„Progrese şi Perspective în Medicina Veterinară” - Lucrări ştiinţifice vol. 50 161

Sex hormones and environment involved in sex determination and growth in eels, Anguilla Anguilla - basic and applied aspects

DEGANI G. MIGAL Galilee Technology Center - Israel

In animal growth, many biological processes are involved, including internal and external parameters, some apparently more important than others. Over the twenty years, during which the growth of the European eel, Anguilla Anguilla, has been researched, many aspects influencing eel growth have been documented. In the present paper, we propose a model, which describes the relationship between growth, sex hormones, environment and sex determination. Much of the model’s supporting data has been collected in our laboratory.

Key Words: sex hormones, Anguilla

INTRODUCTION

Increased pollution and over-fishing of rivers in Europe are bringing about a decrease in the natural eel population, and an increase in the value of the European eel (Anguilla anguilla). The European eel is a catadromic teleost species with a complex life cycle, which takes place both in sea and freshwater environments (Tesch 1977). Significant differences between growth rate and size have been observed between the two sexes, with females growing faster and larger than males (Degani et al., 2003b). The environment has been found to affect the growth and sex determination in eels; females continue to grow, while the males cease. Some males are between 15-200 g. Many theoretical and applied aspects of the European eel have been investigated (Degani and Gallagher 1995). In this paper, a model is presented, which suggests that the relationship between the environment, hormones and sex determination affects growth. The relevance of such a model, not only in providing basic information, but also with respect to aquaculture, is described.

RESULTS and DISCUSSION – THE MODEL

A high variation in growth was discovered in European eels grown under aquacultural conditions (Degani and Levanon 1983). Under natural conditions, the sex ratio in silver eels varies considerably between localities, ranging between 0 to100% females (Tesch 1977). Under aquaculture conditions, starting from the glass eel stage, the majority of eels will develop male gonads (Tzchori et al., 2004a), which possibly results from the high density in which eels are reared in captivity. Indeed, it has been shown that low densities of captive eels favor female differentiation, while high densities support male differentiation, implicating that social factors play an important role in sex determination (Degani and Kushnirov 1992). These results support the environmental effect on sex determination in the model (Fig.1 - A to C). It was postulated that sex determination in the eel, as in other fish, is regulated by the ratio of androgen to estrogen levels at the critical time of sex determination, and steroid intervention at this period alters this balance. About 95% feminization was obtained by

oral treatments with estrogen (Colombo and Grandi 1990). These results, confirmed by Degani and Kushnirov (1992) and Tzchori et al. (2004a) in our laboratory, indicate that the density of the

162 UNIVERSITATEA DE ŞTIINŢE AGRICOLE ŞI MEDICINĂ VETERINARĂ IAŞI

eels, during their growth, affects their steroid production. High density favors androgen secretion and low density encourages estrogen secretion, as suggested in the model (Fig.1 - A to C). However, more direct studies are necessary to support this hypothesis.

The steroid, 17β – estradiol (E2), which is produced by the female gonad (Fig.1- F), and which

controls vitellogenesis, augments the growth rate (Degani 1986) (Fig.1 - G) and affects oogenesis (Fig.1- H). When E2 was provided in the diet, there was an increase in the growth rate of the eels. Aromatase (P450) is the key enzyme that transforms testosterone into estradiol in vertebrates. The cloned complementary deoxyribonucleic acid (cDNA) of P450 from the European eel contains an open reading frame of 1539 bp, encoding a deduced protein of 513 residues (Tzchori et al., 2004b). Its mRNA expression during sex determination was significantly higher in females than in males. All of these results support the model (Fig.1- A to C). Treatment of the eels with a P450 inhibitor reduced the female percentage during sex determination. These data support the E2 contribution to growth, which is significantly higher in female than in male European eels. Follicle stimulation hormone (FSH), which controls vitellogenesis by induction of E2 secretion, seems to

have an indirect effect on growth, as described in the model (Fig.1- L, M, N, H). The FSH β subunit cDNA, which consists of 1068 bp, encodes a 108 amino acid peptide (Degani et al., 2003a). The expression of FSH and luteinizing hormone (LH) mRNA were examined in males and females after sex determination, and the mRNA levels were found to be significantly higher in females than in males. In conclusion, these results suggest that FSH secretion from the pituitary has an effect on E2 secretion, which in turn has an effect on the increase in the growth rate, as presented in the model. In the male, 11-ketotestosterone (11KT) is the main androgen, and apparently this hormone does not affect growth, but rather influences spermatogenesis (Sbaihi et al., 2001) (Fig.1.C)

Growth hormone (GH) is secreted from the pituitary of the eel, as supported by a study of

Degani et al. (2003b), in which the GH gene was cloned and its transcription level determined in males and females (Fig. 1- D), as well as its influence on eel growth demonstrated (Degani and Gallagher 1985). According to the mRNA expression of the cloned and sequenced cDNA of the European eel GH, females exhibited a significantly higher GH transcription than males. Insulin administration to the eel diet significantly increased the growth rate of eels (Degani and Abraham 1992) (Fig.1 - K). These results support the pathway which is described in the model (Fig.1 - I to K) that GH is affected directly by the insulin like growth factor (IGF), although more detailed research is

required. Montero et al. (1998) have determined the localization of cyclase-activating polypeptide (PACAP)-immunoreactive neurons in the central nervous system of the European eel, using an antiserum raised against PACAP27. The data of this study revealed that pituitary PACAP stimulates GH secretion from cultured eel pituitary cells (Montero et al., 1998) (Fig.1- I). The gonadotropin-releasing hormone (GnRH), did not alter GH release in the European eel (Rousseau et al., 1999).

In conclusion, the proposed model, regarding the relationship between the complex

phenomena of eel growth and the hormones involved in its reproduction, may clarify the interaction between the two complex systems (growth and reproduction) and the environment.

„Progrese şi Perspective în Medicina Veterinară” - Lucrări ştiinţifice vol. 50 163

Supported by results from Degani group studies

Supported by results from other studies

PACAP

Hypothalamus

Pituitary

LH FSH

Gonads

E2

Eel Growth

GH Liver IGF

GnRHEnvironmental Sex Determination

Low

densitiesHigh

densities

E2

Females Males

11KTOogenesis

A

B

C

D

E

F

GH

I J

KL

M

N

I

Figure 1. Model proposing hormonal involvement in sex determination, growth, gonad development and

oogenesis in the European eel. 17β – estradiol (E2), follicle stimulation hormone (FSH), luteinizing hormone (LH), growth hormone (GH), cyclase-activating polypeptide (PACAP), gonadotropin-releasing hormone

(GnRH), 11-ketotestosterone (11KT), insulin like growth factor (IGF).

REFERENCES

1. COLOMBO G., G. GRANDI, 1990 - Gonad sex differentiation of Anguilla anguilla by sex steroid; International Reviews in Hydrobiology 75:763-773.

2. DEGANI G., 1986 - Growth and behavior of six species of amphibian larvae in winter pond in Israel; Hydrobiologia; 140:5-10.

3. DEGANI G., M. ABRAHAM, 1992 - Effect of insulin in the diet on the growth of European eels (Anguilla anguilla (L.); Fish Physiol Biochem; 10:223-227

4. DEGANI G., M.L. GALLAGHER, 1985 - Effects of dietary 17-I-methyltestosterone and bovine growth hormone on growth and food conversion of slow and normally growing Amercan elvers (Anguilla rostrata); Can J Fish Aquatic Sci; 42:610-629.

5. DEGANI G., M.L. GALLAGHER. 1995 - Growth and Nutrition of Eels; Israel: Laser Pages Publishing.

6. DEGANI G., D. GOLDBERG, I. TZCHORI, A. HURVITZ, S. YOM DIN, K. JACKSON, 2003a - Cloning of European eel (Anguilla anguilla) FSH-beta subunit, and expression of FSH-beta and LH-beta in males and females after sex determination; Comp Biochem Physiol B Biochem Mol Biol; 136: 283-293.

7. DEGANI G., D. KUSHNIROV, 1992 - Effects of 17a-estradiol and grouping on sex determination of European eels; Prog Fish Cult; 54:88-91.

8. DEGANI G., D. LEVANON, 1983 - The influence of low density on food adaptation, cannibalism, and growth of eels [Anguilla anguilla (L.)]; Isr J Aquacult Bamidgeh; 35: 53-60.

9. DEGANI G., I. TZCHORI, S. YOM-DIN, D. GOLDBERG, K. JACKSON, 2003b - Growth differences and growth hormone expression in male and female European eels [Anguilla anguilla (L.)]; Gen Comp Endocrinol; 134:88-93.

10. MONTERO M., L. YON, K. ROUSSEAU, A. ARIMURA, A. FOURNIER, S. DUFOUR, et al., 1998 - Distribution, characterization, and growth hormone-releasing activity of pituitary adenylate cyclase-activating polypeptide in the European eel, Anguilla anguilla; Endocrinology; 139:4300-4310.

11. ROUSSEAU K., N. LE BELLE, J. MARCHELIDON, S. DUFOUR, 1999 - Evidence that corticotropin-releasing hormone acts as a growth hormone-releasing factor in a primitive teleost, the European eel (Anguilla anguilla); J Neuroendocrinol; 11:385-392.

12. SBAIHI M., M. FOUCHEREAU-PERON, F. MEUNIER, P. ELIE, I. MAYER, E. BURZAWA-GERARD, et al., 2001 - Reproductive biology of the conger eel from the south coast of Brittany, France and comparison with the European eel; Journal of Fish Biology; 59:302-318.

13. TESCH F. 1977 - The Eel Biology and Management of Anguillid Eel; London: Chapman and Hall.

14. TZCHORI I., G. DEGANI, R. ELISHA, R. ELIYAHU, A. HURVITZ, J. VAYA, et al., 2004a - The influence of phytoestrogens and oestradiol-17β on growth and sex determination in the European eel (Anguilla anguilla) Aquaculture Research; 35:1213–1219

15. TZCHORI I., G. DEGANI, A. HURVITZ, B. MOAV, 2004b - Cloning and developmental expression of the cytochrome P450 aromatase gene (CYP19) in the European eel (Anguilla anguilla); Gen Comp Endocrinol; 138:271-280.