Low-latitude Populations Research Articles

Enhanced assimilation rate and water use efficiency with latitude through increased photosynthetic capacity and internal conductance in balsam poplar (Populus balsamifera L.)

In outdoor common gardens, high latitude populations of deciduous tree species often display higher assimilation rates (A) than low latitude populations, but they accomplish less height. To test whether trends in A reflect adaptation to growing season length or, alternatively, are garden growth artefacts, we examined variation in height increment and ecophysiological traits in a range-wide collection of Populus balsamifera L. populations from 21 provenances, during unconstrained growth in a greenhouse. Rooted cuttings, maintained without resource limitation under 21 h photoperiod for 90 d, displayed increasing height growth, A, leaf mass per area and leaf N per area with latitude whereas stomatal conductance (g(s)) showed no pattern. Water-use efficiency as indicated by both gas exchange and delta(13)C increased with latitude, whereas photosynthetic nitrogen-use efficiency decreased. Differences in delta(13)C were less than expected based on A/g(s), suggesting coextensive variation in internal conductance (g(m)). Analysis of A-C(i) curves on a subset of populations showed that high latitude genotypes had greater g(m) than low-latitude genotypes. We conclude that higher peak rates of height growth in high latitude genotypes of balsam poplar are supported by higher A, achieved partly through higher g(m), to help compensate for a shorter growing season.

Age structure, growth, mortality, and density of belugas (Delphinapterus leucas) in the Canadian Arctic: responses to environment?

Environmental constraints on life-history traits are expected to increase with seasonality in resources such as food and appropriate breeding habitat. Seasonality is highest at polar latitudes, where environmental constraints can be stronger than biotic factors, such as density and its effect on intraspecific competition. In this study, the age structure, body-length distribution, mortality, and density were studied and compared among five beluga populations of the Canadian Arctic: eastern Beaufort Sea (EBS), Baffin Bay (BB), Cumberland Sound (CS), western (WHB), and eastern Hudson (EHB) Bay, to test the prediction that density-dependent effects on these life-history traits should be inversely related to latitude. Growth, but not mortality, showed a significant positive relationship with latitude. Winter density also increased with winter latitude, consistent with the prediction of greater risk of mortality associated with density-independent effects, such as ice entrapment in winter. Age distributions differed among populations, with animals harvested at the highest-latitude population (EBS) being the oldest and attaining the longest adult body lengths, compared to lower-latitude populations (WHB and EHB). The latitudinal variation in growth, adult body size, and winter density is congruent with the hypothesis that environmental seasonality may impose stronger constraints on life-history traits of beluga with increasing latitude.

Ecotypic variations in phosphoenolpyruvate carboxylase activity of the cordgrass Spartina densiflora throughout its latitudinal distribution range

This study compared the specific activity of phosphoenolpyruvate carboxylase (PEPC) of Spartina densiflora Brongn., collected from four populations along its latitudinal distribution range. Spartina densiflora is a halophyte with C(4) photosynthesis that has a very wide latitudinal distribution, from Patagonia to the southwest Iberian Peninsula. The basis of intraspecific differences in PEPC activity were analysed by recording the phosphorylation state and amount of the enzyme, comparing leaf anatomy and evaluating leaf gas exchange. S. densiflora individuals from Patagonia had 60% higher PEPC specific activity than plants from the other three populations due to higher levels of PEPC protein that coincided with lower activation mediated by phosphorylation, yielding similar net photosynthesis rate (c. 29 micromol CO(2)xm(-2)xs(-1)). Patagonian plants had a higher area of photosynthetic mesophyll relative to total chlorophyll than plants from north Argentina and the southwest Iberian Peninsula. Ecotypic differentiation in PEPC activity and leaf anatomy were found, distinguishing a higher-latitude ecotype from lower-latitude populations. The higher PEPC protein levels of the Patagonian ecotype seemed to be a response to lower light activation level of the enzyme, as judged by the low PEPC phosphorylation state.

Are high-latitude individuals superior competitors? A test with Rana temporaria tadpoles

Species with a wide distribution over latitudinal gradients often exhibit increasing growth and development rates towards higher latitudes. Ecological theory predicts that these fast-growing genotypes are, in the absence of trade-offs with fast growth, better competitors than low-latitude conspecifics. While knowledge on key ecological traits along latitudinal clines is important for understanding how these clines are maintained, the relative competitive ability of high latitude individuals against low latitude conspecifics has not been tested. Growth and development rates of the common frog Rana temporaria increase along the latitudinal gradient across Scandinavia. Here we investigated larval competition over food resources within and between two R. temporaria populations originating from southern and northern Sweden in an outdoor common garden experiment. We used a factorial design, where southern and northern tadpoles were reared either as single populations or as mixes of the two populations at two densities and predator treatments (absence and non-lethal presence of Aeshna dragonfly larvae). Tadpoles from the high latitude population grew and developed faster and in the beginning of the experiment they hid less and were more active than tadpoles from the low latitude population. When raised together with high latitude tadpoles the southern tadpoles had a longer larval period, however, the response of high latitude tadpoles to the competition by low latitude tadpoles did not differ from their response to intra-population competition. This result was not significantly affected by density or predator treatments. Our results support the hypothesis that high latitude populations are better competitors than their low latitude conspecifics, and suggest that in R. temporaria fast growth and development trade off with other fitness components along the latitudinal gradient across Scandinavia.

A test of the latitudinal defense hypothesis: herbivory, tannins and total phenolics in four North American tree species

AbstractIt is widely believed that insect herbivory is less intense at higher latitudes, due to winter mortality which would tend to keep insect herbivores from reaching density‐limitation of their populations. One prediction of this theory is that plants should tend to be better defended at lower latitudes. Here we investigated latitudinal trends in herbivory and tannins, in four species of common North American trees. Our comparisons spanned 15° of latitude in Acer rubrum, Fagus grandifolia, and Quercus alba, and 10° latitude in Liquidambar styraciflua. Sun leaves on forest edges were sampled, at phenologically equivalent times of year. Analysis revealed significant differences between populations, including those at similar latitudes, but no significant latitudinal trend in herbivory, condensed and hydrolyzable tannins, or total phenolics measured as Folin–Denis reactives in any of the four species. Our findings contradict the theory that low latitude plants are better defended, in that lower latitude populations of the four tree species showed no greater amounts of phenolics. The possible implications for community ecology are discussed.

Heat shock protein 101 effects in A. thaliana: genetic variation, fitness and pleiotropy in controlled temperature conditions.

The Hsp100/ClpB heat shock protein family is ancient and required for high temperature survival, but natural variation in expression and its phenotypic effects is unexplored in plants. In controlled environment experiments, we examined the effects of variation in the Arabidopsis cytosolic AtHsp101 (hereafter Hsp101). Ten wild-collected ecotypes differed in Hsp101 expression responses across a 22 to 40 degrees C gradient. Genotypes from low latitudes expressed the least Hsp101. We tested fitness and pleiotropic consequences of varying Hsp101 expression in 'control' vs. mild thermal stress treatments (15/25 degrees C D/N vs. 15/25 degrees D/N plus 3 h at 35 degrees C 3 days/week). Comparing wild type and null mutants, wt Columbia (Col) produced approximately 33% more fruits compared to its Hsp101 homozygous null mutant. There was no difference between Landsberg erecta null mutant NIL (Ler) and wt Ler; wt Ler showed very low Hsp101 expression. In an assay of six genotypes, fecundity was a saturating function of Hsp101 content, in both experimental treatments. Thus, in addition to its essential role in acquired thermal tolerance, Hsp101 provides a substantial fitness benefit under normal growth conditions. Knocking out Hsp101 decreased fruit production, days to germination and days to bolting, total dry mass, and number of inflorescences; it increased transpiration rate and allocation to root mass. Root : total mass ratio decayed exponentially with Hsp101 content. This study shows that Hsp101 expression is evolvable in natural populations. Our results further suggest that Hsp101 is primarily an emergency high-temperature tolerance mechanism, since expression levels are lower in low-latitude populations from warmer climates. Hsp101 expression appears to carry an important trade-off in reduced root growth. This trade-off may select for suppressed expression under chronically high temperatures.

Intraspecific Divergence in Seed Germination Traits between High- and Low-Latitude Populations of the Arctic-Alpine Annual Koenigia islandica

ABSTRACTPopulations of arctic-alpine plant species inhabit a range of environments, to which their life-history characters are expected to have adapted; yet geographic differentiation among populations is seldom studied. Brief growing seasons mean that germination characters are critical fitness traits, especially for annual species. In this study, striking differences in germination traits among three geographically distinct populations of the widely distributed arctic-alpine annual Koenigia islandica are found. The seeds (achenes) of the Colorado population, which experience the lowest summer temperatures, are conditionally dormant; only scarification breaks dormancy. This germination pattern is consistent in all four investigated subpopulations from Colorado, with no significant differences among them. In contrast, seeds originating in the Yukon, which experience extreme winter temperatures but a relatively warm summer, germinate readily after cold stratification, a pattern consistent with that of summer annuals. Seeds from the Norway population, which experience the mildest climate, germinate readily even if untreated. Cold stratification decreases germination fraction in the Norway population, a pattern characteristic of winter annuals. The strong population differentiation found here provides evidence for divergent selection operating among arctic-alpine habitats and suggests that further investigation of its adaptive significance is merited.

Microsatellite markers for the invasive plant species white sweetclover (Melilotus alba) and yellow sweetclover (Melilotus officinalis)

AbstractWe describe specific primers that amplify nine microsatellite DNA loci from Melilotus alba and Melilotus officinalis, both invasive plant species (Fabaceae) throughout North America. Allelic diversity was slightly lower for M. alba than for M. officinalis, as was expected heterozygosity. For both species, heterozygote deficit was observed at several loci. Genotypic diversity was very high for both species; the 29 plant samples of each species all had different multilocus genotypes. These markers will be used to determine the origins of the sweetclover invasion in Alaska and to compare patterns of diversity between subarctic and lower latitude populations.

Lack of genetic structure among ecologically adapted populations of an Australian rainforest Drosophila species as indicated by microsatellite markers and mitochondrial DNA sequences

Although fragmented rainforest environments represent hotspots for invertebrate biodiversity, few genetic studies have been conducted on rainforest invertebrates. Thus, it is not known if invertebrate species in rainforests are highly genetically fragmented, with the potential for populations to show divergent selection responses, or if there are low levels of gene flow sufficient to maintain genetic homogeneity among fragmented populations. Here we use microsatellite markers and DNA sequences from the mitochondrial ND5 locus to investigate genetic differences among Drosophila birchii populations from tropical rainforests in Queensland, Australia. As found in a previous study, mitochondrial DNA diversity was low with no evidence for population differentiation among rainforest fragments. The pattern of mitochondrial haplotype variation was consistent with D. birchii having undergone substantial past population growth. Levels of nuclear genetic variation were high in all populations while F(ST) values were very low, even for flies from geographically isolated areas of rainforest. No significant differentiation was observed between populations on either side of the Burdekin Gap (a long-term dry corridor), although there was evidence for higher gene diversity in low-latitude populations. Spatial autocorrelation coefficients were low and did not differ significantly from random, except for one locus which revealed a clinal-like pattern. Comparisons of microsatellite differentiation contrasted with previously established clinal patterns in quantitative traits in D. birchii, and indicate that the patterns in quantitative traits are likely to be due to selection. These results suggest moderate gene flow in D. birchii over large distances. Limited population structure in this species appears to be due to recent range expansions or cycles of local extinctions followed by recolonizations/expansions. Nevertheless, patterns of local adaptation have developed in D. birchii that may result in populations showing different selection responses when faced with environmental change.

Membrane lipid physiology and toxin catabolism underlie ethanol and acetic acid tolerance inDrosophila melanogaster

Drosophila melanogaster has evolved the ability to tolerate and utilize high levels of ethanol and acetic acid encountered in its rotting-fruit niche. Investigation of this phenomenon has focused on ethanol catabolism, particularly by the enzyme alcohol dehydrogenase. Here we report that survival under ethanol and acetic acid stress in D. melanogaster from high- and low-latitude populations is an integrated consequence of toxin catabolism and alteration of physical properties of cellular membranes by ethanol. Metabolic detoxification contributed to differences in ethanol tolerance between populations and acclimation temperatures via changes in both alcohol dehydrogenase and acetyl-CoA synthetase mRNA expression and enzyme activity. Independent of changes in ethanol catabolism, rapid thermal shifts that change membrane fluidity had dramatic effects on ethanol tolerance. Cold temperature treatments upregulated phospholipid metabolism genes and enhanced acetic acid tolerance, consistent with the predicted effects of restoring membrane fluidity. Phospholipase D was expressed at high levels in all treatments that conferred enhanced ethanol tolerance, suggesting that this lipid-mediated signaling enzyme may enhance tolerance by sequestering ethanol in membranes as phophatidylethanol. These results reveal new candidate genes underlying toxin tolerance and membrane adaptation to temperature in Drosophila and provide insight into how interactions between these phenotypes may underlie the maintenance of latitudinal clines in ethanol tolerance.

The northern geographic range limit of the intertidal limpetCollisella scabra: a test of performance, recruitment, and temperature hypotheses

A decline in abundance towards a species' range boundary is often interpreted as evidence of a decline in individual success, and is usually assumed to reflect a decline in suitable environmental conditions. Gradual declines towards high latitude range boundaries are frequently attributed to limitations on organismal tolerance of cold temperature. Rarely have these two assumptions been empirically tested. The intertidal gastropodCollisella scabradeclines monotonically in abundance from 435 to <1 m−2over the northern 300 km of its geographic distribution. I examined temperature, adult performance (survival, growth, reproduction), and recruitment at five locations in this region of decline. Mortality ranged from 4.9 to 11.2% per month, but was highest at the lowest latitude study site. Growth rates ranged from 0 to 5.2 mm yr−1, but were generally lower at lower latitude sites. Gonad development was high in the three populations examined, but the possibility of infrequent spawning at high latitude sites could not be excluded. Finally, a comparison of performance differences among populations with temperature revealed clear effects of temperature on both growth and mortality; however, the patterns were not consistent with a hypothesis of cold stress at the range boundary. Overall there was little evidence for either reduced performance or increasing cold stress in low density high latitude populations. Over the same 300 km, recruitment declined monotonically from a mean of six recruits per 625 cm2to less than one; suggesting that limitations on recruitment, rather that adult performance, are responsible for low abundance in marginal populations. Several hypotheses for the decline in recruitment are discussed in the paper and the most likely explanation appears to be an increase in the distance between populations at the range margin, reducing the chances that dispersing larvae find suitable habitat for settlement.

Introduced Bullfrogs (Rana catesbeiana) in Western Canada: Has Their Ecology Diverged?

Organisms can diverge in life history when introduced outside their native range due to release from predators, competitors, and parasites, and also due to genetic drift and local adaptation. We studied the ecology of the American Bullfrog (Rana catesbeiana) in its introduced range in British Columbia (BC). To assess differences between introduced and native populations, we compared the population ecology of BC bullfrogs to published life-history parameters from the bullfrog's extensive native range in eastern North America. In BC, bullfrogs emerged from hibernation in late April and early May. Breeding choruses developed fully when air temperatures exceeded 20°C. Eggs were laid from mid-June to mid-July when the mean water temperature was 20°C. Mean egg mass size was 13,014 ± 7,296 eggs (mean ± standard deviation). Tadpoles hatched in 3 to 5 days and overwintered the first year as tadpoles. Approximately 68% of the tadpoles metamorphosed at the end of the following summer, but the remaining spent a second winter in the pond. Bullfrogs were large enough to attain sexual maturity 2 yr after metamorphosis. Bullfrog population density among ponds varied from 4.1 to 530 frogs/ha. Terrestrial insects were the primary prey item of bullfrogs < 150 g, whereas frogs were the primary prey item of larger bullfrogs. The life-history parameter values estimated for BC bullfrogs were within the range observed for bullfrogs in their native habitats. Due to milder weather conditions in southwestern BC, the seasonal pattern and growth rate of bullfrogs were similar to lower latitude populations in Kentucky and Missouri. We found no evidence to support the hypothesis that when released from native predators and parasites bullfrogs build up to unusually high population densities or attain significantly larger sizes in their introduced range.

Variation of Germination Response to Temperature in Formosan Lily (Lilium formosanum Wall.) Collected from Different Latitudes and Elevations in Taiwan

Germination characteristics at different temperatures were examined in Formosan lily (Lilium formosanum Wall.) seeds collected at different latitudes (22º 46’–24º 47’ N) and elevations (50-3000 m) in Taiwan. Germination was sensitive to temperature and varied among the populations, especially at 5ºC and 25ºC. All tested populations germinated fastest at 18ºC. At this temperature, it took 8-14 days for initiation of germination (Gin) and 10-19 days to reach 50% of final germination percentage (G50). Days to Gin and G50, were 10-21 and 14-41, respectively, at 10ºC, 42-82 and 51-136, respectively, at 5ºC and 10-39 and 15-100, respectively, at 25ºC. In the low elevation (50-880 m) populations, seeds from higher latitude populations germinated more rapidly at 5ºC but more slowly at 25ºC than those from a lower latitude. Furthermore, seeds from the lower latitude populations had significantly higher final germination percentages than those from the higher latitude populations at 25ºC. Some germination characteristics of seeds from higher elevation habitats at the middle latitude were very similar to those from lower elevation habitats at the lower latitude despite of the difference in air temperature between the habitats. The eco-physiological germination characteristics of Formosan lily in response to temperatures were discussed in relation to the climatic conditions of each habitat.

Postglacial expansion of the southern red‐backed vole (Clethrionomys gapperi) in North America

Dynamic climatic oscillations of the Pleistocene dramatically changed the distributions of high latitude species. Molecular investigations of a variety of organisms show that processes of postglacial colonization of boreal regions were more complex than initially thought. Phylogeographical and coalescent analyses were conducted on partial sequences of the cytochrome b gene (600 bp) from 64 individuals of Clethrionomys gapperi from North Carolina, Pennsylvania, Minnesota, Idaho, Washington, British Columbia, Northwest Territories, and Alaska to test hypotheses relating to Pleistocene refugia and postglacial colonization routes. Three divergent clades (east, west, central) were identified with highest net divergence (dA = 5.2%) between the eastern and western clades. Populations from the recently deglaciated higher latitudes of Canada and Alaska are closely related to lower latitude populations of the central clade (dA = 1.2%) suggesting recent expansion from this midwestern region. No representatives from the east or west clade were found at latitudes higher than 50 degrees N, indicating that postglacial colonization occurred through a midcontinental route. The high latitude population from the Northwest Territories exhibited demographic patterns and genetic diversity consistent with a stable noncolonizing population. This population is found near the Mackenzie range, where the two continental ice sheets were believed to have coalesced. Molecular variation observed in this population may be the result of leading edge population diversifying in the continental corridor or may reflect the signal of a high latitude refugial population.

Embryonic responses to variation in oviductal oxygen in the lizard Sceloporus undulatus from New Jersey and South Carolina, USA

Viviparity in reptiles is hypothesized to evolve in cold climates at high latitudes and high elevations through selection for progressively longer periods of egg retention. Oxygen consumption of embryos increases during development and therefore longer periods of egg retention should be associated with maternal or embryonic features that enhance embryonic oxygen availability. We tested the hypotheses that embryos of the oviparous lizard Sceloporus undulatus from a high-latitude population in New Jersey are oviposited at more advanced developmental stages and have a higher growth rate at low oxygen partial pressures (pO2) than embryos from a low-latitude population in South Carolina. These hypotheses were rejected; embryos from the two populations did not differ in embryonic stage at oviposition, survival, rate of differentiation or growth in mass when incubated under simulated in utero conditions at low oxygen concentrations. We also estimated the effective pO2 experienced by lizard embryos in utero. At an effective pO2 of 8.6 kPa (9% O2), development of S. undulatus embryos is arrested at Dufaure and Hubert stage 30 and at a dry mass of 0.8 mg. Physiological and morphological features of gravid females, embryos, or both, that facilitate oxygen uptake for developing embryos appear to be a critical early step during the evolution of reptilian viviparity. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 83, 289–299.

Interpopulation Variation in Developmental Titers of Vitellogenin, but Not Storage Proteins, in Lubber Grasshoppers

We examined simultaneous plastic and latitudinal interpopulation variation in the time course of hemolymph protein titers during egg production in the lubber grasshopper. Our goal was to gain insight into possible evolutionary changes in the physiology underlying reproductive plasticity. We used lubbers from three locations in the United States (Florida [FL], Louisiana [LA], and Georgia [GA]), each offered three daily food rations. Previous genetic analysis indicated that grasshoppers from FL (the low-latitude population) and GA (the high-latitude population) were phylogenetically closer to each other than to LA grasshoppers (the intermediate-latitude population). The ages at maximum titers of vitellogenin (Vg(max)) and three storage proteins that were referred to as major hemolymph proteins (MHP(max)) were used as indices of the progress of oocyte development. Age at Vg(max) was affected significantly both by diet and by population. Perhaps most importantly, age at Vg(max) was less for GA grasshoppers than for FL and LA grasshoppers; this pattern differs from the phylogenetic relationships of the populations. Age at MHP(max) was significantly affected only by diet and not by population. Hence, the regulation of these proteins may differ across populations. Finally, we found no evidence that plasticity of reproductive investment in response to food availability differs across populations (as indicated by nonsignificant interactions of population and feeding environment).

LATITUDINAL VARIATION IN GENETIC DIVERGENCE OF POPULATIONS AND THE POTENTIAL FOR FUTURE SPECIATION

The increase in biological diversity with decreasing latitude is widely appreciated but the cause of the pattern is unknown. This pattern reflects latitudinal variation in both the origin of new species (cladogenesis) and the number of species that coexist. Here we address latitudinal variation in species origination, by examining population genetic processes that influence speciation. Previous data suggest a greater number of speciation events at lower latitudes. If speciation events occur more frequently at lower latitudes, we predicted that genetic divergence among populations within species, an important component of cladogenesis, should be greater among lower latitude populations. We tested this prediction using within-species patterns of mtDNA variation across 60 vertebrate species that collectively spanned six continents, two oceans, and 119 degrees latitude. We found greater genetic divergence of populations, controlling for geographic distance, at lower latitudes within species. This pattern remained statistically significant after removing populations that occur in localities previously covered by continental glaciers during the last glaciation. Results suggest that lower latitude populations within species exhibit greater evolutionary independence, increasing the likelihood that mutation, recombination, selection, and/or drift will lead to divergence of traits important for reproductive isolation and speciation. Results are consistent with a greater influence of seasonality, reduced energy, and/or glacial (Milankovitch) cycles acting on higher latitude populations, and represent one of the few tests of predictions of latitudinal variation in speciation rates using population genetic data.

High autumn temperature delays spring bud burst in boreal trees, counterbalancing the effect of climatic warming.

The effect of temperature during short-day (SD) dormancy induction was examined in three boreal tree species in a controlled environment. Saplings of Betula pendula Roth, B. pubescens Ehrh. and Alnus glutinosa (L.) Moench. were exposed to 5 weeks of 10-h SD induction at 9, 15 and 21 degrees C followed by chilling at 5 degrees C for 40, 70, 100 and 130 days and subsequent forcing at 15 degrees C in a 24-h photoperiod for 60 days. In all species and with all chilling periods, high temperature during SD dormancy induction significantly delayed bud burst during subsequent flushing at 15 degrees C. In A. glutinosa, high temperature during SD dormancy induction also significantly increased the chilling requirement for dormancy release. Field experiments at 60 degrees N with a range of latitudinal birch populations revealed a highly significant correlation between autumn temperature and days to bud burst in the subsequent spring. September temperature alone explained 20% of the variation between years in time of bud burst. In birch populations from 69 and 71 degrees N, which ceased growing and shed their leaves in August when the mean temperature was 15 degrees C, bud burst occurred later than expected compared with lower latitude populations (56 degrees N) in which dormancy induction took place more than 2 months later at a mean temperature of about 6 degrees C. It is concluded that this autumn temperature response may be important for counterbalancing the potentially adverse effects of higher winter temperatures on dormancy stability of boreal trees during climate warming.

Interpopulation Variability in Temperature-Dependent Sex Determination of the Tidewater Silverside Menidia peninsulae (Pisces: Atherinidae)

Abstract The existence of temperature-dependent sex determination (TSD) and its latitudinal variability among populations were examined experimentally in Menidia peninsulae. Along the Florida coast, TSD was detected only in a northern population (Apalachee Bay: 29°55′N), wherein the proportion of females declined dramatically with increasing rearing temperature. In lower-latitude populations (New Smyrna Beach: 29°05′N, Vero Beach: 27°46′N, and Grassy Key: 24°45′N), however, offspring sex ratios were temperature-independent and approximated 1:1 at all temperatures examined (17–32 C), indicating genetic sex determination (GSD). The shift from TSD to GSD with decreasing latitude in M. peninsulae occurs coincidentally with a shift from annual to semiannual breeding and life cycle patterns. Combined with the previous studies of the northern congener Menidia menidia (∼32–47°N), the level of TSD in the genus Menidia is generally highest at intermediate latitudes (∼32–38°N), whereas nearly pure GSD is displayed a...

Comparison of growth and RNA: DNA ratios in three populations of juvenile turbot reared at two salinities

Salinity significantly influenced growth and RNA: DNA ratios in three populations (Iceland. Norway and France) of juvenile turbot Scophthalmus maximus as these variables were higher at 15 than at 33.5‰ salinity. Furthermore, growth and RNA: DNA ratios of the two high latitude populations (Iceland and Norway) were higher than those of the low latitude population (France): the Norwegian and the Icelandic populations had RNA: DNA ratios of 6.028 and 5.91, respectively, whereas the ratio of the French population was 5.01. It is concluded that in the upper temperature range for turbot, growth can be improved by rearing at intermediate salinities at least during the first year.