Variation in Melanin Content of Lizard Livers: Hybrids Turning to the Dark Side.

AbstractPigments such as melanin are most often associated with the surface of an organism, providing functions such as coloration and protection from UV radiation. However, the internal organs of some species also contain melanin. Internal melanin may also perform protective functions when cellular stress is experienced. We tested liver tissue of two tree lizard species that experienced introgression of their mitochondria to see whether melanin was present and whether it was at higher concentrations in the types of lizards thought to be under cellular stress. Previous work found that mitochondria from livers of hybrid tree lizards (those with introgressed mitochondrial DNA) had higher levels of ATP production at higher temperatures than both parental species and showed indications of dysfunction. Therefore, we predicted that if internal melanin functions to mediate the impact of damaging by-products of metabolism, such as excessive reactive oxygen species (ROS), melanin would be highest in livers of lizards with introgressed mitochondria. To test this, we used a melanin-specific stain on liver tissue sections and measured melanin concentration with spectrophotometry of heavy-membrane fractions from whole-liver homogenates of both parental species and their hybrids with introgressed mitochondria. Slides of liver sections treated with a melanin-specific stain revealed that hybrids contained melanin and had significantly higher levels of it than either parental species. Spectrophotometry gave the same result. This distribution of internal melanin matches the pattern expected if melanin functions as a compensatory mechanism to deal with higher ATP production and the subsequent high levels of ROS expected in these hybrid lizards. Future studies will examine this mechanistically.

Functional Divergence of Mitochondria and Coevolution of Genomes: Cool Mitochondria in Hot Lizards.

Mitochondria play a key role in the ecology and evolution of species through their influence on aerobic metabolism. Mitochondrial DNA (mtDNA) and nuclear genomes must interact for optimal functioning of oxidative phosphorylation to produce ATP, and breakdown of coadaptation components from each may have important evolutionary consequences for hybridization. Introgression of mitochondria in natural populations through hybridization with unidirectional backcrossing allows the testing of coadaptation of mitochondria to different nuclear backgrounds. We compared the function of mitochondria isolated from two species of Urosaurus lizards and hybrid populations. Due to past introgression, hybrids contain the nuclear genome of the "hot-adapted" species (U. graciosus) but the mtDNA of the less heat-tolerant species (U. ornatus). It was found that the function of the parental forms of mitochondria had significantly diverged with the hot-adapted species. There was significant genotype × genotype × environment interactions for mitochondrial membrane potential and genotype × genotype interactions for ATP production. Membrane potential decreased less at a higher temperature, while ATP production was higher at both temperatures in introgressed mitochondria. Oxygen consumption was lower in U. graciosus than in U. ornatus parental-type mitochondria, indicating a likely response to living in hotter environments. Respiratory control ratio values, which provide an indication of the functional quality of isolated mitochondria, were lower in introgressed mitochondria than in parental U. ornatus types, indicating a negative impact on biological function in introgressed mitochondria.

Introgression of mtDNA in Urosaurus lizards: historical and ecological processes.

Introgression of mtDNA appears common in animals, but the implications of acquiring a novel mitochondrial genome are not well known. This study investigates mito-genome introgression between the lizard species Urosaurus graciosus, a thermal specialist, and U. ornatus, a species that occupies a wider range of thermal environments. As ectotherms, their metabolic rate is strongly influenced by the thermal environment; with mitochondria being linked to metabolic rates, overall energy budgets could be impacted by introgression. I use mitochondrial gene trees, inferred from Bayesian analyses of Cyt-B and ND1 gene sequences, along with morphology and microsatellites from nineteen populations of these two species to address if the direction and location of mito-nuclear discordance match predictions of introgression resulting from past population expansions. MtDNA is expected to move from resident species into expanding or invading species. Second, does having a heterospecific form of mitochondria impact body size, a trait strongly associated with fitness? Multiple independent introgression events of historic origin were detected. All introgression was unidirectional with U. ornatus-type mtDNA found in U. graciosus parental type individuals. This result was consistent with population expansions detected in U. graciosus but not U. ornatus. Females with heterospecific mtDNA were significantly smaller than homospecific forms, and heterospecific males had a different relationship of body mass to body length than those with homospecific mtDNA. These changes indicate a potential selective disadvantage for individuals with heterospecific mitochondria and are consistent with the theoretical expectation that deleterious alleles are more likely to persist in expanding populations.

Isolation and characterization of microsatellite markers in tree lizards (Urosaurus ornatus).

Tree lizards (Urosaurus ornatus) are a highly accessible species useful in testing theories of life-history evolution and behavioural ecology. A polymerase chain reaction-based method was used to isolate nine polymorphic tetranucleotide microsatellite loci from the genome of these lizards. The isolated loci displayed 5 to 9 alleles in the populations screened. Observed heterozygosity ranged from 0.333 to 0.900. Results indicate these loci now provide a basis to study mate choice, parentage, and population genetic structure within this species.

Hormones, performance and fitness: Natural history and endocrine experiments on a lizard (Sceloporus undulatus).

We used the "morphology-performance-fitness" paradigm (Arnold, 1983) as our framework to investigate endocrine control of performance and fitness in Sceloporus undulatus (Eastern Fence Lizard). Focusing on males, we used the "natural experiments" of seasonal, sexual, and developmental variation in growth and in exercise endurance to identify testosterone and corticosterone as potential modulators of performance and related traits of interest. We followed with experimental manipulations of testosterone to investigate functional relationships, both in the laboratory and in the field. Further, we used focal observations and demographic studies, coupled with genetic determination of paternity, to test associations between performance and fitness, measured as reproductive success. We found that in males, endurance and plasma concentrations of testosterone and corticosterone are at their peaks in the spring breeding season, when lizards are most actively engaged in patrolling home ranges and in reproductive behavior. At that time, plasma concentrations of testosterone are correlated with body size; plasma concentrations of corticosterone and parameters of home range, including area and the number of overlapped females, are correlated with home-range overlap between males and females. During prereproductive development, males (but not females) experience a maturational increase in plasma testosterone. At about the same time, they become more active, expand their home ranges, and grow less quickly than do females, suggesting a trade-off in the allocation of energy, mediated by testosterone. Experimentally, testosterone has positive effects on fitness by stimulating endurance and reproductive activity and increasing home-range area, but it exacts costs in fitness by increasing ectoparasitism, decreasing growth, and decreasing survivorship. We found evidence of selection on body size, endurance, and home-range size (and thus access to potential mates). Despite having positive effects on performance traits, plasma concentrations of testosterone were not correlated with number of offspring sired by males. However, we found a strong correlation between the level of plasma corticosterone and the number of offspring sired. We also found evidence of size-assortative mating, indicating that for males, both the number and the size (and thus, fecundity) of their mates increase with body size. Our studies exemplify the power of natural history combined with experimental endocrine manipulations to identify hormonal regulators of performance and linkages to fitness. Furthermore, our results illustrate ecological and evolutionary significance of individual variation in endocrine traits.