Seasonal effects on circulating leptin in the lizard Sceloporus undulatus from two populations

Characterizing leptin's structure and function in mammals has been the subject of thousands of studies since 1994. Recently, the study of leptin has expanded to include its distribution in non-mammalian taxa, and the role that leptin plays in the reproductive axis. We demonstrated in a previous study that Sceloporus undulatus, fence lizards (ectotherms), express a leptin-like protein. In the current study we quantified seasonal variation in this putative leptin among free-ranging fence lizards from two populations characterized by early and late reproductive maturation (after one or two years, respectively). Immunoblots were performed on whole blood samples to detect leptin and estimate its titer. Leptin titers were higher in the reproductive population of S. undulatus (early maturing: 2.5 ± 0.2 μg/mL; late-maturing 2.2 ± 0.3 μg/mL; mean ± 2 S.E.), but both populations showed the same seasonal pattern. Leptin titers were lowest in fall when fat stores are expected to be highest (spring: 2.6 ± 0.3 μg/mL; summer: 2.6 ± 0.3; μg/mL; fall: 1.8 ± 0.3 μg/mL), consistent with findings of seasonal variation in free-ranging mammals. Our data support previous work asserting that lizards express leptin and that it has a similar physiological function in endotherms and ectotherms. Our long-term goal is to use leptin to manipulate age at maturity and to test fundamental questions in the evolution of life-history strategies.     

Incubation temperature and phenotypic traits of Sceloporus undulatus: implications for the northern limits of distribution

Cold environmental temperature is detrimental to reproduction by oviparous squamate reptiles by prolonging incubation period, negatively affecting embryonic developmental processes, and by killing embryos in eggs directly. Because low soil temperature may prevent successful development of embryos in eggs in nests, the geographic distributions of oviparous species may be influenced by the thermal requirements of embryos. In the present study, we tested the hypothesis that low incubation temperature determines the northern distributional limit of the oviparous lizard Sceloporus undulatus. To compare the effects of incubation temperature on incubation length, egg and hatchling survival, and hatchling phenotypic traits, we incubated eggs of S. undulatus under temperature treatments that simulated the thermal environment that eggs would experience if located in nests within their geographic range at 37°N and north of the species’ present geographic range at latitudes of 44 and 42°N. After hatching, snout–vent length (SVL), mass, tail length, body condition (SVL relative to mass), locomotor performance, and growth rate were measured for each hatchling. Hatchlings were released at a field site to evaluate growth and survival under natural conditions. Incubation at temperatures simulating those of nests at 44°N prolonged incubation and resulted in hatchlings with shorter SVL relative to mass, shorter tails, shorter hind limb span, slower growth, and lower survival than hatchlings from eggs incubated at temperatures simulating those of nests at 37 and 42°N. We also evaluated the association between environmental temperature and the northern distribution of S. undulatus. We predicted that the northernmost distributional limit of S. undulatus would be associated with locations that provide the minimum heat sum (∼495 degree-days) required to complete embryonic development. Based on air and soil temperatures, the predicted northern latitudinal limit of S. undulatus would lie at ∼40.5–41.5°N. Our predicted value closely corresponds to the observed latitudinal limit in the eastern United States of ∼40°N. Our results suggest that soil temperatures at northern latitudes are not warm enough for a sufficient length of time to permit successful incubation of S. undulatus embryos. These results are consistent with the hypothesis that incubation temperature is an important factor limiting the geographic distributions of oviparous reptile species at high latitudes and elevations.     

Reproductive ecology of red-chinned lizards (Sceloporus undulatus erythrocheilus) in Southcentral Colorado: comparisons with other populations of a wide-ranging species

Summary We studied the reproductive ecology of a population of Sceloporus undulatus erythrocheilus near Walsenburg, CO during the summers of 1987–1990. Reproductive activity commences soon after emergence in early May and continues until the middle of July. Females mature in their second year following hatching at an age of 20–21 months and a size of 60–63 mm snout-vent length (SVL). Two clutches, averaging10.9 eggs, are produced per year. Larger females produce larger clutches with an increase of one egg per 3 mm SVL. Average SVL of females was 71 mm. Eggs are reasonably large (0.32 g) and relative clutch mass was the largest reported for the species (34% of body mass). A principal component analysis was used to examine relationships among reproductive characters in 12 populations of Sceloporus undulatus. The analysis suggested that patterns of reproduction in this wide-ranging species result from several factors reflecting both adaptive and phylogenetic sources of variation.     

Thermal and reproductive biology of high and low elevation populations of the lizard Sceloporus scalaris: implications for the evolution of viviparity

Viviparity in squamate reptiles is presumed to evolve in cold climates by selection for increasingly longer periods of egg retention. Longer periods of egg retention may require modifications to other reproductive features associated with the evolution of viviparity, including a reduction in eggshell thickness and clutch size. Field studies on the thermal and reproductive biology of high (HE) and low (LE) elevation populations of the oviparous lizard, Sceloporus scalaris, support these expectations. Both day and night-time temperatures at the HE site were considerably cooler than at the LE site, and the activity period was 2 h shorter at the HE than at the LE site. The median body temperature of active HE females was 2°C lower than that of LE females. HE females initiated reproduction earlier in the spring than LE females, apparently in order to compensate for relatively low temperatures during gestation. HE females retained eggs for about 20 days longer than LE females, which was reflected by differences in the degree of embryonic development at the time of oviposition (stages 35.5–37.0 versus stages 31.0–33.5, respectively). These results support the hypotheses that evolution of viviparity is a gradual process, and is favored in cold climates. Females in the HE population exhibited other traits consistent with presumed intermediate stages in the evolution of viviparity; mean eggshell thickness of HE eggs (19.3 m) was significantly thinner than that of LE eggs (26.6 m) and the size-adjusted clutch sizes of HE females (9.4) were smaller than those of LE females (11.2).     

Thyroxine affects behavioral thermoregulation but not growth rate among populations of the western fence lizard (Sceloporus occidentalis)

We investigated whether thyroxine influences hatchling growth rate of the western fence lizard (Sceloporus occidentalis) throught its effects on thermoregulatory behaviors. We reared control and thyroxine-injected hatchlings from three populations of S. occidentalis that differ in growth rate in a thermal gradient. We also measured the daily changes in body temperature and activity level (proportion of time spent out of retreat sites) of control and thyroxine-injected lizards. Previous studies have shown that within and among population differences in growth rate of the western fence lizard are correlated with the maintenance of high activity levels (proportion of time spent outside of retreat sites) and high body temperatures throughout the day. Growth rate was not influenced by injections of thyroxine. However, injections of thyroxine did elevate average daily body temperature and daily activity. Although administration of thyroxine uniformly increased the probability of activity throughout the day, it did not appear to alter the daily changes in activity. Previous studies have shown that the slower-growing hatchlings from northern populations exhibit a decline in activity during the later part of the thermal cycle, whereas the faster growing southern hatchlings maintain the same level of high activity throughout the thermal cycle. The decline in activity of northern populations was not prevented by thyroxine injection used in our current study. Northern lizards receiving exogenous thyroxine were still less active later in the day compared to early in the day, even though activity level throughout the day was elevated. Thus, the effects of thyroxine on temperature regulation observed in our study (general increase in activity level) appear to be unrelated to those aspects of temperature regulation (e.g., daily changes in behavioral thermoregulation) that are correlated with among population differences in growth rate. We also raised hatchlings in a cycling thermal regime (forced thermal cycle of 34°C:15°C, 12L:12D) where behavioral thermoregulation is not possible. The growth rate of lizards forced to cycle between 34°C:15°C on a daily basis was significantly lower than those lizards allowed to behaviorally thermoregulate, further underscoring the importance of the circadian pattern of thermoregulation for growth.Abbreviations GR growth rate - MR metabolic rate - SMR standard metabolic rate - SVL snout-vent length - T₄ thyroxine - T _b body temperature - T _e environmental temperature     

Phylogenetic comparative analysis of life-history variation among populations of the lizard Sceloporus undulatus: an example and prognosis

Over the past 15 years, phylogenetic comparative methods (PCMs) have become standard in the study of life-history evolution. To date, most studies have focused on variation among species or higher taxonomic levels, generally revealing the presence of significant phylogenetic effects as well as residual variation potentially attributable to adaptive evolution. Recently, population-level phylogenetic hypotheses have become available for many species, making it possible to apply PCMs directly to the level at which experiments are typically used to test adaptive hypotheses. In this study, we present the results of PCMs applied to life-history variation among populations of the widespread and well-studied lizard Sceloporus undulatus. Using S. undulatus (which may represent four closely related species) as an example, we explore the benefits of using PCMs at the population level, as well as consider the importance of several thorny methodological problems including but not limited to nonindependence of populations, lack of sufficient variation in traits, and the typically small sample sizes dictated by the difficulty of collecting detailed demographic data. We show that phylogenetic effects on life-history variation among populations of S. undulatus appear to be unimportant, and that several classic trade-offs expected by theory and revealed by many interspecific comparisons are absent. Our results suggest that PCMs applied to variation in life-history traits below the species level may be of limited value, but more studies like ours are needed to draw a general conclusion. Finally, we discuss several outstanding problems that face studies seeking to apply PCMs below the species level.     

Characterization of gastrointestinal chitinase in the lizard Sceloporus undulatus garmani (Reptilia: Phrynosomatidae)

Most studies on chitinase activity in lizards have been concerned with Palaearctic (European) and Laurasian (Middle Eastern and Asian) taxa. Several genera of Old World lizards, Anguis, Uromastix, Chamaeleo and Lacerta, have been shown to possess chitinolytic activity. To date, only one New World lizard, Anolis carolinensis, has been reported to exhibit chitinolytic activity. In the present study, chitinase activity was characterized in a second New World taxon, Sceloporus undulatus garmani, a New World, phrynosomatid lizard. Chitinolytic activity was measured by incubating tissue extracts with a radioactive chitin substrate, acetyl-[H³]chitin and determining acid soluble radioactivity as an estimate for chitin hydrolysis. Chitinolytic activity was present in stomach, small intestine and pancreas extracts, with the stomach and pancreas having the highest specific activities. Chitinolytic activity was higher at pH 4.5 than at pH 7.5. The stomach chitinase is immunologically similar to the gastric chitinase previously described for rainbow trout. Western blot analysis showed anti-chitinase cross-reactivity in the extracts of the stomach, but no cross-reactivity in the pancreatic or intestinal extracts, suggesting different isoforms of chitinase. There was no detected lysozyme activity (less than 0.01 mg/ml lysozyme) present in the extracts of the stomach, small intestine and pancreas. The localization of chitinolytic activity in S. u. garmani is in agreement with earlier reptilian reports on the distribution of chitinase.     

Convergent evolution of embryonic growth and development in the eastern fence lizard (Sceloporus undulatus)

Theory predicts that cold environments will select for strategies that enhance the growth of ectotherms, such as early emergence from nests and more efficient use of resources. We used a common garden experiment to detect parallel clines in rates of embryonic growth and development by eastern fence lizards (Sceloporus undulatus). Using realistic thermal conditions, we measured growth efficiencies and incubation periods of lizards from five populations representing two distinct clades. In both clades, embryos from cold environments (Indiana, New Jersey, and Virginia) grew more efficiently and hatched earlier than embryos from warm environments (Florida and South Carolina). Because eggs from cold environments were larger than eggs from warm environments, we experimentally miniaturized eggs from one population (Virginia) to determine whether rapid growth and development were caused by a greater maternal investment. Embryos in miniaturized eggs grew as efficiently and incubated for the same duration as embryos in unmanipulated eggs. Taken together, our results suggest countergradient variation has evolved at least twice in S. undulatus.     

Steroid correlates of multiple color traits in the spiny lizard, <Emphasis Type="Italic">Sceloporus pyrocephalus</Emphasis>

Conspicuous coloration in females is less well studied compared to that in males. Adult female Mexican boulder spiny lizards (Sceloporus pyrocephalus) have conspicuously colored throat, or gular, regions, ranging from red to yellow, while adult males only weakly express such color in their gular region. Both sexes have dark blue–black gular stripes and venter stripes. Understanding proximate mechanisms underlying trait expression can aid in understanding trait function. To characterize the proximate mechanisms potentially influencing color variation among field-captured male and female S. pyrocephalus, we quantified three aspects of color (hue, saturation, brightness) for three body locations (gular region, gular stripes, venter stripes) and then assessed how color was related to reproductive state and concentrations of the plasma steroid hormones testosterone (T) and corticosterone (CORT) in males and T, CORT, and 17-β estradiol (E₂) in females. Testes volume was not related to variation in color or in hormones, perhaps because most males were in peak reproductive condition. Large vitellogenic follicles as opposed to oviductal eggs were associated with higher E₂ in females. Males with more dull gular stripes and females with dull venter stripes had significantly higher CORT. Females with red gular regions and pale grey gular stripes had higher T and E₂ concentrations compared to females with a more yellow gular region and darker gular stripes. Thus, gular region color in females could communicate reproductive state; dull gular stripes in males and dull venter stripes in females could communicate stress status.     

Maternal thermoregulation influences offspring viability in a viviparous lizard

Gravid females of the viviparous scincid lizard Eulamprus heatwolei were maintained in the laboratory, with some females allowed to bask for 8 h/day and others for only 2 h/day. Maternal basking regimes influenced the gestation period, and significantly affected the body shapes, activity levels and running speeds of the offspring born to these females. Neonates from females with lesser basking opportunities were relatively short and fat, were very active, and were relatively fast runners. Effects of the embryos' thermal regime on the young lizards' morphology and running speed were still detectable two months after birth. Thus, much of the morphological and behavioral variation among neonatal reptiles may arise from phenotypically plastic responses to the thermal environments experienced during embryonic development, rather than from heritable genetic differences among individuals. Hence, selection on maternal thermoregulatory behavior may be an important avenue for adaptive modifications to neonatal phenotypes in reptiles.     

Variation and covariation among life-history traits in Rumex acetosella from a successional old-field gradient

In this study morphological variation and the potential for competition to affect biomass and seedling selection of the families of five populations of Rumex acetosella L. sampled along a successional old-field gradient have been investigated. Seeds from 25 families were submitted to four competitive regimes: no competition (one plant per pot), medium competition (two plants/ pot taking plants from the same population), high within-population competition (four individuals from the same population in a pot) and high between-population competition (four individuals from two different populations in a pot). Eight traits were analysed after 3 months of growth for variation among families within populations. A significant difference among families within the two older populations was recorded for sexual biomass and related components. High sensitivity of these traits to density was observed in all populations except the youngest, suggesting specialization to particular environmental conditions in late successional populations, and a good adaptive capacity to buffer environmental variation in the pioneer population. Little significant interaction between competitive regimes and families within populations was found, i.e. genotypes within each population showed little variation in their response to environmental variation. Genotypic variance decreased with increasing competitive conditions for the majority of the traits. However, the percentage of variance in sexual reproduction explained by family was stable among treatments. Tradeoffs between vegetative reproduction and sexual reproduction were recorded at the population level along the successional gradient, with increasing competitive conditions. As succession proceeds, we observed a decrease in sexual reproduction and an increase in vegetative reproduction. At the family level, correlation among traits were similar when plants were grown in the absence of competition and at high density, with a significant negative correlation between sexual reproduction and vegetative reproduction. For both sprout number and sexual biomass, the performance of families grown under all the treatments was positively correlated. Together these results indicate allocational constraints on the reproductive biology of R. acetosella that may be favoured by natural selection and have influenced population differentiation along the successional gradient. However, they also revealed that the potential exists for evolutionary specialization through plasticity, in response to variation in environmental conditions.     

An experimental study of the effects of weed invasion on lizard phenotypes

We examined how a weed affected the basking and activity of a diurnal lizard, and the potential cascading effects of these shifts for life history strategies and expression of morphology. Hatchlings of the diurnal lizard Lampropholis delicata were raised to maturity in outdoor enclosures that mimicked high, moderate and low invasion by a sprawling plant (blue periwinkle, Vinca major). Skinks depend on sunlight for growth and maintenance. Periwinkle differs from displaced grassland by being structurally complex and blocking sunlight. Lizards restricted to the enclosure floor achieved preferred body temperatures only when exposure to periwinkle was moderate or low. However, lizards in high invasion enclosures could reach preferred body temperatures by climbing plants and basking on exposed canopy. This shift in basking strategy resulted in lizards growing longer hind limbs compared with animals that rarely (moderate invasion) and never (low invasion) climbed plants. Consequently, lizards reared in high invasion enclosures sprinted faster than conspecifics reared in lower invasion environments. Throughout the study there was no significant variation among treatments in the tendency of animals to be moving when they were not hidden. However, lizards in high invasion treatments hid more often during the day, were lighter in body mass, and females had lighter clutch masses and offspring than did those from moderate and low invasion enclosures. Thus, microhabitat degradation can drive a cascade of changes to an animal’s ecology.     

Thermal dependence of reproductive allocation in a tropical lizard

In ectotherms, environmental temperature is the most prominent abiotic factor that modulates life-history traits. We explored the influence of environmental temperature on reproduction in the Madagascar ground gecko (Paroedura picta) by measuring reproductive traits of females at constant temperatures (24, 27, 30 °C). Females of this species lay clutches of one or two eggs within short intervals. For each female, we measured egg mass for the first five clutches. For one clutch, we also measured the energetic content of eggs via bomb calorimetry. Temperature positively influenced the rate of egg production, but females at 30 °C laid smaller eggs than did females at either 24 or 27 °C. Dry mass of eggs scaled allometrically with wet mass, but this relationship was similar among thermal treatments. Females at all temperatures produced eggs with similar energy densities. Females at 24 °C allocated less energy per time unit (≈8 mW) to reproduction than did females from higher temperatures (≈12 mW). However, females at either 24 or 27 °C allocated significantly more energy per egg than did females at 30 °C. Our results demonstrate that a complex thermal sensitivity of reproductive rate can emerge from distinct thermal sensitivities of egg size, egg composition and clutch frequency.     

Age specific fecundity and size of offspring in the prosobranch snail,Viviparus ater

Summary Age specific fecundity and size of offspring were compared in two populations of Viviparus ater, one from Lake Zürich (Switzerland) and one from Lake Maggiore (Italy). Females were caged and their offspring sampled at weekly intervals. Average fecundity per female over one year was 19.6 and 14.7 offspring in Lake Zürich and Lake Maggiore, respectively. Average shell widths of offspring at birth in the two lakes were 8.5 mm and 7.4 mm. Fecundity did not change with age in Lake Zürich but decreased with increasing age of the snails in Lake Maggiore. Fecundity and offspring size were positively correlated with the size of the females in both lakes, but fecundity was higher and offspring were larger in Lake Zürich than in Lake Maggiore irrespective of the size of the females. An analysis of covariance, correcting for differences between the two lakes in the total fresh weight of the offspring produced annually, showed that females from the two lakes distributed their reproductive investment differently. Females in Lake Zürich produced relatively fewer but larger offspring than females in Lake Maggiore. This trade-off was hidden because females in Lake Zürich produced twice as much offspring biomass as females in Lake Maggiore. The possibility that different selection regimes might account for the observed differences is discussed.     

Seasonal alterations in adrenocortical cell function associated with stress-responsiveness and sex in the eastern fence lizard (Sceloporus undulatus)

We characterized steroidogenic properties of dispersed adrenocortical cells from field-active male and female eastern fence lizards (Sceloporus undulatus) to investigate whether alterations in cell function could, in part, explain seasonal variation in baseline and stress-induced plasma corticosterone (B). Lizards were collected during the breeding and postbreeding seasons and shortly prior to hibernation. Dispersed cells in vitro produced B, aldosterone (ALDO), and progesterone in response to 8-Br-cAMP, 25-(OH)cholesterol, adrenocorticotropin (ACTH; as little as 100 fM), and angiotensin II. Maximal progesterone, B, and ALDO responses to ACTH were roughly 1000%, 500%, and 100% greater than corresponding basal values. Angiotensin II was an effective steroidogenic stimulant but much less so than ACTH. Corticosteroid production exhibited considerable steroid-specific variation among seasons. Maximal ACTH-induced B production was lower in the postbreeding season than at either of the other two measurement points, essentially opposite to the pattern for ALDO. Males and females generally produced B at similar rates, but ALDO and progesterone showed numerous sex differences that usually covaried between the two steroids. Cellular sensitivity to 25-(OH)cholesterol and angiotensin II showed few sex differences or seasonal changes. In contrast, sensitivity to ACTH decreased markedly from the breeding to the postbreeding season in males, corresponding to the decrease in stress-responsiveness, and in both sexes was considerably lower prior to hibernation than during the breeding season. Under some conditions, plasma B may be limited by the production capacity of adrenocortical cells. In summary, seasonal variations in body condition, reproductive activity, and baseline and stress-induced plasma B may be attributed at least in part to alterations in adrenocortical cell steroidogenic function.     

Temperature-dependent rate of clutch production in a tropical lizard (Paroedura picta: Gekkonidae): Intraspecific test of the metabolic theory of ecology

Recently, a general model based on scaling of metabolic rate and reaction kinetics that predicts dependence of various biological rates on temperature and body size has been proposed as a core of the “Metabolic Theory of Ecology” (MTE). However, its thermal component has been rarely explicitly tested and its usefulness for prediction of thermal effect on key life-history traits such as reproductive rate and hence fitness is still questionable. Here, we tested its applicability to temperature-dependent rate of clutch production in a tropical gecko. The thermal effects on reproductive rates in reptiles are only poorly known and difficult to estimate, because most species lay clutches largely infrequently. Females of the Madagascar ground gecko (Paroedura picta) lay clutches in unusually short intervals, which allowed us to use this species as a model for estimation of dependence of rate of clutch production on temperature. We kept adult females at three constant temperatures (24, 27, 30 °C) and recorded their reproductive characteristics. Increasing temperature positively influences rate of clutch production, but in a manner not predicted by a simple model of reaction kinetics. The results in P. picta suggest that predictions of fitness consequences of shifts in thermal environment can be more complicated than expected under the general relationship of the MTE.     

Effects of freshwater and marine overwintering environments on life histories of threespine sticklebacks: evidence for adaptive variation between anadromous and resident freshwater populations

Summary Life history theory predicts that migratory fishes should delay reproduction, be larger at first reproduction, and have higher fecundities than nonmigrants. We tested this hypothesis by comparing life histories of anadromous (estuary) and resident freshwater (upstream) threespine sticklebacks (Gasterosteus aculeatus L.) from the Navarro River, California, USA. Using a split-brood, two-environment breeding design, families from cach population were divided and reared in both freshwater and seawater overwintering environments. In both treatments, the more migratory estuary sticklebacks were larger at first reproduction and had large initial clutch sizes; in the freshwater treatment, the estuary sticklebacks matured later than the upstream fish. Population means varied little across treatments, indicating that the average effects of the different overwintering conditions were slight. The responses of individual families to a given overwintering treatment were highly variable in both populations, as reflected in significant family x treatment effects for all traits. Phenotypic correlations among life history traits were significant and positive for most traits, and were similar in magnitude in both populations. Differences in the relative degree of specialization for migration may in part explain variation in life history between these populations.     

Evolution of thermal physiology and growth rate between populations of the western fence lizard (Sceloporus occidentalis)

Summary Hatchling Sceloporus occidentalis from northern populations (central Oregon) grow more slowly than hatchlings from southern populations (southern California) in nature. In this study, I determine whether this difference in growth rate results from differences in thermal environment and/or in thermoregulatory behavior. To determine the degree to which the thermal environment affects growth rate among populations, I reared hatchings from the northern and southern populations in a cycling thermal regime in one of three experimental treatments differing in access to radiant heat (6, 9, or 12 h radiant heat; remainder of 24 h at 15°C). I also measured the body temperature that each individual voluntarily selected over the course of the daily activity cycle. Growth rate varied positively with duration of access to radiant heat. Within the three treatments, individual growth rate was positively correlated with body temperature. Moreover, the difference in growth rate between the northern and southern populations was due in part to differences in behavior — individuals from northern populations selected lower body temperatures. I found that significant variation in body temperature was associated with family membership, suggesting that thermal physiology has a genetic basis. Moreover, growth rate was correlated with body temperature among families in each population suggesting a genetic correlation underlies the phenotypic correlations. Thus, genetically based variation in thermal physiology contributes to differences in growth rate among individuals within a population as well as to differences among populations.     

Phenotypic variation in the breeding phenology of the woodlouse Armadillidium vulgare

Summary The breeding phenology of temperate wood-lice is strongly seasonal, the result of physiological constraints and precise environmental cues for reproduction. The adaptive value of such mechanisms is that the release of offspring coincides with favourable conditions for growth and survival (Willows 1984). We recorded the breeding phenology of Armadillidium vulgare (Latreille) on two grassland sites in Great Britain and found between-site and between-year variation in the onset of reproduction, the duration of reproductive activity, the release of offspring, the size of reproductive females and the number of broods per female. Between 82.7 and 97.7% of gravid females sampled were semelparous at 23 months, with the remainder iteroparous, producing a second brood after 35 months. On one site (Weeting Health) improved growth conditions during 1984 allowed some females (19.3% of gravid females sampled in that year) to produce a brood after 11 months. There was also an increase in the number of 3-year-old females found to be gravid. An experimental manipulation of the same habitat confirmed that such changes in life history tactics could be phenotypic responses. The observed phenotypic variation was sufficient to produce a range of life history tactics within a population. Mixtures of life history tactics within a population may be typical of invasive species and populations at the edge of the species range. Our results support the idea that phenotypic plasticity can be an appropriate tactic to maximise fitness in a fluctuating environment (Caswell 1983, 1989).     

Variability in life history traits of the aphid,Acyrthosiphon pisum (Harris), from sexual and asexual populations

Many aphid species have shown remarkable adaptability by invading new habitats and agricultural crops, although they are parthenogenetic and might be expected to show limited genetic variation. To determine if the mode of reproduction limits the level of genetic variation in adaptively important traits, we assess variation in 15 life history traits of the pea aphid, Acyrhosiphon pisum (Harris), for five populations sampled along a north-south transect in central North America, and for three traits for three populations from eastern Australia. The traits are developmental times and rates as affected by temperature, body weights as affected by temperature, fecundity, measures of migratory tendency, and photoperiodic responses. The most southerly population from North America is shown to be obligately parthenogenetic, as are the Australian populations, and the four more northerly North American populations are facultatively parthenogenetic with the number of parthenogenetic generations per year increasing from north to south. The broad-sense heritabilities of life history traits varied from 0.36 to 0.71 for nine quantitive traits based on a comparison of within-and between-lineage variances. Using these traits, 7–13 distinct genotypes (i.e. clones) were identified among each of the 18 lines sampled from the North American populations, but the number did not differ significantly among populations. The level of genetic variation differed from trait to trait. For 4 of 12 quantitative traits, the level of variation in the obligately parthenogenetic population from North America was lowest, but significantly lower than all the sexual populations for only 1 trait. The obligately parthenogenetic population had the highest level of genetic variation for two traits, and had intermediate levels for the others. The most northerly population, which was sexual and had relatively few parthenogenetic generations each year, had the lowest level of variation for 5 of 12 traits and the highest level of variation for 2 traits. There was no decline in variability from north to south correlated with the increase in the annual number of parthenogenetic generations. The Australian populations showed no less variation than the North American populations for two of three traits, although the pea aphid was introduced to Australia only 5 years prior to the study, whereas the aphid has been in North America for at least 100 years. The mode of reproduction has not had a substantial impact on the level of genetic variation in life history traits of the pea aphid, but there are population-specific factors that effect the level of variation in certain traits.