The goal of this study was to compare the reproductive physiology of
triploid and diploid European sea bass (Dicentrarchus labrax L.). Gonads of diploid and
triploid fish (males and females) were examined both microscopically and macroscopically, together with the plasma levels of the major sex
steroids produced (
testosterone and
estradiol-17beta) when fish were adults. Prior to sexual maturation, the gonadosomatic index (GSI) of
triploid males was similar to that of diploids. However, the GSI in 4-year-old adult
triploid males was 1.8 times lower than that of diploids (P < 0.05). All diploid males exhibited normal gonadal development. In contrast, in
triploid males spermatogenesis was impaired during late meiosis, affecting severely spermiogenesis. This was achieved by an increasing imbalance in the amount of
DNA present in daughter cells of the same type as spermatogenesis progressed, as demonstrated by abnormal cell sizes, culminating in inviable spermatids. Thus, no spermiating
triploid fish were observed during 4 years, which included three full consecutive maturation cycles. Furthermore, the germ cells from
triploids were significantly larger than those from diploids (P < 0.001). Seasonal profiles of plasma levels of
testosterone in 4-year-old males were essentially similar in both ploidies. On the other hand,
triploid females had rudimentary ovaries containing oogonia and primary oocytes that were arrested during meiotic prophase I, while diploid females exhibited all stages of ovarian development. Diploid females showed levels of
testosterone and
estradiol-17beta significantly higher than those of
triploids (P < 0.05), in which no endocrine signs of maturation were observed at all. Regarding sex ratios,
triploids had 10% more females than diploids (P < 0.05) but in both ploidies males predominated, as is usually found in this species under culture conditions. These results show that
triploidy blocked the initial phases of meiosis in females and the latter ones in males, resulting in the absence of or reduced gonadal development, respectively. In conclusion, we provide an explanation for the lack of gonadal development in
triploid male fish, and, to the best of our knowledge, we report for the first time a case in which induced
triploidy completely blocks meiosis in both sexes, thus conferring functional
sterility in the sea bass.