Growth hormone differentially regulates growth and growth-related gene expression in closely related fish species

Zebrafish (Danio rerio) have become an important model organism for developmental biology and human health studies. We recently demonstrated differential growth patterns between the zebrafish and a close relative the giant danio (Danio aequipinnatus), where the giant danio appears to exhibit indeterminate growth similar to most fish species important for commercial production, while zebrafish exhibit determinate growth more similar to mammalian growth. This study focused on evaluating muscle growth regulation differences in adult zebrafish and giant danio utilizing growth hormone treatment as a mode of growth manipulation. Growth hormone treatment resulted in increased overall growth in giant danio, but failed to increase growth in the zebrafish. Growth hormone treatment increased muscle IGF-I and GHrI gene expression in both species, but to a larger degree in the giant danio. In contrast, zebrafish exhibited a larger increase in IrA and IGF-IrB gene expression in muscle in response to GH treatment. In addition muscle myostatin levels were differentially regulated between the two species, with a down-regulation observed in rapidly growing, GH-treated giant danio and an up-regulation in zebrafish not actively growing in response to GH. This is the first report of differential expression of growth-regulating genes in closely related fish species exhibiting opposing growth paradigms. These results further support the role that the zebrafish and giant danio can play important model organisms for determinate and indeterminate growth.     

Size-independent growth in fishes: patterns, models and metrics

A combination of a dynamic energy budget (DEB) model, field data on Atlantic salmon Salmo salar and brown trout Salmo trutta and laboratory data on Atlantic salmon was used to assess the underlying assumptions of three different metrics of growth including specific growth rate (G), standardized mass‐specific growth rate (G_S) and absolute growth rate in length (G_L) in salmonids. Close agreement was found between predictions of the DEB model and the assumptions of linear growth in length and parabolic growth in mass. Field data comparing spring growth rates of age 1+ year and 2+ year Atlantic salmon demonstrated that in all years the larger age 2+ year fish exhibited a significantly lower G, but differences in growth in terms of G_S and G_L depended on the year examined. For brown trout, larger age 2+ year fish also consistently exhibited slower growth rates in terms of G but grew at similar rates as age 1+ year fish in terms of G_S and G_L. Laboratory results revealed that during the age 0+ year (autumn) the divergence in growth between future Atlantic salmon smolts and non‐smolts was similar in terms of all three metrics with smolts displaying higher growth than non‐smolts, however, both G_S and G_L indicated that smolts maintain relatively fast growth into the late autumn where G suggested that both smolts and non‐smolts exhibit a sharp decrease in growth from October to November. During the spring, patterns of growth in length were significantly decoupled from patterns in growth in mass. Smolts maintained relatively fast growth though April in length but not in mass. These results suggest G_S can be a useful alternative to G as a size‐independent measure of growth rate in immature salmonids. In addition, during certain growth stanzas, G_S may be highly correlated with G_L. The decoupling of growth in mass from growth in length over ontogeny, however, may necessitate a combination of metrics to adequately describe variation in growth depending on ontogenetic stage particularly if life histories differ.     

WHY DO DINOFLAGELLATES HAVE LOWER GROWTH RATES?1

Dinoflagellates have substantially lower growth rates than other taxa of similar size. These low growth rates have been suggested to reflect the lower chlorophyll a to carbon ratio (Chl a:C) in dinoflagellates, but that speculation has never been widely tested. This study tests if the variations in growth rates among taxa are related to differences in Chl a:C using published data. I collected 92 data entries from the literature representing 31 species, mostly from two divisions (Chrysophyta and Pyrrophyta), and found a significant relation (r²= 0.39) between growth and Chl a:C. Since Chl a:C is almost independent of C content, I also developed a growth model using both C and Chl a:C. Together, the two variables explain 68% of variation in algal growth. However, a further 6.4% of the variance in growth can still be attributed to phyletic differences. Low Chl a:C is only a partial explanation for the low growth rates of the dinoflagellates.     

入侵植物薇甘菊新枝在海南的月生长动态

【背景】薇甘菊是入侵性和危害性极强的外来入侵植物,对生物多样性和农林业生产造成严重影响。观测薇甘菊新枝的月生长动态,了解薇甘菊快速生长扩散的原因,能够为薇甘菊的合理防治提供参考。【方法】采用定点定时观测法,历时2年对海南7个市县薇甘菊新枝的月生长量进行测定。【结果】薇甘菊在海南全年可生长,一新枝月平均生长66 cm,全年累计平均生长795 cm。薇甘菊生长高峰期在6月和8月,其中6月平均生长120 cm,8月平均生长106 cm。薇甘菊最小生长量在1月和12月,平均生长量均为21 cm。薇甘菊在6—10月生长较快,一新枝累计平均生长489 cm,占全年生长量的61.5%;在4—5月和11月生长次之,一新枝累计平均生长196 cm,占全年生长量的24.7%;在1—3月和12月生长缓慢,一新枝累计平均生长110 cm,占全年生长量的13.8%。薇甘菊的花果期在10月至次年3月,一些种群在1—3月再次开花或在3—5月开花。薇甘菊的生长量与温度呈极显著正相关,相关系数为0.91;与降雨量呈显著正相关,相关系数0.56。【结论与意义】薇甘菊是多分枝植物,一新枝年生长量可达795 cm,能迅速生长形成种群。温度是薇甘菊生长的主要因素,降雨在一定程度促进了薇甘菊的生长。     

Studies on the hormonal relationships of algae in pure culture

Summary The effect of potential precursors of Indole-3-acetic acid (IAA) on the growth (increase in dry weight) of Anacystis nidulans, Chlorogloea fritschii, Phormidium foveolarum, Nostoc muscorum and Tolypothrix tenuis have been investigated under as sterile conditions as possible.Tryptamine showed a marked stimulation of growth indicating its possible conversion to IAA. Tryptophan at a hormonal concentration promoted growth in only 1 species, Chlorogloea fritschii. Indole stimulated the growth of Chlorogloea fritschii, Nostoc muscorum and Tolypothrix tenuis. Anthranilic acid promoted the growth of Nostoc muscorum, it failed to stimulate the growth of Chlorogloea fritschii. Anthranilonitrile promoted the growth of Nostoc muscorum and Chlorogloea fritschii. -Alanine promoted the growth of Nostoc muscorum, Tolypothrix tenuis and Chlorogloea fritschii at hormonal concentrations.     

Sapling growth as a function of light and landscape-level variation in soil water and foliar nitrogen in northern Michigan

Interspecific differences in sapling growth responses to soil resources could influence species distributions across soil resource gradients. I calibrated models of radial growth as a function of light intensity and landscape-level variation in soil water and foliar N for saplings of four canopy tree species, which differ in adult distributions across soil resource gradients. Model formulations, characterizing different resource effects and modes of influencing growth, were compared based on relative empirical support using Akaike’s Information Criterion. Contrary to expectation, the radial growth of species associated with lower fertility (Acer rubrum and Quercus rubra) was more sensitive to variation in soil resources than the high fertility species Acer saccharum. Moreover, there was no species tradeoff between growth under high foliar N versus growth under low foliar N, which would be expected if growth responses to foliar N mediated distributions. In general, there was functional consistency among species in growth responses to light, foliar N, and soil water availability, respectively. Foliar N influenced primarily high-light growth in F. grandifolia, A. rubrum, and Q. rubra (but was not significant for A. saccharum). In A. saccharum and A. rubrum, for which soil water availability was a significant predictor, soil water and light availability simultaneously limited growth (i.e., either higher light or water increased growth). Simple resource-based models explained 0.74–0.90 of growth variance, indicating a high degree of determinism. Results suggest that nitrogen effects on forest dynamics would be strongest in high-light early successional communities but that water availability influences growth in both early successional and understory environments.     

Growth rate adjustment of two Drosophila parasitoids in response to the developmental stage of hosts

1. Generalist koinobiont parasitoids often exhibit high flexibility in their development; their larvae shorten or prolong the developmental period depending on the host quality at parasitisation. However, flexibility of the growth rate of parasitoid larvae has rarely been investigated so far. 2. This study investigated how the koinobiont parasitoid wasps Asobara japonica and Leptopilina ryukyuensis regulate their larval growth when they parasitise host Drosophila larvae with varying larval periods. 3. In both parasitoid species, the preimaginal period was longer when they parasitised 1‐day‐old larvae of Drosophila rufa than when they parasitised older larvae of D. rufa or when they parasitised larvae of Drosophila simulans, a species with a shorter larval period than D. rufa. After host pupariation, A. japonica accelerated its growth, thereby showing a biphasic growth curve. On the other hand, L. ryukyuensis did not accelerate its growth after host pupariation. 4. Growth retardation of parasitoid larvae in 1‐day‐old D. rufa larvae would contribute to avoiding excess growth before host pupariation, because the excess growth of parasitoid larvae would have negative effects on host growth. The growth rate acceleration of A. japonica after host pupariation suggests that they enhance resource utilisation in a host that has reached maximum body mass. It remains uncertain as to why L. ryukuensis does not show clear accelerated growth after host pupariation. Nonetheless, these results suggest that parasitoid larvae have the ability to detect the developmental stage of hosts in a species‐specific manner.     

RELATIONSHIP BETWEEN CELL CYCLE AND LIGHT‐LIMITED GROWTH RATE IN OCEANIC PROCHLOROCOCCUS (MIT9312) AND SYNECHOCOCCUS (WH8103) (CYANOBACTERIA)1

The relationships between growth rate, cell‐cycle parameters, and cell size were examined in two unicellular cyanobacteria representative of open‐ocean environments: Prochlorococcus (strain MIT9312) and Synechococcus (strain WH8103). Chromosome replication time, C, was constrained to a fairly narrow range of values (～4–6 h) in both species and did not appear to vary with growth rate. In contrast, the pre‐ and post‐DNA replication periods, B and D, respectively, decreased with increasing growth rate from maxima of ～30 and 10–20 h to minima of ～4–6 and 2–3 h, respectively. The combined duration of the chromosome replication and postreplication periods (C+D), a quantity often used in the estimation of Prochlorococcus in situ growth rates, varied ～2.4‐fold over the range of growth rates examined. This finding suggests that assumptions of invariant C+D may adversely influence Prochlorococcus growth rate estimates. In both strains, cell mass was the greatest in slowly growing cells and decreased 2‐ to 3‐fold over the range of growth rates examined here. Estimated cell mass at the start of replication appeared to decrease with increasing growth rate, indicating that the initiation of chromosome replication in Prochlorococcus and Synechococcus is not a simple function of cell biomass, as suggested previously. Taken together, our results reflect a notable degree of similarity between oceanic Synechococcus and Prochlorococcus strains with respect to their growth‐rate‐specific cell‐cycle characteristics.     

Drought advances spring growth phenology of the Mediterranean shrub Erica multiflora

Current climate projections predict drier and warmer conditions in the Mediterranean basin over the next century. While advanced spring growth due to warming has been described in the literature, few data are available on the effects of drought on phenology. Hence, the phenology and growth of two Mediterranean shrubs, Erica multiflora and Globularia alypum, was studied in a rainfall exclusion field experiment to simulate spring drought in a natural shrubland. We estimated the onset of growth in spring by monitoring the appearance of new stems, and the end of growth in summer by following the elongation of stems. Drought treatment caused earlier onset of the spring growing season in E. multiflora, whereas no advance was observed in G. alypum. However, growth cessation was not affected in E. multiflora. Drought reduced the growth of both shrubs, as reflected in less stem elongation. The results show that a drier climate might affect not only growth but also spring phenology of some Mediterranean species. We suggest that a reduction in the cooling effect of transpiration may have analogous effects to warming and might advance the start of growth in E. multiflora, a species whose phenology has been described as warming‐sensitive. The lengthening of the growing season resulting from advanced growth did not imply higher productivity, as growth was restricted by drought.     

Functions of the SLC36 transporter Pathetic in growth control

Neurons exhibit extreme diversity in size, but whether large neurons have specialized mechanisms to support their growth is largely unknown. Recently, we identified the SLC36 transporter Pathetic (Path) as a factor required for extreme dendrite growth in neurons. Path is broadly expressed, but only neurons with large dendrite arbors or small neurons that are forced to grow large require path for their growth. To gain insight into the basis of growth control by path, we generated additional alleles of path and further examined the apparent specificity of growth defects in path mutants. Here, we confirm our prior finding that loss of path function imposes an upper limit on neuron growth, and additionally report that path likely limits overall neurite length rather than dendrite length alone. Using a GFP knock-in allele of path, we identify additional tissues where path likely functions in nutrient sensing and possibly growth control. Finally, we demonstrate that path regulates translational capacity in a cell type that does not normally require path for growth, suggesting that path may confer robustness on growth programs by buffering translational output. Altogether, these studies suggest that Path is a nutrient sensor with widespread function in Drosophila.     

Contrasting ‘Top-Down’ Effects of Crustacean Zooplankton Grazing on Bacteria and Phytoflagellates

The combined effects of grazing and nutrient regeneration by Daphnia and Eudiaptomus on the growth of Rhodomonas and heterotrophic bacteria was assessed experimentally. The responses of Rhodomonas and bacteria to the grazers were measured as net specific growth rate over the entire experimental periods, as well as production and specific production at the end of the experiments. Both zooplankton species had a negative effect on Rhodomonas net specific growth rate due to grazing and a positive effect on specific primary production due to nutrient regeneration. Daphnia had no effect on bacterial net specific growth rate, bacterial production or specific bacterial production in one of two experiments. In the other experiment, however, both bacterial growth rate and production decreased as a result of grazing. Furthermore, Daphnia had a negative effect on specific bacterial production, but Eudiaptomus had a positive effect on all bacterial parameters due to nutrient regeneration, probably of phosphorus. Positive effects of copepods on bacterial growth has previously been attributed to trophic cascades via protozoa. However, the present experiments show that regeneration of nutrients, especially phosphorus, may account for a large part of the stimulation of bacterial growth.     

The Effects of Temperature and pH on the Growth of Eight Enteric and Nine Glucose Non-Fermenting Species of Gram-Negative Rods

We studied the heat resistance and the range of growth temperature o gram-negative rods to find one of the bacterial factors governing their infectivity in exogenous and endogenous infections in predisposed patients. Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, Pseudomonas aeruginosa, and Acinetobacter calcoaceticus grew equally well at 25, 30, 37, and 42 C. Among other sugar non-fermenting gram-negative rods, six species showed suppressed growth at either or both ends of the incubation temperature range. All the bacterial species tested were killed within 30 min at 60 or 70 C. At 10 C, none of the bacterial strains multiplied, but all survived for 6 hr. Of 17 bacterial species tested, E. coli had the widest range of growth temperature (18–47 C), and also the shortest time necessary for growth to a certain population. Among the sugar non-fermenting rods, A. calcoaceticus had the widest range of growth temperature (20–45 C) and also multiplied rapidly. Pseudomonas strains exhibited slower growth at all temperatures and also had a narrower range of growth temperature than Enterobacteriaceae. Among Pseudomonas species, P. aeruginosa had the widest range of growth temperature (25–42 C) and also showed rapid growth. Pseudomonas cepacia, Achromobacter xylosoxidans, and Alcaligenes faecalis had a narrow range of growth temperature (28–37 C), and Pseudomonas fluorescens, Flavobacterium meningosepticum, and Moraxella grew most rapidly at 30 C. The above results are correlated fairly well with the incidence of clinical cases of infection. The growth attitude of a species of bacteria in response to temperature was considered to be one of the factors affecting the establishment of infection.     

Studies on the Germination of Pine Pollen (Pinus mugo) in vitro. I. Growth Conditions and Effects of pH and Temperature on Germination, Tube Growth and Respiration

The ability of pine pollen to grow in vitro is discussed in relation to its in situ. Optimal conditions of growth in vitro were investigated. All the experiments were made with pollen of Pinus mugo Turra. A satisfactory medium is described as that which at pH 5.2 and at 29 °C can lead to a tube growth bigger than in vivo. The initial growth (0–30) hours) in terms of tube growth and respiration is described in detail. It is shown that a correlation exists between tube growth and respiration. A method for estimating the metabolism and growth of the pollen tube, based on the uptake of ³²p-labelled phosphate, is described. Addition of known stimulators of plant growth and extension does not stimulate tube growth. Temperature studies show that under 20°C germination does not take place.     

A preliminary investigation on the growth requirement for monovalent cations, divalent cations and medium ionic strength of marine actinomycete Salinispora

In this paper, we report that three species of Salinispora, S. arenicola, S. tropica, and S. pacifica, require magnesium and calcium, for growth, with S. pacifica having the most stringent growth requirement for these ions. Interaction between these ions in supporting the growth of Salinispora was observed. We also demonstrated that the absolute requirement of sodium to support the growth of Salinispora has not been established as all three species of Salinispora can use either potassium or lithium to replace sodium to support maximum growth. While lithium can replace sodium to support maximum growth of Salinispora, it is more toxic to S. arenicola than S. tropica and S. pacifica, inhibiting the growth of S. arenicola at 189 mM but without effect on the growth of S. tropica and S. pacifica. Using both sodium chloride-based and lithium chloride-based media, we showed that Salinispora has a growth requirement for divalent ions, magnesium and calcium as well as growth requirement for ionic strength (8.29 to 15.2 mS/cm). S. arenicola has a lower growth requirement for ionic strength than S. tropica and S. pacifica.     

Drivers of growth for Atlantic cod (Gadus morhua L.) in Icelandic waters – A Bayesian approach to determine spatiotemporal variation and its causes

This study assesses spatiotemporal and sex-specific growth of Atlantic cod Gadus morhua in Icelandic waters. We use a Bayesian approach which lends itself to fitting and comparing nested models such as these. We then compare fitted parameters of these models to potential explanatory variables using a redundancy analysis (RDA) to look for drivers of growth in G. morhua. Results indicate that models that incorporate differences in growth among time, space and sex are the best-fitting models according to deviance information criterion (DIC). Results from RDA indicate that capelin Mallotus villosus recruitment and biomass is highly correlated with deviations in the von Bertalannfy growth parameter k and that L_∞ is correlated with G. morhua landings in the model that uses year to account for time-varying growth and estimated G. morhua recruitment in the model that uses cohort to account for time-varying growth.     

GROWTH REGULATION OF rRNA CONTENT IN PROCHLOROCOCCUS AND SYNECHOCOCCUS (MARINE CYANOBACTERIA) MEASURED BY WHOLE‐CELL HYBRIDIZATION OF rRNA‐TARGETED PEPTIDE NUCLEIC ACIDS1

The cyanobacteria Synechococcus and Prochlorococcus are important primary producers in marine ecosystems. Because currently available approaches for estimating microbial growth rates can be difficult to apply in the field, we have been exploring the feasibility of using quantitative rRNA measurements as the basis for making such estimates. In this study we examined the relationship between rRNA and growth rate in several Synechococcus and Prochlorococcus strains over a range of light‐regulated growth rates. Whole‐cell hybridization with fluorescently labeled peptide nucleic acid (PNA) probes was used in conjunction with flow cytometry to quantify rRNA on a per cell basis. This PNA probing technique allowed rRNA analysis in a phycoerythrin‐containing Synechococcus strain (WH7803) and in a non–phycoerythrin‐containing strain and in Prochlorococcus. All the strains showed a qualitatively similar tri‐phasic relationship between rRNA·cell^?1 and growth rate, involving relatively little change in rRNA·cell^?1 at low growth rates, linear increase at intermediate growth rates, and a plateau and/or decrease at the highest growth rates. The onset of each phase was associated with the relative, rather than absolute, growth rate of each strain. In the Synechococcus strains, rRNA normalized to flow cytometrically measured forward angle light scatter (an indicator of size) was well‐correlated with growth rate across strains. These findings support the idea that cellular rRNA may be useful as an indicator of in situ growth rate in natural Synechococcus and Prochlorococcus populations.     

Prenatal data impacts common bottlenose dolphin (Tursiops truncatus) growth parameters estimated by length‐at‐age curves

Compilation of marine mammal demographic data is central to management efforts. However, marine mammal length‐at‐age growth curves demonstrate limitations. Physiological growth parameters of terrestrial mammals are typically estimated using curvilinear models fit to size‐at‐age data along a time series from conception to senescence. The difficulty of collecting and aging prenatal cetaceans is addressed here, and growth parameters of common bottlenose dolphins (Tursiops truncatus) along coastal Texas were estimated using length‐at‐age information from a broader scope of age classes, including late‐term fetuses. A Gompertz growth curve fit to pre‐ and postnatal data underestimated size parameters, but demonstrated similar growth rate constants (k) to an exclusively postnatal model. However, when growth parameters were broken out, the absolute growth rate (G) and rate of growth decay (g) decreased (0.44 from 0.27 and 0.55 from 0.39, respectively), which underscores the importance of reporting k in its expanded form (G/g). Although the Gompertz fits most age classes well, it cannot explain growth in all age classes. We argue that a novel sigmoidal model would be more useful for inference.     

Variability and seasonality of the growth of some corticolous pleurocarpous mosses

Abstract

Field measurements of the elongation of shoots of Hypnum cupressiforme and lsothecium myosuroides reveal considerable variation. Differences in growth rates in the following situations were found to be significant: at different heights on a trunk, on different sides of sloping trunks, on different species of tree and, over a period, on different trees of a single species at a site.

Monthly growth measurements of H. cupressiforme and Platygyrium repens in Wytham Wood, Berkshire, show that H. cupressiforme grows faster, but that the growth patterns of these species are similar. Most of the annual growth for 1970 and 1971 took place during the autumn and winter. Growth between April and August was very slight. The seasonality of the growth of four pleurocarpous mosses at Wychwood, Oxfordshire, was very similar to the species measured at Wytham. Comparison of seasonal growth records with climatic records for stations near Oxford shows a close correlation Qetween growth and wetness. Monthly rainfall minus potential evapotranspiration (Rf–Et) is particularly closely related to monthly shoot growth.

At Roborough, North Devon, growth of H. cupressiforme was measured every 3 months. Considerable growth occurred during autumn and winter. At this site the summers are wetter and this appears to permit an extension of the growing period, considerable growth occurring between May and October.

In 1970 and 1971 summer growth at several localities was compared. Summer growth was again closely related to Rf–Et. The summer growth in wetregions does not seem to be accompanied by a proportionally high winter growth rate. The wetter the climate, the earlier in the year the peak period of bryophyte growth occurs and the more closely the annual pattern of growth correlates with temperature and day length.     

Ontogeny and the evolution of adult body size dimorphism in apes

This analysis investigates the ontogeny of body size dimorphism in apes. The processes that lead to adult body size dimorphism are illustrated and described. Potential covariation between ontogenetic processes and socioecological variables is evaluated. Mixed-longitudinal growth data from 395 captive individuals (representing Hylobates lar [gibbon], Hylobates syndactylus [siamang], Pongo pygmaeus [orangutan], Gorilla gorilla [gorilla], Pan paniscus [pygmy chimpanzee], and Pan troglodytes [“common” chimpanzee]) form the basis of this study. Results illustrate heterogeneity in the growth processes that produce ape dimorphism. Hylobatids show no sexual differentiation in body weight growth. Adult body size dimorphism in Pongo can be largely attributed to indeterminate male growth. Dimorphism in African apes is produced by two different ontogenetic processes. Both pygmy chimpanzees (Pan paniscus) and gorillas (Gorilla gorilla) become dimorphic primarily through bimaturism (sex differences in duration of growth). In contrast, sex differences in rate of growth account for the majority of dimorphism in common chimpanzees (Pan troglodytes). Diversity in the ontogenetic pathways that produce adult body size dimorphism may be related to multiple evolutionary causes of dimorphism. The lack of sex differences in hylobatid growth is consistent with a monogamous social organization. Adult dimorphism in Pongo can be attributed to sexual selection for indeterminate male growth. Interpretation of dimorphism in African apes is complicated because factors that influence female ontogeny have a substantial effect on the resultant adult dimorphism. Sexual selection for prolonged male growth in gorillas may also increase bimaturism relative to common chimpanzees. Variation in female growth is hypothesized to covary with foraging adaptations and with differences in female competition that result from these foraging adaptations. Variation in male growth probably corresponds to variation in level of sexual selection. © 1995 Wiley-Liss, Inc.     

Bistability of the <Emphasis Type="Italic">lac</Emphasis> Operon During Growth of <Emphasis Type="Italic">Escherichia coli</Emphasis> on Lactose and Lactose + Glucose

The lac operon of Escherichia coli can exhibit bistability. Early studies showed that bistability occurs during growth on TMG/succinate and lactose + glucose, but not during growth on lactose. More recently, studies with lacGFP-transfected cells show bistability during growth on TMG/succinate, but not during growth on lactose and lactose + glucose. In the literature, these results are invariably attributed to variations in the destabilizing effect of the positive feedback generated by induction. Specifically, during growth on TMG/succinate, lac induction generates strong positive feedback because the permease stimulates the accumulation of intracellular TMG, which in turn, promotes the synthesis of even more permease. This positive feedback is attenuated during growth on lactose because hydrolysis of intracellular lactose by β-galactosidase suppresses the stimulatory effect of the permease. It is attenuated even more during growth on lactose + glucose because glucose inhibits the uptake of lactose. But it is clear that the stabilizing effect of dilution also changes dramatically as a function of the medium composition. For instance, during growth on TMG/succinate, the dilution rate of lac permease is proportional to its activity, e, because the specific growth rate is independent of e (it is completely determined by the concentration of succinate). However, during growth on lactose, the dilution rate of the permease is proportional to e ² because the specific growth rate is proportional to the specific lactose uptake rate, which in turn, proportional to e. We show that: (a) This dependence on e ² creates such a strong stabilizing effect that bistability is virtually impossible during growth on lactose, even in the face of the intense positive feedback generated by induction. (b) This stabilizing effect is weakened during growth on lactose + glucose because the specific growth rate on glucose is independent of e, so that the dilution rate once again contains a term that is proportional to e. These results imply that the lac operon is much more prone to bistability if the medium contains carbon sources that cannot be metabolized by the lac enzymes, e.g., succinate during growth on TMG/succinate and glucose during growth on lactose + glucose. We discuss the experimental data in the light of these results.