromosomes like many fish and reptiles.ResultsWe use DArTseq as a novel approach to identify sex-linked markers in the North American green frog (Rana clamitans melanota) using lab-reared tadpoles as well as wild-caught adults from seven ponds either in undeveloped, forested habitats or suburban ponds known to be subject to contamination by anthropogenic chemicals. The DArTseq methodology identified 13 sex-linked SNP loci and eight presence-absence loci associated with males, indicating an XX-XY system. Both alleles from a single locus show partial high sequence homology to Dmrt1, a gene linked to sex determination and differentiation throughout Metazoa. Two other loci have sequence similarities to regions of the chimpanzee and human X-chromosome as well as the chicken Z-chromosome. Several loci also show geographic variation in sex-linkage, possibly indicating sex chromosome recombination. While all loci are statistically sex-linked, they show varying degrees of female heterozygosity and male homozygosity, providing further evidence that some markers are on regions of the sex chromosomes undergoing higher rates of recombination and therefore further apart from the putative sex determining locus.ConclusionThe ease of the DArTseq platform provides a useful avenue for future research on sex reversal and sex chromosome evolution in vertebrates, particularly for non-model species with homomorphic or cryptic or nascent sex chromosomes.

Diverse sources for endocrine disruption in the wild

ABSTRACT We are increasingly realizing that diverse human activities are associated with endocrine disruption in wildlife populations. Most field research on endocrine disruption tends to narrowly target particular endocrine disrupting chemicals (EDCs) and contaminant sources as the dominant culprit of observed endocrine disruption. Recently, we and our colleagues assessed patterns of endocrine disruption in metamorphosing frogs in ponds along a forest-suburban land use gradient. In conjunction, we assayed for a diversity of putative of EDCs from an array of possible sources. We found that endocrine disruption in suburban frog populations was associated with a cocktail of different EDCs including classical estrogens, phytoestrogens, and metalloestrogens. These results indicate that endocrine disruption in suburban frogs is likely to be the product of multiple chemicals and sources. Our findings have implications for other systems where endocrine disruption is attributed to a limited set of contaminants or sources without necessarily exploring the contributions of other EDC pathways. Here, we discuss our findings in the context of other field endocrine disruption research and encourage future studies to assess the broader EDC landscape which may be impacting wildlife populations.

Clover root exudate produces male-biased sex ratios and accelerates male metamorphic timing in wood frogs

In amphibians, abnormal metamorph sex ratios and sexual development have almost exclusively been considered in response to synthetic compounds like pesticides or pharmaceuticals. However, endocrine-active plant chemicals (i.e. phytoestrogens) are commonly found in agricultural and urban waterways hosting frog populations with deviant sexual development. Yet the effects of these compounds on amphibian development remain predominantly unexplored. Legumes, like clover, are common in agricultural fields and urban yards and exude phytoestrogen mixtures from their roots. These root exudates serve important ecological functions and may also be a source of phytoestrogens in waterways. I show that clover root exudate produces male-biased sex ratios and accelerates male metamorphosis relative to females in low and intermediate doses of root exudate. My results indicate that root exudates are a potential source of contaminants impacting vertebrate development and that humans may be cultivating sexual abnormalities in wildlife by actively managing certain plant species.

Hormonally-active phytochemicals and vertebrate evolution

Living plants produce a diversity of chemicals that share structural and functional properties with vertebrate hormones. Wildlife species interact with these chemicals either through consumption of plant materials or aquatic exposure. Accumulating evidence shows that exposure to these hormonally active phytochemicals (HAPs) often has consequences for behavior, physiology, and fecundity. These fitness effects suggest there is potential for an evolutionary response by vertebrates to HAPs. Here, we explore the toxicological HAP–vertebrate relationship in an evolutionary framework and discuss the potential for vertebrates to adapt to or even co‐opt the effects of plant‐derived chemicals that influence fitness. We lay out several hypotheses about HAPs and provide a path forward to test whether plant‐derived chemicals influence vertebrate reproduction and evolution. Studies of phytochemicals with direct impacts on vertebrate reproduction provide an obvious and compelling system for studying evolutionary toxicology. Furthermore, an understanding of whether animal populations evolve in response to HAPs could provide insightful context for the study of rapid evolution and how animals cope with chemical agents in the environment.

Habitat Features Determine the Basking Distribution of Introduced Red-Eared Sliders and Native Western Pond Turtles

Abstract We quantified basking site usage by native western pond turtles (Emys [ =  Actinemys] marmorata) and introduced red-eared sliders (Trachemys scripta elegans) in an urbanized waterway. A lower frequency of human disturbance, steeper slopes, shallower water adjacent to basking sites, and a concrete substrate were all associated with a higher abundance of native threatened pond turtles relative to introduced sliders. These differences suggest new habitat management practices that could favor native pond turtles in the face of competition from invasive sliders.

Septic systems, but not sanitary sewer lines, are associated with elevated estradiol in male frog metamorphs from suburban ponds

Suburban neighborhoods are a dominant type of human land use. Many housing regions globally rely on septic systems, rather than sanitary sewers, for wastewater management. There is evidence that septic systems may contaminate waterbodies more than sewer lines. There is also mounting evidence that human activities contaminate waterways with endocrine-disrupting chemicals (EDCs), which alter wildlife sexual development. While endocrine disruption is often associated with intense activities such as agriculture or wastewater treatment plant discharges, recent evidence indicates that endocrine disruption is pervasive in frogs from suburban neighborhoods. In conjunction with other putative EDC sources, one hypothesis is that wastewater is contaminating suburban waterways with EDCs derived from pharmaceuticals or personal care products. Here, we measure estradiol (E2) in metamorphosing green frogs (Rana clamitans) from forested ponds and suburban ponds adjacent to either septic tanks or sanitary sewers. We show that E2 is highest in male frogs from septic neighborhoods and that E2 concentrations are significantly lower in male frogs from forested ponds and from ponds near sewers. These results indicate that septic tanks may be contaminating aquatic ecosystems differently than sewer lines. This pattern contrasts prior work showing no difference in EDC contamination or morphological endocrine disruption between septic and sewer neighborhoods, implying that suburbanization may have varying effects at multiple biological scales like physiology and anatomy.

Large-scale experimental removal of non-native slider turtles has unexpected consequences on basking behavior for both conspecifics and a native, threatened turtle

The red-eared slider turtle (Trachemys scripta elegans; RES) is one of the world’s most invasive species. Native to the central United States, RES are now widely established in freshwater habitats across the globe, largely due to release of unwanted pets. Laboratory and mesocosm experiments suggest that introduced RES are competitively dominant to native turtles, but such competition remains untested in the wild. Here, we experimentally removed introduced RES to test whether they compete for critical basking habitat with native, threatened western pond turtles (Emys marmorata; WPT), a species being considered for listing under the U.S. Endangered Species Act. Following removal, we found that both the remaining RES as well as WPT altered their basking distribution but in a manner inconsistent with strong interspecific competition. However, these findings suggest strong intraspecific competition for basking sites amongst RES and that interspecific competition between WPT and introduced RES likely occurs at higher RES densities. Our works suggests RES influence the behavior of native species in the wild and indicates that RES removal may be most beneficial at high RES densities. This experiment highlights the importance of considering experimental venue when evaluating competition between native and non-native species and should encourage conservation biologists to treat removal efforts as experiments.

Reconstruction of female heterogamety from admixture of XX-XY and ZZ-ZW sex-chromosome systems within a frog species

Sex‐determining mechanisms change repeatedly throughout evolution, and it is difficult to track this continual process. The Japanese soil‐frog Glandirana rugosa is a remarkable evolutionary witness to the ongoing process of the evolution of sex‐determining modes. The two geographic groups, designated XY and Neo‐ZW, have homologous sex chromosomes, yet display opposite types of sex chromosomes, XX‐XY and ZZ‐ZW, respectively. These two groups are sympatric at the edges of their respective ranges in Central Japan. In this study, we discovered molecular evidence that the eastern part of the Neo‐ZW group (Neo‐ZW2 subgroup), which is found near the sympatric area, shares mitochondrial haplotypes with the XY group. By analysing single nucleotide polymorphism (SNP) loci, we have also discovered that the representative nuclear genome of the Neo‐ZW2 subgroup shares allele clusters with both the XY group and another part of the Neo‐ZW group (Neo‐ZW1 subgroup), indicating a hybrid origin of the Neo‐ZW2. Further analysis of sex‐linked SNP loci revealed that the alleles on the W chromosomes of the Neo‐ZW2 were derived mostly from X chromosomes, while alleles on the Z chromosomes originated from the Z chromosomes of the Neo‐ZW1 subgroup and partly from the Y chromosomes of the XY group. Our study revealed that admixture of the two opposite sex‐chromosome systems reconstructed a female heterogametic system by recycling the X chromosomes into new W chromosomes. This work offers an illustrative example of how de novo sex‐chromosome systems can arise by recycling material from ancestral sex chromosomes.