Suzanne E. McGaugh

The cavefish genome reveals candidate genes for eye loss

Natural populations subjected to strong environmental selection pressures offer a window into the genetic underpinnings of evolutionary change. Cavefish populations, Astyanax mexicanus (Teleostei: Characiphysi), exhibit repeated, independent evolution for a variety of traits including eye degeneration, pigment loss, increased size and number of taste buds and mechanosensory organs, and shifts in many behavioural traits. Surface and cave forms are interfertile making this system amenable to genetic interrogation; however, lack of a reference genome has hampered efforts to identify genes responsible for changes in cave forms of A. mexicanus. Here we present the first de novo genome assembly for Astyanax mexicanus cavefish, contrast repeat elements to other teleost genomes, identify candidate genes underlying quantitative trait loci (QTL), and assay these candidate genes for potential functional and expression differences. We expect the cavefish genome to advance understanding of the evolutionary process, as well as, analogous human disease including retinal dysfunction.

Genomic impacts of chromosomal inversions in parapatric Drosophila species

Chromosomal inversions impact genetic variation and facilitate speciation in part by reducing recombination in heterokaryotypes. We generated multiple whole-genome shotgun sequences of the parapatric species pair Drosophila pseudoobscura and Drosophila persimilis and their sympatric outgroup (Drosophila miranda) and compared the average pairwise differences for neutral sites within, just outside and far outside of the three large inversions. Divergence between D. pseudoobscura and D. persimilis is high inside the inversions and in the suppressed recombination regions extending 2.5 Mb outside of inversions, but significantly lower in collinear regions further from the inversions. We observe little evidence of decreased divergence predicted to exist in the centre of inversions, suggesting that gene flow through double crossovers or gene conversion is limited within the inversion, or selection is acting within the inversion to maintain divergence in the face of gene flow. In combination with past studies, we provide evidence that inversions in this system maintain areas of high divergence in the face of hybridization, and have done so for a substantial period of time. The left arm of the X chromosome and chromosome 2 inversions appear to have arisen in the lineage leading to D. persimilis approximately 2 Ma, near the time of the split of D. persimilis–D. pseudoobscura–D. miranda, but likely fixed within D. persimilis much more recently, as diversity within D. persimilis is substantially reduced inside and near these two inversions. We also hypothesize that the inversions in D. persimilis may provide an empirical example of the ‘mixed geographical mode’ theory of inversion origin and fixation, whereby allopatry and secondary contact both play a role.

Recombination Modulates How Selection Affects Linked Sites in Drosophila

Recombination rate in Drosophila species shapes the impact of selection in the genome and is positively correlated with nucleotide diversity.

Contrasting demographic and genetic estimates of dispersal in the endangered Coahuilan box turtle: a contemporary approach to conservation

The evolutionary viability of an endangered species depends upon gene flow among subpopulations and the degree of habitat patch connectivity. Contrasting population connectivity over ecological and evolutionary timescales may provide novel insight into what maintains genetic diversity within threatened species. We employed this integrative approach to evaluating dispersal in the critically endangered Coahuilan box turtle (Terrapene coahuila) that inhabits isolated wetlands in the desert‐spring ecosystem of Cuatro Ciénegas, Mexico. Recent wetland habitat loss has altered the spatial distribution and connectivity of habitat patches; and we therefore predicted that T. coahuila would exhibit limited movement relative to estimates of historic gene flow. To evaluate contemporary dispersal patterns, we employed mark–recapture techniques at both local (wetland complex) and regional (intercomplex) spatial scales. Gene flow estimates were obtained by surveying genetic variation at nine microsatellite loci in seven subpopulations located across the species’ geographical range. The mark–recapture results at the local spatial scale reveal frequent movement among wetlands that was unaffected by interwetland distance. At the regional spatial scale, dispersal events were relatively less frequent between wetland complexes. The complementary analysis of population genetic substructure indicates strong historic gene flow (global FST = 0.01). However, a relationship of genetic isolation by distance across the geographical range suggests that dispersal limitation exists at the regional scale. Our approach of contrasting direct and indirect estimates of dispersal at multiple spatial scales in T. coahuila conveys a sustainable evolutionary trajectory of the species pending preservation of threatened wetland habitats and a range‐wide network of corridors.

Inheritance of nesting behaviour across natural environmental variation in a turtle with temperature-dependent sex determination

Nesting behaviour is critical for reproductive success in oviparous organisms with no parental care. In organisms where sex is determined by incubation temperature, nesting behaviour may be a prime target of selection in response to unbalanced sex ratios. To produce an evolutionary change in response to sex-ratio selection, components of nesting behaviour must be heritable. We estimated the field heritability of two key components of nesting behaviour in a population of painted turtles (Chrysemys picta) with temperature-dependent sex determination by applying the ‘animal model’ to a pedigree reconstructed from genotype data. We obtained estimates of low to non-detectable heritability using repeated records across all environments. We then determined environment-specific heritability by grouping records with similar temperatures for the winter preceding the nesting season, a variable known to be highly associated with our two traits of interest, nest vegetation cover and Julian date of nesting. The heritability estimates of nest vegetation cover and Julian date of nesting were qualitatively highest and significant, or nearly so, after hot winters. Additive genetic variance for these traits was not detectable after cold winters. Our analysis suggests that the potential for evolutionary change of nesting behaviour may be dependent on the thermal conditions of the preceding winter, a season that is predicted to be especially subject to climate change.

State-dependent physiological maintenance in a long-lived ectotherm, the painted turtle (Chrysemys picta).

SUMMARY Energy allocation among somatic maintenance, reproduction and growth varies not only among species, but among individuals according to states such as age, sex and season. Little research has been conducted on the somatic (physiological) maintenance of long-lived organisms, particularly ectotherms such as reptiles. In this study, we examined sex differences and age- and season-related variation in immune function and DNA repair efficiency in a long-lived reptile, the painted turtle (Chrysemys picta). Immune components tended to be depressed during hibernation, in winter, compared with autumn or spring. Increased heterophil count during hibernation provided the only support for winter immunoenhancement. In juvenile and adult turtles, we found little evidence for senescence in physiological maintenance, consistent with predictions for long-lived organisms. Among immune components, swelling in response to phytohemagglutinin (PHA) and control injection increased with age, whereas basophil count decreased with age. Hatchling turtles had reduced basophil counts and natural antibodies, indicative of an immature immune system, but demonstrated higher DNA repair efficiency than older turtles. Reproductively mature turtles had reduced lymphocytes compared with juvenile turtles in the spring, presumably driven by a trade-off between maintenance and reproduction. Sex had little influence on physiological maintenance. These results suggest that components of physiological maintenance are modulated differentially according to individual state and highlight the need for more research on the multiple components of physiological maintenance in animals of variable states.

Effective heritability of targets of sex-ratio selection under environmental sex determination.

Selection is expected to maintain primary sex ratios at an evolutionary equilibrium. In organisms with temperature‐dependent sex determination (TSD), targets of sex‐ratio selection include the thermal sensitivity of the sex‐determining pathway (hereafter, sex determination threshold) and nest‐site choice. However, offspring sex may be canalized for nests located in thermally extreme environments; thus, genetic variance for the sex determination threshold is not expressed and is invisible to direct selection. The concept of ‘effective heritability’ accounts for this dependence and provides a more realistic prediction of the expected evolutionary response to selection in the wild. Past estimates of effective heritability of the sex determination threshold, which were derived from laboratory data, suggested that the potential for the sex determination threshold to evolve in the wild was extremely low. We re‐evaluated original estimates of this parameter by analysing field‐collected measures of nest temperatures, vegetation cover and clutch sex ratios from nests in a population of painted turtles (Chrysemys picta). We coupled these data with measurements of broad‐sense heritability of the sex determination threshold in C. picta, using an experiment that splits clutches of eggs between a constant temperature (i.e. typical laboratory incubation) and a daily fluctuating temperature (i.e. similar to natural nests) with the same mean. We found that (i) the effective heritability of the sex determination threshold appears to have been historically underestimated and the effective heritability of nest‐site choice has been overestimated and (ii) significant family‐by‐incubation treatment interaction exists for sex for C. picta between constant‐ and fluctuating‐temperature regimes. Our results suggest that the thermal sensitivity of the sex‐determining pathway may play a larger, more complex role in the microevolution of TSD than traditionally thought.

Rapid molecular evolution across amniotes of the IIS/TOR network

Significance Comparative analyses of central molecular networks uncover variation that can be targeted by biomedical research to develop insights and interventions into disease. The insulin/insulin-like signaling and target of rapamycin (IIS/TOR) molecular network regulates metabolism, growth, and aging. With the development of new molecular resources for reptiles, we show that genes in IIS/TOR are rapidly evolving within amniotes (mammals and reptiles, including birds). Additionally, we find evidence of natural selection that diversified the hormone-receptor binding relationships that initiate IIS/TOR signaling. Our results uncover substantial variation in the IIS/TOR network within and among amniotes and provide a critical step to unlocking information on vertebrate patterns of genetic regulation of metabolism, modes of reproduction, and rates of aging. The insulin/insulin-like signaling and target of rapamycin (IIS/TOR) network regulates lifespan and reproduction, as well as metabolic diseases, cancer, and aging. Despite its vital role in health, comparative analyses of IIS/TOR have been limited to invertebrates and mammals. We conducted an extensive evolutionary analysis of the IIS/TOR network across 66 amniotes with 18 newly generated transcriptomes from nonavian reptiles and additional available genomes/transcriptomes. We uncovered rapid and extensive molecular evolution between reptiles (including birds) and mammals: (i) the IIS/TOR network, including the critical nodes insulin receptor substrate (IRS) and phosphatidylinositol 3-kinase (PI3K), exhibit divergent evolutionary rates between reptiles and mammals; (ii) compared with a proxy for the rest of the genome, genes of the IIS/TOR extracellular network exhibit exceptionally fast evolutionary rates; and (iii) signatures of positive selection and coevolution of the extracellular network suggest reptile- and mammal-specific interactions between members of the network. In reptiles, positively selected sites cluster on the binding surfaces of insulin-like growth factor 1 (IGF1), IGF1 receptor (IGF1R), and insulin receptor (INSR); whereas in mammals, positively selected sites clustered on the IGF2 binding surface, suggesting that these hormone-receptor binding affinities are targets of positive selection. Further, contrary to reports that IGF2R binds IGF2 only in marsupial and placental mammals, we found positively selected sites clustered on the hormone binding surface of reptile IGF2R that suggest that IGF2R binds to IGF hormones in diverse taxa and may have evolved in reptiles. These data suggest that key IIS/TOR paralogs have sub- or neofunctionalized between mammals and reptiles and that this network may underlie fundamental life history and physiological differences between these amniote sister clades.

Molecular phylogeography of Apalone spinifera (Reptilia, Trionychidae)

The molecular evaluation of morphologically based species delimitations of many herpetofauna has improved the understanding of evolutionary processes and the rigor of conservation efforts. Previous evidence for a deep lineage divide between south‐eastern + northern subspecies of the softshell turtle Apalone spinifera (A. s. aspera + A. s. hartwegi + A. s. spinifera) and western subspecies (A. s. pallida + A. s. emoryi + A. s. guadalupensis) was re‐examined with a broader sampling using mitochondrial and nuclear markers. The south‐eastern + northern clade and the western clade maintained mitochondrial reciprocal monophyly. We molecularly confirmed a geographical boundary between these two clades in Oklahoma, and developed a phylogeographical hypothesis that invokes stream capture. We evaluated whether these mitochondrial lineages represent distinct species by surveying these clades for divergence at the nuclear intron R35 and two nuclear genes, Cmos and recombination activating gene 1 (RAG‐1). The nuclear loci showed no phylogenetic resolution and only the nuclear intron exhibited significant nearest neighbour statistics. Taken together, the nuclear data suggest that taxonomic elevation of the two mitochondrial clades would be currently unjustified.

Color Variation among Habitat Types in the Spiny Softshell Turtles (Trionychidae: Apalone) of Cuatrociénegas, Coahuila, Mexico

Abstract Ground coloration is highly variable in many reptile species. In turtles, ground color may correspond well to the background coloration of the environment and can change over time to match new surroundings in the laboratory. Variable carapace and plastron coloration across three habitat types were investigated in the Black Softshell Turtle, Apalone spinifera atra, by measuring individual components of the RGB (Red, Green, Blue) color system. In general, A. s. atra carapaces were darker in turtles from lagoons than in turtles from playa lakes. Red and green values were significantly different among all pairs of habitat types, but blue values differed only between the playa lakes and lagoons. Mean color components (RG only) for each population were significantly correlated with corresponding values for the bottom substrate, indicating a positive association of carapace and habitat substrate color components. In contrast, plastron ground color RGB channels showed no significant differences between habitat types and no significant correlations with substrate RGB. These results suggest that dorsal background matching in A. s. atra may be responsible for some of the variation in this key taxonomic trait.