Gonadal differentiation and hormonal sex reversal in arctic charr (Salvelinus alpinus)

Abstract

The purpose of this study was to develop a practical protocol for the production of female populations of Arctic charr (Salvelinus alpinus). Achieving this required knowledge of the timing of gonadal differentiation. Undifferentiated gonads were observed microscopically to be present by 194 degrees C-days post-hatch and definitive germ cells by 346 degrees C-days post-hatch, where " degrees C-days" denote acquired thermal units calculated as the product of temperature and days. Some of the gonads had developed a lumen by 510 degrees C-days post-hatch, and by 681 degrees C-days post-hatch anatomical divergence into two types of gonads was clear. Two protocols (immersion and feeding) were tested for hormonal sex reversal of genotypic females using the synthetic androgen 17alpha-methyldihydrotestosterone (MDHT). Six-hour MDHT immersions (0.5, 1, 3, 5 and 10 mg/L) were carried out weekly from hatch to first feeding (140 degrees C-days post-hatch), whereas daily feeding treatments (0.5 mg/kg) went from 140 to 600 degrees C-days post-hatch. The sex ratios of all immersion experimental groups were significantly different from the control, with the proportion of presumptive males increasing as MDHT concentration increased. The highest immersion treatment, 10 mg/L, yielded a population of 90% presumptive males and 10% with atypical gonads. However, the most effective treatment, yielding a population of 90% presumptive males and no fish with atypical gonads, was the feeding treatment. Given that female salmonid fishes are homogametic, sex-reversed (masculinized) genotypic females produced in this way can serve as broodstock for the creation of all-female charr populations for aquaculture.

Gonadal differentiation in Atlantic cod, <I>Gadus morhua</I> L., and haddock, <I>Melanogrammus aeglefinus</I> (L.)

“…Serial sections were cut to 5µm and stained with Azocarmine B and Hubbs II (Humason 1972). Cross-sections were compared to photomicrographs from the salmonid literature (Takashima et al 1980, van den Hurk and Slof 1981, Nakamura 1982, Sacobie and Benfey 2005, Chiasson and Benfey 2007 and from the gadoid literature (Morrison 1990, Rommens 1997 to determine when gonadal differentiation occurred. The terminology of Patiño and Takashima (1995) is used to describe the cellular stages of differentiation.…”

Sex differentiation in grayling (Salmonidae) goes through an all-male stage and is delayed in genetic males who instead grow faster

“…Within the Salmoninae, differentiated gonochorism where primordial germ cells develop directly into testis or ovarian tissues has been described in, for example, Arctic charr ( Salvelinus alpinus ) 4 , brook charr ( Salvelinus fontinalis ) 50 , brown trout ( Salmo trutta ) 51 , rainbow trout ( O. mykiss ) 52 , and coho salmon ( O. kisutch) 53 . The European grayling as representative of the Thymallinae instead shows a rare form of undifferentiated gonochorism, since undifferentiated gonochoristic species usually go through an all-female stage before they differentiate into testis and ovaries 1 .…”

“…In the gonochoristic species, primordial germ cells are typically formed during embryo or early larval development and subsequently differentiate into male or female gonads under the influence of genetic mechanisms and/or endocrine, environmental, or behavioural signals 1 . This process can be direct, as in the so-called “differentiated” gonochoristic species 3 where primordial germ cells develop without any detour into testicular or ovarian tissues, for example, in Arctic charr ( Salvelinus alpinus ) 4 and some cyprinid fishes 5, 6 . In “undifferentiated” gonochoristic species, the typical pattern is that individuals first develop ovarian tissues that may subsequently degenerate, followed by a masculinization of the gonads that finally leads to normal testes, as in zebrafish ( Danio rerio ) and some other cyprinids 7, 8 .…”

Controlling preharvest maturity in farmed Arctic char: A review from the Canadian perspective

“…Given that all salmonid fishes have female-homogametic sex determination (equivalent to the mammalian XX-female/XY-male system), crossing a neomale with a normal female results in all-female offspring (Baroiller & D'Cotta, 2016;Piferrer, 2001). This approach to producing all-female populations is used extensively in the rainbow trout industry (Krisfalusi & Cloud, 1999) and has also been demonstrated to be effective in other salmonid and nonsalmonid species, such as Atlantic salmon, Salmo salar L. (Lee, King, & Pankhurst, 2003), Arctic charr, S. alpinus Linnaeus (M. Chiasson & Benfey, 2007), brook char, Salvelinus fontinalis Mitchill (Galbreath, Adams, & Sherrill, 2003), and Atlantic halibut, Hippoglossus hippoglossus L. (Lin, Benfey, & Martin-Robichaud, 2012). The key steps for optimizing the production of all-female populations are (a) determining the developmental stage at which gonadal differentiation occurs and is therefore malleable, (b) optimizing dosage and duration of treatment with masculinizing agents, and (c) distinguishing neomales from normal males for breeding purposes.…”

“…(Fatima et al, 2016;Galbreath et al, 2003;Krisfalusi & Cloud, 1999;Lee et al, 2003), little information is available for Salvelinus spp. (M. Chiasson & Benfey, 2007;Gillet, Vauchez, & Haffray, 2001). In the only published study conducted on the timing of gonadal differentiation and the production of neomales in Arctic char, M. Chiasson and Benfey (2007) noted anatomical differentiation into two distinct morphologies at 681 C-dph (degree celsius days post hatch), followed by cytological differentiation and the appearance of primary oocytes in some gonads by 839 C-dph.…”