Graham C Mair

Monosex male production in finfish as exemplified by tilapia: applications, problems, and prospects

Abstract The use of monosex fish is intrinsically desirable in a variety of fish species in a range of aquaculture production systems. The potential advantages sought from their use may include one or more of the following features: achievement of higher average growth rate, elimination of reproduction, reduction of sexual/territorial behaviour, reduction of variation in harvest size, and reduction of risk of environmental impact resulting from escapes of exotic species. Fish as a group have systems of sex determination which are of considerable biological interest and significance for studies in evolutionary biology. However, they are very variable, relatively poorly understood and give rise to much variation in sex ratio between, and within, species. Enough is known, however, to enable us to say that these systems are often employed in ways which sharply distinguish the fishes from groups such as mammals, birds and reptiles. As a consequence, manipulations of sexual phenotype designed to produce monosex populations are not straightforward and the results are not necessarily predictable. This paper reviews the techniques for production of monosex males, and considers in detail the case of the YY/GMT technology in the Nile tilapia, which is the only example of a genetic technology for the production of monosex males so far widely adopted by the aquaculture industry. The considerable benefits accruing from the use of GMT are described. An attempt at projecting future developments in this area of aquaculture is made.

Environmental sex determination: the effect of temperature and salinity on sex ratio in Oreochromis niloticus L

Abstract This paper reports the effects of environmental conditions during the period of sex differentiation on the sex ratio of the Nile tilapia (Oreochromis niloticus). Different sex genotypes were exposed to varying temperatures (putative all-female, all-male and all-YY males) and salinities (putative all-female progeny only) for a minimum period of 21 days after first feeding and were on grown prior to sexing by gonad squash. The majority of the putative all-female progeny exposed to high temperature (36.54±0.39°C) produced significantly higher percentages of males compared to controls reared at ambient temperature (27.87±1.40°C). Similarly, at high temperature, some of the all-male and YY male progenies had significantly lower percentage of males compared to controls. Sex differentiation in YY males appears to be more labile than in normal XY males although this could possibly be attributable to different levels of inbreeding. Low temperature (25.78±0.24°C) and varying levels of salinity (11.30 to 26.65 ppt) did not significantly affect sex ratios. The apparent sensitivity of sex differentiation to some environmental factors is considered in the context of a predominantly monofactorial genetic sex determining mechanism. Implications for sex control technologies are discussed.

Chromosome-set manipulation in tilapia — techniques, problems and prospects

Abstract In recent years a great deal of research has been conducted on chromosome-set manipulation (polyploidy and gynogenesis) in a number of tilapia species. The rationale for this work has been to produce sterile progeny for culture; for sex manipulation; and to produce highly inbred or cloned lines for crossbreeding and for research purposes. Haploid, gynogenetic diploid, triploid and tetraploid genotypes have been produced in a number of laboratories and yet this research has been of little or no discernible benefit to the world-wide tilapia culture industry. Techniques for induction and verification of chromosome-set manipulation are reviewed and the potential applications of these are critically discussed.

Influence of sexual phenotype and genotype, and sex ratio on growth performances in tilapia, Oreochromis niloticus

In tilapia, precocious sexual maturity associated with continuous and asynchronous reproductive activity often results in unwanted reproduction and overcrowding in ponds where the food supply is limited. This constitutes a major limitation in the culture of most tilapia species. Furthermore, divergent growth performances linked to sex are regularly observed. Males present better growth performances than females. The first experiment of the present study analysed the influence of sexual genotype on phenotypic male growth rates. Males with three different sexual genotypes were produced: YY100 (YY male×YY female), XY100 (YY male×XX female) and XX100 (hormonally sex-inverted females). They were compared with classic (XY male×XX female) or all female (XX male×XX female) progenies. The objectives of the second experiment were to study the influence of social interactions through the use of different sex ratios: F100, F75, F50, F25, F0 (F100=100% females in the population). These two experiments were carried out in hapas implanted in pond with “Egypt-UCS” strain of Oreochromis niloticus. At the end of the first experiment (140 days post-fertilisation), the XY genotype presented the best growth performances and YY genotype the lowest. Final body weights of neomale (XX100) were significantly (P<0.05) higher than those of YY100 males. According to our results, the involvement of a genetic component in sex-related growth difference between males and females is strongly suspected. We stressed upon the involvement of genes bound to the sexual genotype (especially XY in comparison to YY and XX) independently of the sexual phenotype. Furthermore, sexual maturity is not the only phenomenon involved in the sexual dimorphism. From 102 days post-fertilisation until the end of the second experiment, females of the F25 population presented the best growth performances. Their final body weights were significantly higher than those of all the males except F50 population males. In general, female body weight tended to increase inversely to their proportion in the population. Males of the F50 population presented better growth rate than the other males. Thus, tilapias seems to be very sensitive to the effects of social interactions. This social behaviour could strongly influence growth differences between males and females.

Sex determination and the feasibility of genetically male tilapia production in the Thai-Chitralada strain of Oreochromis niloticus (L.)

Abstract This paper presents the findings from investigations on sex determination and attempts to produce monosex male tilapia through the use of novel YY-males in the Thai-Chitralada strain of Oreochromis niloticus cultured commercially in Thailand. To determine the underlying variability in sex ratio, 95 progeny groups, derived from single pair matings of normal broodstock, were sexed over a period of 2 years. In addition, diallel-type crosses were generated from seven females and five males. Of the 7822 progeny sexed, there were 3951 males (50.5%), with the overall sex ratio being not significantly different from 1:1 (P>0.25). However, progeny sex ratios ranged widely from 15.5% to 100% male. The data were highly heterogeneous (P

An evaluation of triploidy as a potential solution to the problem of precocious sexual maturation in common carp, Cyprinus carpio, in Karnataka, India

Abstract Precocious maturation and unwanted reproduction have been identified as potential constraints on yields of common carp in aquaculture and culture-based fisheries in Karnataka state, southern India. Both males and females can attain sexual maturation well before reaching a marketable size. Even if fish do not spawn and produce fry, the gonadosomatic index (GSI) can exceed 20% of the harvested weight of an individual fish. The use of sterile triploids may provide a solution to this problem. Based on modifications to previously published protocols, triploidy was induced using heat shocks (40 °C for 1.5 min starting 1–3 min post-fertilization). The culture performance of diploid and triploid fish was compared in a series of trials involving separate or communal stocking and monoculture or polyculture (with catla and rohu). Gonad development in triploids was typical of that seen in other species (very much reduced in females, rather less reduced in males, while diploid counterparts were mature). Diploid fish were generally heavier than triploid fish at the end of most of these trials, and female fish were generally heavier than male fish. However, when gutted weight was considered, the significant differences between diploid and triploid fish were removed, although females were still heavier than males in most trials. Thus, although these trials continued for several months beyond the typical time of first maturation in diploid common carp, triploid fish did not show any improvement over diploids apart from higher dressout percentages.

Genotypic effects on comparative growth performance of all-male tilapia Oreochromis niloticus (L.)

An evaluation of the comparative growth performance of sex reversed male tilapia (SRT) and genetically male tilapia (GMT) of two strains of Nile tilapia Oreochromis niloticus was carried out in triplicate cages immersed in an 800 m2 fertilized pond over a period of 196 days. SRT of a Thai strain (EA–SRT), SRT of a strain introduced from the Philippines (ES–SRT) and an inter-strain SRT of the two strains (EA×ES–SRT) were produced by oral administration of 17 α-methyltestosterone. GMT of the Philippine strain (ES–GMT) and inter-strain GMT (EA×ES–GMT) were derived from crosses between novel YY-males of the Philippine strain with normal females (XX) of Philippine and Thai strains, respectively. Significant differences in mean body weights between treatments were observed at final harvest, EA–SRT and EA×ES–SRT having grown significantly larger (P<0.001) than the other crosses. At harvest, mean weight gain of EA–SRT was 55.6% and 61.9% greater than those of ES–SRT and ES–GMT, respectively; and the mean weight gain of EA×ES–SRT was 41.4% greater than that of EA×ES–GMT. It seems likely that these differences are due to a combination of the superior growth of the Thai strain compared to the introduced Philippine strain in the trial environment and the low sex ratio of the inter-strain GMT. The results are discussed in the context of options for transfer of the YY male technology and the application of monosex techniques in commercial culture of tilapia.