Is Aging an Inevitable Characteristic of Organic Life or an Evolutionary Adaptation?

Aging is an evolutionary paradox. Several hypotheses have been proposed to explain it, but none fully explains all the biochemical and ecologic data accumulated over decades of research. We suggest that senescence is a primitive immune strategy which acts to protect an individual's kin from chronic infections. Older organisms are exposed to pathogens for a longer period of time and have a higher likelihood of acquiring infectious diseases. Accordingly, the parasitic load in aged individuals is higher than in younger ones. Given that the probability of pathogen transmission is higher within the kin, the inclusive fitness cost of infection might exceed the benefit of living longer. In this case, programmed lifespan termination might be an evolutionarily stable strategy. Here, we discuss the classical evolutionary hypotheses of aging and compare them with the pathogen control hypothesis, discuss the consistency of these hypotheses with existing empirical data, and present a revised conceptual framework to understand the evolution of aging.

The intersection of stress, sex and immunity in fishes

While sexual dimorphism in immune responses has been documented in other vertebrates, evidence for a similar phenomenon in fish is lacking. Here, we review the relationship between immunity, stress, spawning, and sex hormones in fish to gain a better understanding of sex-based differences in fish immune responses and its consequences for aquaculture. It is well known that there is a strong link between the stress response and immune function in fish. In addition, research to date has demonstrated that sexual dimorphism in the stress response exists in many species; yet, the relationship between the sexual dimorphic stress responses and immune function has rarely been explored together. Aside from stress, spawning is also known to trigger changes in fish immune responses. Estrogens and androgens have been shown to modulate the immune system which could account for differences between the two sexes of fish when spawning; however, evidence regarding the sexual dimorphism of these changes varies between fishes and is likely related to the spawning strategy employed by a given species. Sex hormones are also used in aquaculture practices to produce monosex populations, and exposure to these hormones early in development has been shown to impact the development of immune organs in several fishes. While female fish are generally thought to be more robust than males, aquaculture practices should also consider the role that maternal stress has on the immune function of the offspring and what role this plays in compromising the immune response of farmed fish.

Hormones and fish monosex farming: A spotlight on immunity

Aquaculture is a promising and developing industry worldwide. One of the first step in monosex culturing, particularly in Nile tilapia, is the production of all-male fry; hormones are widely used in this respect. It is known that exogenous treatment with hormones disrupts various systems in the body including the immune and endocrine systems. There has been a growing interest in how hormones shape the biology of the fish. Many researchers all over the world explored how androgen can interact with many of the body systems; however, rarely any of them tried to improve the hormonal method or to find an alternative. The gate is open for research in this field. This review focusses on the potential effects of hormones, particularly androgens on fish immunity, and the up to date solutions (however, they are rare).

Reproductive and health assessment of fathead minnows (Pimephales promelas) inhabiting a pond containing oil sands process-affected water

Previous laboratory based studies have shown that oil sands process-affected waters (OSPWs) containing high concentrations of naphthenic acids (>25 mg/l) have adverse effects on the reproductive physiology of fish. The purpose of this study was to assess the reproductive development and health of a wild population of fathead minnows (Pimephales promelas) inhabiting an OSPW pond that has moderate concentrations of naphthenic acids (~10 mg/l). Fathead minnows were collected at various times during the period of 2006 through 2008 from Demonstration Pond (OSPW) located at Syncrude Canada Ltd., and two reference sites, Beaver Creek reservoir and Poplar Creek reservoir, which are all north of Fort McMurray, AB, Canada. Condition factor, gill histopathology, gonadosomatic indices, liver somatic indices, male secondary sexual characteristics, and plasma sex steroids were examined. Depending on the time of year that fathead minnows were collected, there were differences in the condition factor, gonadosomatic indices, liver somatic indices, and secondary sexual characteristics of fathead minnows (in males) from Demonstration Pond when compared to the fathead minnows from the reference sites. In comparison to reference fish, lower concentrations of 11-ketotestosterone were measured in the plasma of male fathead minnows collected from Demonstration Pond in June 2006 and July 2007. Black spot disease and Ligula intestinalis were prevalent in fathead minnows from the reference sites but were not observed in fathead minnows from Demonstration Pond. The opercula of fathead minnows from Demonstration Pond also differed from those of reference fish. An examination of the gills of fathead minnows from Demonstration Pond revealed that were a number of proliferative and degenerative alterations relative to reference fish. Even though the fathead minnow population has been maintained in this OSPW pond since 1993, the results of this study demonstrate that the OSPW continues to affect the reproductive development and health of the fathead minnows compared to fish collected at reference sites.

Factors influencing the structure, function and biota of the salmonid epidermis

The present contribution shows the extent and the limitations of our knowledge of the structure, functions and biota of the epidermis of just one family of teleost fish, the Salmonidae. Even within this group it is clear that the epidermis can respond, with remarkable rapidity in some cases, to changes in the environment and to internal stimuli, presumably mediated via the fish's endocrine system. Against this background of constantly changing structure it is believed that the emphasis of the multiple functions of the epidermis must also be changing. Thus, future studies on the skin offish must take into account the possibility of such variations and relate the biology of the skin and its inhabitants to broader aspects of the biology of the fish.

Effects of sex steroids, sex, and sexual maturity on cortisol production: an in vitro comparison of chinook salmon and rainbow trout interrenals

Sex steroids appear to be responsible for hyperactivation of the hypothalamus-pituitary-interrenal (HPI) axis that occurs in mature semelparous Pacific salmon as a prelude to post-spawning (programmed) death. This study was undertaken to examine the direct effects of sex steroids on interrenal activity of semelparous (chinook salmon) and iteroparous (rainbow trout) salmonids using an in vitro incubation system. In addition, phenotypic sex differences in cortisol production by interrenals of sexually mature (spawning) rainbow trout and chinook salmon were investigated. Interrenal tissue from juvenile and sexually mature chinook salmon and rainbow trout was incubated for 48 h in culture medium containing either no steroid (controls), 1 microM estradiol (E2) or 1 microM 11-ketotestosterone (11-KT). This tissue was then challenged for 3h with either pregnenolone, dibutyryladenosine 3('):5(')-cyclic monophosphate (dbcAMP) or forskolin, or synthetic human adrenocorticotropic hormone (ACTH(1-24)). Sex differences in in vitro interrenal cortisol production were assessed using separate tissue pools challenged with the same agents. Cortisol in media was measured by radioimmunoassay. E2 suppressed the ability of juvenile chinook salmon interrenals to utilize pregnenolone as substrate for cortisol synthesis. In mature female chinook salmon the suppressive effect of E2 was less pronounced, but was observed as a reduced response of interrenals to both pregnenolone and dbcAMP. E2 did not affect ACTH(1-24) stimulated cortisol production. Immature and mature rainbow trout interrenals were both relatively insensitive to E2. 11-KT did not affect cortisol production by juvenile chinook salmon and juvenile or mature rainbow trout, and had only minor effects in male and female spawning chinook salmon. In mature chinook salmon and rainbow trout, the interrenals of females were more responsive to ACTH stimulation and showed a greater utilization of pregnenolone as a substrate than interrenals of males. Mature female rainbow trout were also more responsive to dbcAMP stimulation than males. The results of this study suggest that the onset of sexual maturation and gonadal steroid production may contribute to sexually dimorphic cortisol responses in vitro.

Cutaneous biology and diseases of fish

The normal structure and function of the piscine integument reflects the adaptation of the organism to the physical, chemical, and biological properties of the aquatic environment, and the natural history of the organism. Because of the intimate contact of fish with the environment, cutaneous disease is relatively more common in fish than in terrestrial vertebrates and is one of the primary disease conditions presented to the aquatic animal practitioner. However, cutaneous lesions are generally nonspecific and may be indicative of disease that is restricted to the integument or a manifestation of systemic disease. Regardless, a gross and microscopic examination of the integument is simple to perform, but is highly diagnostic and should always be included in the routine diagnostic effort of the aquatic animal practitioner, especially since various ancillary diagnostic procedures are either not practical or lack predictive value in fish. The purpose of this article is to provide an overview of normal cutaneous biology prior to consideration of specific cutaneous diseases in fish.

Branchial Na(+)K(+)ATPase activity in brook charr (Salvelinus fontinalis): effect of gonadal development in hypo- and hyperosmotic environments

Changes in gill Na(+)K(+)ATPase activity were examined following the transfer of brook charr (Salvelinus fontinalis) from fresh water (FW) to seawater (SW). Gonadal development was altered at the hatching stage using three doses of ionizing radiation (IR): 6.2, 7.8, and 11.4 Gray (Gy). A non-irradiated control group was also included in the experimental set-up. Following 15 and 19 months of growth in FW, assessment of gill activity in regard to gonadal status (sterile vs. mature) and level of IR exposure was realized by conducting two estuarine challenge tests. A first introduction was performed during June (period of highest osmoregulatory capacities for this species) (summer experiment). A second introduction was conducted during October (period of diminished osmoregulatory capacities) (fall experiment). Gill Na(+)K(+)ATPase activity and water content were measured at different times and two FW control samplings were added in October and January. In the summer experiment (June-December), normal gonadal development of female brook charr was related to reduced gill Na(+)K(+)ATPase activity during the spawning period as compared to sterile fish (4.0+/-1.5 and 7.2+/-1.9 micromole Pi. mg protein(-1). hr(-1)) (P<0.0002). Similar results were not observed in FW conditions, implying that a lack of gonadal growth does not initiate a significant advantage when the osmoregulatory system including the gills are not highly in demand, i.e. in a FW environment. Ionizing radiation exposure of < or =11.4 Gy at the hatching stage had no significant negative or positive effect on Na(+)K(+)ATPase activity either in FW or SW conditions.

11-Ketotestosterone stimulates growth of heart and red muscle in rainbow trout

The relative ventricular mass (RVM; 100 × ventricle mass/body mass) doubles in male rainbow trout (Oncorhynchus mykiss) as they mature and enter the breeding season, while the RVM of mature females remains unchanged. To examine if this increase in RVM was caused by elevated levels of 11-ketotestosterone (11-KT) in the males, we implanted immature trout of both sexes with Silastic pellets containing either 11-KT or testosterone. Control fish received blank implants. Implantation with 11-KT resulted in high plasma levels of 11-KT that were sustained for the duration of the experiment and were similar to those of maturing male trout. Increases in RVM in male and female fish treated with 11-KT and testosterone were comparable to increases seen in naturally maturing males. Furthermore, the cross-sectional area of the lateral red muscle increased in androgen-treated fish. These results suggest that heart growth of maturing males is stimulated by high plasma titres of 11-KT and testosterone just prior to and during the breeding season. However, the increase in RVM in males but not in females must be due to the interaction of high levels of 11-KT with other, unknown factors. The increase in RVM and red muscle area of the steroid-treated fish suggests that one role of androgens is to increase the swimming endurance of maturing males.

Influence of dietary lipid and carbohydrate levels and chronic 3,5,3'-triiodo-L-thyronine treatment on thyroid function in immature rainbow trout, Oncorhynchus mykiss

The influence of varying dietary levels of nonprotein energy sources (lipid, L; carbohydrate, C) and 3,5,3'-triiodo-L-thyronine (T3) on thyroid function in immature rainbow trout was studied. Three diets of equivalent available energy content and identical nutrient composition, except for dissimilar concentrations of L and C (diet 1, L = 7%, C = 28.3%; diet 2, L = 13%, C = 14.9%; diet 3, L = 19%, C = 1.5%), were each supplemented with 0, 4, 8, or 12 ppm T3 and fed to satiation to trout at 6.5 +/- 0.5 degrees on a 12-hr photoperiod for 12 weeks. Dietary L and C concentrations did not influence plasma total L-thyroxine (T4) or T3 levels, indices of free T4 or free T3 levels, hepatic T4 5'-monodeiodinase (5'D) activity, capacity or affinity of hepatic nuclear T3 receptors, or thyroid follicle epithelial cell height. T3 treatment elevated total and free T3 levels and decreased 5'D activity (Vmax) in approximate proportion to T3 dose, and without effect on plasma total or free T4 levels or T3 receptor properties. However, thyroid follicle epithelial cell height was depressed at 8 or 12 ppm dietary T3. In trout reverted for 20 days to a T3-free diet from a T3 (12 ppm) diet, plasma total T3 levels fell to 30% of those of control trout (0 ppm T3 throughout). It was concluded that, under our experimental conditions, (i) trout thyroid function was refractory to dietary concentrations of L and C, (ii) the primary response to T3 supplementation was suppressed hepatic 5'D level and T3 production, which was sustained for at least 20 days after T3 treatment ceased, and (iii) despite causing a probable indirect decrease in thyroidal secretion, T3 did not modify the set point of the hypothalamo-hypophyseal-thyroid axis based on plasma total or free T4 levels.

Action of cortisol on the proliferation of rainbow trout fibroblasts

The effect of cortisol on the proliferation of the rainbow trout fibroblast cell line, RTG-2, was examined in synchronous and asynchronous cultures. When the transition from G1 to S was synchronized by restoring serum to serum-deprived cultures, the addition of cortisol at the time of serum restoration delayed the entry of cells into S phase. However, if cortisol was added 24 h after serum restoration, at the G1/S transition point, the subsequent peak of DNA synthesis was unaffected. In asynchronous cultures cortisol inhibited [3H]-thymidine and [3H]-uridine but not [3H]-leucine incorporation into acid-insoluble material. If the exogenous nucleoside concentration was raised, [3H]-thymidine but not [3H]-uridine incorporation continued to be inhibited by cortisol. This suggested that cortisol's effect on [3H]-thymidine incorporation reflected a change in entry into S phase and not just on thymidine uptake and metabolism. Cortisol inhibited the proliferation of RTG-2 in asynchronous cultures. At 1000 ng/ml of cortisol a reduction in cell number became apparent before the RTG-2 cultures were confluent, whereas at 100 ng/ml the reduction only became evident in confluent cultures. The synthetic antiglucocorticoid, RU 486, which acts at the level of the corticosteroid receptor, blocked the growth inhibition by cortisol. These results suggest that cortisol regulates rainbow trout fibroblast proliferation via the corticosteroid receptor and that the G1/S transition is one point at which this regulation occurs.

Seasonal variation in specific plasma- and target-tissue binding of androgens, relative to plasma steroid levels, in the brown trout, Salmo trutta L

The circulating levels of the plasma androgens, testosterone and 11-ketotestosterone, and their specific binding to skin cytosol, skin nuclear extract, and plasma were determined in mature male and immature male and female brown trout during a single spawning cycle. 11-Ketotestosterone was not bound by any of the fractions examined whereas testosterone was bound with high affinity to plasma (kD = 32.6 nM), skin cytosol (kD = 16.9 nM), and skin nuclear extract (kD = 2.6 nM). The binding capacity of each fraction varied independently with time. In mature male fish an increase in specific binding of testosterone to nuclear extract, from 77 to 269 fmol mg-1 protein, occurred between September and November, coincident with peak androgen levels. Following the spawning period and the decline in androgen levels, nuclear-binding capacity in mature fish dropped to a level similar to that of immature fish by June. Nuclear binding in immature fish remained in the range 25-75 fmol mg-1 protein throughout. Plasma-binding capacity of both mature and immature fish declined during the spawning period, from 190 to 125 nM in mature fish and from 360 to 125 nM in immature fish. Plasma-binding capacity in both mature and immature fish increased following spawning to reach levels of 340 nM (mature) and 250 nM (immature). Little change was observed in cytosol-binding capacity of either mature or immature fish. The results suggest that androgen-induced structural changes in the integument are predominantly testosterone stimulated, are initiated by an increase in the concentration of a specific testosterone-binding protein within the nucleus, may be potentiated by a drop in plasma testosterone-binding capacity, and that a cytosol-binding protein of intermediate affinity for testosterone may maintain a high intracellular concentration of steroid.

Androgen binding in the skin of mature male brown trout, Salmo trutta L

Specific, saturable binding of [3H]testosterone (Kd = 12.5 nM, Nmax = 1.1 pmol mg-1 protein) has been identified in skin cytosol of mature male brown trout. Binding with this affinity and capacity resembles more closely that observed in the plasma of mature male brown trout (Kd = 20.6 nM, Nmax = 6.6 pmol mg-1 protein) than that expected of a specific steroid receptor. However, the fraction of skin cytosol precipitating with 30% ammonium sulphate has a higher affinity for testosterone (Kd = 1.9 nM) and a lower capacity (Nmax = 96.3 fmol mg-1 protein) than does whole cytosol. Furthermore, the high-salt extract of crude skin nuclear pellet binds [3H]testosterone with affinity (Kd = 1.3 nM) and capacity (Nmax = 69.4 fmol mg-1 protein) similar to those of ammonium sulphate-precipitated cytosol. Specific binding in all three fractions of skin is abolished by treatment with proteolytic enzymes and a component of both cytosol and nuclear extract, which binds [3H]testosterone specifically, is retained on DNA-cellulose columns, eluting with high-salt buffer. Specifically bound [3H]testosterone is displaced most efficiently by testosterone. 5 alpha-dihydrotestosterone, estradiol-17 beta, and 11-ketotestosterone also compete but are 5- to 10-fold less potent. Consistent and reproducible binding of [3H]11-ketotestosterone to skin cytosol fractions or nuclear extract could not be demonstrated. It is concluded that testosterone binding with the above characteristics partially fulfills the criteria required of a putative steroid hormone receptor. The results are discussed with reference to the relative roles of testosterone and 11-ketotestosterone during sexual maturation.

The effects of 11-ketotestosterone and testosterone on the skin structure of brown trout, Salmo trutta L

11-Ketotestosterone and testosterone were administered by intraperitoneal implants to immature brown trout. Physiological levels of circulating androgens were maintained over a period of 4 months and this resulted in changes in skin structure which resembled those occurring in maturing male brown trout. 11-Ketotestosterone promoted both epidermal and dermal thickening, and markedly reduced the number of superficial goblet cells, whereas testosterone stimulated only epidermal thickening. The relative roles of these two androgens in the normal maturational processes of brown trout are discussed.

Epidermal cellular proliferation and differentiation in sexually mature male Salmo salar with androgen levels depressed by oil

Sexually mature male Salmo salar exhibit epidermal thickening and an increase in goblet cell concentration during the spawning season. The ventral skin, which is likely to experience most abrasive contact during the spawning period, has the thickest epidermis and the greatest goblet cell concentration. Following exposure to crude oil there is inhibition of cellular proliferation and elongation associated with epidermal thickening, and also inhibition of mucigenesis. Data on the androgen levels in these fish, and data from earlier studies involving treatment with hormones, indicate that oil-related epidermal effects during the spawning period are most likely systemic in origin, probably arising from reduced plasmatic androgen levels.

Effect of 17 alpha-methyltestosterone on the skin and gonads of freshwater male silver eels

Male silver eels kept in fresh water (FW) were injected with 17 alpha-methyltestosterone (MT). They received 5, 10, 15, or 20 injections (8 micrograms/g every other day) or 6 or 9 injections (4 or 2.5 micrograms/g once a week) and were killed 24 hr after the last injection. Compared to solvent-injected controls, the treated eels appeared darker, the eye diameter was enlarged, the head and the pectoral and caudal fins were black and the melanophore index was maximal. The thickness of the skin (epidermis plus dermis) was significantly increased. Macroscopically, the organs of Syrski (juvenile testes) were slightly larger and less translucent, but the maximal gonosomatic index reached only 1-1.2 in two fishes. Stimulation started in the anterior part of the gonad. Microscopically, all stages of spermatogenesis and spermiogenesis were observed. After 20 injections (high dose, 40 days) or 6 (one case only) and 9 injections (low dose, 57 days), spermatids and spermatozoa were present. These positive results are compared to the negative data previously reported in other countries. The mode of action of the androgen is discussed. It is suggested that it acts for the most part directly on the gonad, but a participation of the pituitary cannot be ruled out, although gonadotrophs appeared only slightly stimulated.

Influence of sex steroids on the catecholamine biosynthesis in chromaffin tissue from rainbow trout, Salmo gairdneri

The in vitro activities of dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyl transferase (PNMT) were determined in the chromaffin tissue (head kidney) of sex steroid-treated rainbow trout, Salmo gairdneri. In addition, the DBH and PNMT activities were measured in fish at different sexual states. Steroid treatment increased the wet weight of the head kidney, while both DBH and PNMT activities decreased. Although no sex difference in the enzyme activities was detected, a possible correlation between DBH and PNMT activities and increased GSI values was indicated. The presented data suggest that changes in sex steroids during development may affect the catecholamine synthesis in the chromaffin tissue in S. gairdneri.

Production of coronary arteriosclerosis with sex hormones and human chorionic gonadotropin (HCG) in juvenile steelhead and rainbow trout, Salmon gairdneri

Coronary arteries were examined for significant changes in myointimal hyperplasia in one-year old juvenile steelhead or rainbow trout treated with estradiol, testosterone or human chorionic gonadotropin (HCG). In one experiment juvenile steelhead trout were injected weekly with 2 microgram/g HCG, 5 microgram/g estradiol cypionate or 5 microgram/g testosterone cypionate for 50 days. Control fish were those killed prior to treatment or injected with fish saline or cottonseed oil. In two experiments juvenile nonanadromous rainbow trout were treated with estrogen and testosterone with the same weekly doses as juvenile steelhead for 42 and 30 days, respectively. In the first experiment juvenile rainbow were injected weekly with 2 microgram/g HCG. Juvenile rainbow treated with estradiol, testosterone and HCG and juvenile steelhead treated with testosterone had larger coronary lesions than untreated or control fish. Coronary hyperplastic nodules resembled morphologically those observed previously in sexually mature steelhead and rainbow trout. These data support the hypothesis that reproductive hormones are causal factors in spontaneously occurring arteriosclerosis in spawning Salmo gairdneri.

Dose response kinetics of serum vitellogenin, liver DNA, RNA, protein and lipid after induction by estradiol-17 beta in male flounders (Platichthys flesus L.)

1. Male flounders receiving 100 micrograms estradiol each second day were fully induced to vitellogenin synthesis within 11 days, while fishes given 5 micrograms doses continued to accumulate vitellogenin in the serum at a progressive rate through 17 days. 2. Liver DNA per unit fish remained constant, while RNA per unit fish in flounders given 100 and 5 micrograms doses attained values 80 and 25% respectively, above the values found in control animals. 3. Liver RNA per unit DNA increased at maximal rate within 6 days in fishes receiving 100 micrograms doses. RNA synthesis continued at a progressive rate through 17 days in fishes given 5 micrograms doses of estradiol. 4. Liver protein per unit DNA elevated at a plateau 60% above control within 6 days with 100 micrograms doses. Doses of 5 micrograms had only little effect on liver protein. 5. Estradiol had a lipogenic effect on the liver. Cellular lipid rose 120 and 60% above control after treatment with 100 and 5 micrograms respectively. 6. Liver dry weight per unit DNA increased 60 and 55% above control with 100 and 5 micrograms doses respectively. Cellular hypertrophy in fishes receiving the smaller dose was primarily associated with an increase in lipid concentration, while protein and lipid contributed almost equally to cellular growth in fishes receiving the high dose.

Coronary degeneration in sexually mature rainbow and steelhead trout, Salmo gairdneri

Correlation of coronary arteriosclerosis with sexual maturation was found in rainbow and steelhead trout. All sexually mature nonanadromous rainbow, precocial male steelhead, and adult spawning steelhead had myointimal hyperplasia but it was infrequent in immature juveniles. Lesion severity in adult rainbow and precocial steelhead approach that of spawning steelhead.