Individual telomere dynamics and their links to life history in a viviparous lizard.

Emerging patterns suggest telomere dynamics and life history are fundamentally linked in endotherms through life-history traits that mediate the processes underlying telomere attrition. Unlike endotherms, ectotherms maintain the ability to lengthen somatic telomeres throughout life and the link between life-history strategies and ectotherm telomere dynamics is unknown. In a well-characterized model system (Niveoscincus ocellatus), we used long-term longitudinal data to study telomere dynamics across climatically divergent populations. We found longer telomeres in individuals from the cool highlands than those from the warm lowlands at birth and as adults. The key determinant of adult telomere length across populations was telomere length at birth, with population-specific effects of age and growth on adult telomere length. The reproductive effort had no proximate effect on telomere length in either population. Maternal factors influenced telomere length at birth in the warm lowlands but not the cool highlands. Our results demonstrate that life-history traits can have pervasive and context-dependent effects on telomere dynamics in ectotherms both within and between populations. We argue that these telomere dynamics may reflect the populations' different life histories, with the slow-growing cool highland population investing more into telomere lengthening compared to the earlier-maturing warm lowland population.

Tail loss and telomeres: consequences of large-scale tissue regeneration in a terrestrial ectotherm.

Large-scale tissue regeneration has potential consequences for telomere length through increases in cell division and changes in metabolism which increase the potential for oxidative stress damage to telomeres. The effects of regeneration on telomere dynamics have been studied in fish and marine invertebrates, but the literature is scarce for terrestrial species. We experimentally induced tail autotomy in a lizard ( Niveoscincus ocellatus) and assessed relative telomere length (RTL) in blood samples before and after partial tail regeneration while concurrently measuring reactive oxygen species (ROS) levels. The change in ROS levels was a significant explanatory variable for the change in RTL over the 60-day experiment. At the average value of ROS change, the mean RTL increased significantly in the control group (intact tails), but there was no such evidence in the regenerating group. By contrast, ROS levels decreased significantly in the regenerating group, but there was no such evidence in the control group. Combined, these results suggest that tail regeneration following autotomy involves a response to oxidative stress and this potentially comes at a cost to telomere repair. This change in telomere maintenance demonstrates a potential long-term cost of tail regeneration beyond the regrowth of tissue itself.

Temperature and telomeres: thermal treatment influences telomere dynamics through a complex interplay of cellular processes in a cold-climate skink.

Telomere dynamics vary fundamentally between endothermic populations and species as a result of differences in life history, yet we know little about these patterns in ectotherms. In ectotherms, the relationships between climate, metabolism and life history suggest that telomere attrition should be higher at relatively high environmental temperatures compared to relatively low environmental temperatures, but these effects may vary between populations due to local adaptation. To address this hypothesis, we sampled reactive oxygen species (ROS) and telomere length of lizards from warm lowland and cool highland populations of a climatically widespread lizard species that we exposed to hot or cold basking treatments. The hot treatment increased relative telomere length compared to the cold treatment independent of climatic origin or ROS levels. Lizards from the cool highland region had lower ROS levels than those from the warm lowland region. Within the highland lizards, ROS increased more in the cold basking treatment than the hot basking treatment. These results are in the opposite direction to those predicted, suggesting that the relationships between temperature, metabolism, ROS and telomere dynamics are not straightforward. Future work incorporating detailed understanding of the thermal reaction norms of these and other linked traits is needed to fully understand these processes.

Seasonal shifts along the oviparity-viviparity continuum in a cold-climate lizard population.

Squamate embryos require weeks of high temperature to complete development, with the result that cool climatic areas are dominated by viviparous taxa (in which gravid females can sun-bask to keep embryos warm) rather than oviparous taxa (which rely on warm soil to incubate their eggs). How, then, can some oviparous taxa reproduce successfully in cool climates - especially late in summer, when soil temperatures are falling? Near the northern limit of their distribution (in Sweden), sand lizards (Lacerta agilis) shift tactics seasonally, such that the eggs in late clutches complete development more quickly (when incubated at a standard temperature) than do those of early clutches. That acceleration is achieved by a reduction in egg size and by an increase in the duration of uterine retention of eggs (especially, after cool weather). Our results clarify the ability of oviparous reptiles to reproduce successfully in cool climates and suggest a novel advantage to reptilian viviparity in such conditions: by maintaining high body temperatures, viviparous females may escape the need to reduce offspring size in late-season litters.

Disentangling sex allocation in a viviparous reptile with temperature-dependent sex determination: a multifactorial approach.

Females are predicted to alter sex allocation when ecological, physiological and behavioural variables have different consequences on the fitness of male and female offspring. Traditionally, tests of sex allocation have examined single causative factors, often ignoring possible interactions between multiple factors. Here, we used a multifactorial approach to examine sex allocation in the viviparous skink, Niveoscincus ocellatus. We integrated a 16-year observational field study with a manipulative laboratory experiment to explore whether the effects of the maternal thermal environment interact with the resources available to females for reproduction to affect sex allocation decisions. We found strong effects of temperature on sex allocation in the field, with females born in warm conditions and males in cold conditions; however, this was not replicated in the laboratory. In contrast, we found no effect of female resource availability on sex allocation, either independently, or in interaction with temperature. These results corresponded with an overall lack of an effect of resource availability on any of the life history traits that we predicted would mediate the benefits of differential sex allocation in this system, suggesting that selection for sex allocation in response to resource availability may be relatively weak. Combined, these results suggest that temperature may be the predominant factor driving sex allocation in this system.

Selection and constraints on offspring size-number trade-offs in sand lizards (Lacerta agilis).

The trade-off between offspring size and number is a central component of life-history theory, postulating that larger investment into offspring size inevitably decreases offspring number. This trade-off is generally discussed in terms of genetic, physiological or morphological constraints; however, as among-individual differences can mask individual trade-offs, the underlying mechanisms may be difficult to reveal. In this study, we use multivariate analyses to investigate whether there is a trade-off between offspring size and number in a population of sand lizards by separating among- and within-individual patterns using a 15-year data set collected in the wild. We also explore the ecological and evolutionary causes and consequences of this trade-off by investigating how a female's resource (condition)- vs. age-related size (snout-vent length) influences her investment into offspring size vs. number (OSN), whether these traits are heritable and under selection and whether the OSN trade-off has a genetic component. We found a negative correlation between offspring size and number within individual females and physical constraints (size of body cavity) appear to limit the number of eggs that a female can produce. This suggests that the OSN trade-off occurs due to resource constraints as a female continues to grow throughout life and, thus, produces larger clutches. In contrast to the assumptions of classic OSN theory, we did not detect selection on offspring size; however, there was directional selection for larger clutch sizes. The repeatabilities of both offspring size and number were low and we did not detect any additive genetic variance in either trait. This could be due to strong selection (past or current) on these life-history traits, or to insufficient statistical power to detect significant additive genetic effects. Overall, the findings of this study are an important illustration of how analyses of within-individual patterns can reveal trade-offs and their underlying causes, with potential evolutionary and ecological consequences that are otherwise hidden by among-individual variation.

Persistence and dispersal in a Southern Hemisphere glaciated landscape: the phylogeography of the spotted snow skink (Niveoscincus ocellatus) in Tasmania.

BACKGROUND: The aim of this research was to identify the effects of Pleistocene climate change on the distribution of fauna in Tasmania, and contrast this with biotic responses in other temperate regions in the Northern and Southern Hemisphere that experienced glacial activity during this epoch. This was achieved by examining the phylogeographic patterns in a widely distributed Tasmanian endemic reptile, Niveoscincus ocellatus. 204 individuals from 29 populations across the distributional range of N. ocellatus were surveyed for variation at two mitochondrial genes (ND2, ND4), and two nuclear genes (ß-globin, RPS8). Phylogenetic relationships were reconstructed using a range of methods (maximum parsimony, Bayesian inference and haplotype networks), and the demographic histories of populations were assessed (AMOVA, Tajima's D, Fu's Fs, mismatch distributions, extended Bayesian skyline plots, and relaxed random walk analyses). RESULTS: There was a high degree of mitochondrial haplotype diversity (96 unique haplotypes) and phylogeographic structure, where spatially distinct groups were associated with Tasmania's Northeast and a large area covering Southeast and Central Tasmania. Phylogeographic structure was also present within each major group, but the degree varied regionally, being highest in the Northeast. Only the Southeastern group had a signature of demographic expansion, occurring during the Pleistocene but post-dating the Last Glacial Maximum. In contrast, nuclear DNA had low levels of variation and a lack of phylogeographic structure, and further loci should be surveyed to corroborate the mitochondrial inferences. CONCLUSIONS: The phylogeographic patterns of N. ocellatus indicate Pleistocene range and demographic expansion in N. ocellatus, particularly in the Southeast and Central areas of Tasmania. Expansion in Central and Southeastern areas appears to have been more recent in both demographic and spatial contexts, than in Northeast Tasmania, which is consistent with inferences for other taxa of greater stability and persistence in Northeast Tasmania during the Last Glacial Maximum. These phylogeographic patterns indicate contrasting demographic histories of populations in close proximity to areas directly affected by glaciers in the Southern Hemisphere during the LGM.

Complex selection associated with Hox genes in a natural population of lizards.

Hox genes are recognized for their explanatory power of bilateral development. However, relatively little is known about natural variation in, and the evolutionary dynamics of, Hox genes within wild populations. Utilizing a natural population of sand lizards (Lacerta agilis), we screened HoxA13 for genetic variation and an association with incidence of offspring malformations. We found significant effects of parental genetic similarity and offspring sex, and their interaction, on risk of hatching malformed as an offspring. We also found within population genetic variation in HoxA13, and identified a significant effect of a three-way interaction among Hox genotype, parental genetic similarity, and offspring sex on the risk of hatching malformation. Since malformed offspring in this population do not survive to maturity, this study reveals complex and ongoing selection associated with Hox genes in a wild reptile population. Importantly, this demonstrates the utility of natural populations in unveiling microevolutionary processes shaping variation in highly conserved genes.

Giving offspring a head start in life: field and experimental evidence for selection on maternal basking behaviour in lizards.

The timing of birth is often correlated with offspring fitness in animals, but experimental studies that disentangle direct effects of parturition date and indirect effects mediated via variation in female traits are rare. In viviparous ectotherms, parturition date is largely driven by female thermal conditions, particularly maternal basking strategies. Our field and laboratory studies of a viviparous lizard (Niveoscincus ocellatus) show that earlier-born offspring are more likely to survive through their first winter and are larger following that winter, than are later-born conspecifics. Thus, the association between parturition date and offspring fitness is causal, rather than reflecting an underlying correlation between parturition date and maternal attributes. Survival selection on offspring confers a significant advantage for increased maternal basking in this species, mediated through fitness advantages of earlier parturition. We discuss the roles of environmentally imposed constraints and parent-offspring conflict in the evolution of maternal effects on parturition date.

Sperm competition and offspring viability at hybridization in Australian tree frogs, Litoria peronii and L. tyleri.

Hybridization between closely related species often leads to reduced viability or fertility of offspring. Complete failure of hybrid offspring (post-zygotic hybrid incompatibilities) may have an important role in maintaining the integrity of reproductive barriers between closely related species. We show elsewhere that in Peron's tree frog, Litoria peronii, males more closely related to a female sire more offspring in sperm competition with a less related rival male. Observations of rare 'phenotypic intermediate' males between L. peronii and the closely related L. tyleri made us suggest that these relatedness effects on siring success may be because of selection arising from risks of costly hybridization between the two species. Here, we test this hypothesis in an extensive sperm competition experiment, which shows that there is no effect of species identity on probability of fertilization in sperm competition trials controlling for sperm concentration and sperm viability. Instead, there was a close agreement between a male's siring success in isolation with a female and his siring success with the same female in competition with a rival male regardless of species identity. Offspring viability and survival, however, were strongly influenced by species identity. Over a 14-day period, hybrid offspring suffered increasing mortality and developed more malformations and an obvious inability to swim and right themselves, leading to compromised probability of survival. Thus, hybridization in these sympatric tree frogs does not compromise fertilization but has a strong impact on offspring viability and opportunity for reinforcement selection on mate choice for conspecific partners.

Evaluation of offspring size-number invariants in 12 species of lizard.

The optimal division of resources into offspring size vs. number is one of the classic problems in life-history evolution. Importantly, models that take into account the discrete nature of resource division at low clutch sizes suggest that the variance in offspring size should decline with increasing clutch size according to an invariant relationship. We tested this prediction in 12 species of lizard with small clutch sizes. Contrary to expectations, not all species showed a negative relationship between variance in offspring size and clutch size, and the pattern significantly deviated from quantitative predictions in five of the 12 species. We suggest that the main limitation of current size-number models for small clutch sizes is that they rely on assumptions of hierarchical allocation strategies with independence between allocation decisions. Indeed, selection may favour alternative mechanisms of reproductive allocation that avoid suboptimal allocation imposed by the indivisible fraction at low clutch sizes.

Maternal effects obscure condition-dependent sex allocation in changing environments.

Climate change increases environmental fluctuations which thereby impact population demography. Species with temperature-dependent sex determination may experience more extreme sex ratio skews, but this has not been considered in species with chromosomally determined sex. However, anticipatory maternal effects cause lifelong physiological changes impacting sex ratios. Here we show, in mice, that more sons were born to mothers in good condition when their breeding environment matched their gestational environment, consistent with theoretical predictions, but mothers in mismatched environments have no condition-sex ratio relationship. Thus, the predicted effect of condition on sex ratio was obscured by maternal effects when the environment changed. This may explain extreme sex ratio skews in reintroduced or translocated populations, and sex ratio skews may become more common and less predictable with accelerating environmental change.

Males with high genetic similarity to females sire more offspring in sperm competition in Peron's tree frog Litoria peronii.

Recent work has confirmed that genetic compatibility among mates can be an important determinant of siring success in sperm competition experiments and in free-ranging populations. Most of this work points towards mate choice of less related mates. However, there may also be the potential for mate choice for intermediate or even genetically similar mates to prevent outbreeding depression or hybridization with closely related taxa. We studied relatedness effects on post-copulatory gametic choice and/or sperm competition in an external fertilizer, Peron's tree frog (Litoria peronii), since external fertilizers offer exceptional control in order to test gametic interaction effects on probability of paternity and zygote viability. Sperm competition experiments were done blindly with respect to genetic relatedness among males and females. Thereafter, paternity of offspring was assigned using eight microsatellite loci. Three hybridization trials between L. peronii and a closely related sympatric species Litoria tyleri were also carried out. In the sperm competition trials, males that are more genetically similar to the female achieved higher siring success compared with less genetically similar males. The hybridization trials confirmed that the two species can interbreed and we suggest that the risk of hybridization may contribute to selection benefits for genetically more similar males at fertilization. To our knowledge, this study is the first to show evidence for post-copulatory selection of sperm from genetically more similar individuals within a natural population.

Gestational experience alters sex allocation in the subsequent generation.

Empirical tests of adaptive maternal sex allocation hypotheses have presented inconsistent results in mammals. The possibility that mothers are constrained in their ability to adjust sex ratios could explain some of the remaining variation. Maternal effects, the influence of the maternal phenotype or genotype on her developing offspring, may constrain sex allocation through physiological changes in response to the gestational environment. We tested if maternal effects constrain future parental sex allocation through a lowered gestational stress environment in laboratory mice. Females that experienced lowered stress as embryos in utero gave birth to female-biased litters as adults, with no change to litter size. Changes in offspring sex ratio was linked to peri-conceptual glucose, as those females that had increasing blood glucose peri-conceptionally gave birth to litters with a higher male to female sex ratio. There was, however, no effect of the lowered prenatal stress for developing male embryos and their sperm sex ratio when adult. We discuss the implications of maternal effects and maternal stress environment on the lifelong physiology of the offspring, particularly as a constraint on later maternal sex allocation.

Testosterone, ticks and travels: a test of the immunocompetence-handicap hypothesis in free-ranging male sand lizards.

The immunocompetence-handicap hypothesis suggests that androgen-dependent male characters constitute honest signals of mate and/or rival quality because of the imposed costs through immune suppression associated with elevated testosterone levels. We demonstrate in a field experiment that male sand lizards (Lacerta agilis) exposed to elevated testosterone suffered from increased mass loss and tick load compared to control males. Although the first of these two results could be due to an elevated basal metabolic rate from increased plasma testosterone levels, the increased parasite load was statistically independent of the loss in body condition and is likely to be due to compromised immune function. Testosterone-treated males showed greater mobility than control males, and greater mobility resulted in higher mating success. Our experiment thus lends support to the immunocompetence-handicap hypothesis, suggesting that male testosterone levels have been moderated by balancing selection for reproductive success and sustained immune function.

Sex allocation and sex determination in squamate reptiles.

Reptiles possess a wide variety of sex determining mechanisms, more so than any other vertebrate group. They offer outstanding opportunities to understand the evolutionary transitions between modes of sex determination. In this review, we argue that sex allocation theory is fundamental for understanding the selective causes of such shifts. Whether selection for biased sex allocation actually results in evolutionary shifts in sex determination depends on the overall strength, direction and consistency of selection and to what extent existing reproductive systems can establish novel links between factors causing sex-specific fitness and mechanisms of sex determination. Perhaps one of the most exciting advances in recent years has been the phylogenetically diverse range in reptile taxa that form the basis of research on the evolution of sex determination. The traditional use of long-lived oviparous species (especially turtles and crocodiles) is now expanded to include a range of short-lived taxa that exhibit both genetic sex determination and environment-/temperature-dependent sex determination (particularly agamid lizards), as well as a greater emphasis on viviparous species. If selection on differential sex allocation is a key selective pressure for the evolution of sex-determining mechanisms, these taxa will provide considerable insights into the integrated fields of sex allocation biology and sex determination.

Asynchronous male and female gonadal cycles and plasma steroid concentrations in a viviparous lizard, Niveoscincus ocellatus (Scincidae), from Tasmania.

The reproductive cycle in males of the skink, Niveoscincus ocellatus, is characterised by testicular development during summer, followed by mating in autumn. Plasma testosterone concentrations show a bimodal seasonal cycle, with the major peak (18.6 +/- 1.2 ng/ml) in late summer/autumn and a minor peak (7.4 +/- 1.0 ng/ml) at spring emergence. In contrast to the males, the females have a gonadal cycle in which mating is temporally dissociated from peak development of the gonads: ovulation occurs in spring and the young are born in summer. Fresh mating marks on females in spring indicate that at least part of the population mates for a second time after spring emergence. In females, plasma estradiol concentrations are significantly elevated (956 +/- 214 pg/ml) through vitellogenesis and are highest (1241 +/- 175 pg/ml) during the preovulatory phase. Plasma progesterone concentrations rise during gestation to 6.5 +/- 1.5 ng/ml, but fall in the final stage of gestation to 1.6 +/- 0.2 ng/ml. There is minimal atresia of vitellogenic follicles, suggesting that clutch size is determined when the follicles are recruited for vitellogenesis.

Offspring-driven local dispersal in female sand lizards (Lacerta agilis).

We report on a field study in which determinants of female breeding dispersal (i.e. the shift in the mean home range coordinates between successive breeding events) was investigated. Offspring were released in full sib groups (or half sib ones if there was within-clutch multiple paternity) at a separation distance from the females that varied between 'families'. This allowed for analysis of 'offspring nearness' effects on maternal dispersal. When a female's offspring were released more closely to her, she responded with greater dispersal. Furthermore, when the data set was truncated at 100 m maternal-offspring separation distance at offspring release (because perception at longer distances is likely to be unrealistic), maternal dispersal resulted in greater separation distance between female and offspring in the following year. A corresponding analysis for juveniles revealed no effect of maternal nearness on offspring dispersal but identified a significant effect of clutch size, to our surprise with dispersal declining with increasing clutch size. We discuss this result in a context of the 'public information hypothesis' (reinterpreted for juveniles in a nonsocial foraging species), suggesting that conspecific abundance perhaps acts as an indicator of local habitat quality. Thus, our analysis suggests a microgeographic structuring of the adult female population driven by genetic factors, either through inbreeding avoidance, or from simply avoiding individuals with a similar genotype regardless of their pedigree relatedness, while a nongenetic factor seems more important in their offspring.