The most extensively studied mode of genetic SD is chromosomal sex determination (CSD) as in mammalian and avian species, for example. In this system, sex is determined by a primary switch located on one or both members of a well-differentiated sex chromosomal pair (see e.g. , , , ). Since in mammals, including humans, sex is determined by CSD, the vast majority of our knowledge on the molecular regulation of vertebrate sex is based on data collected from such systems.
The aim of this study was to perform a detailed analysis on zebrafish sex determination, by combining the power of traditional and molecular technologies. Through analysis of sex ratios in a large number of families, we show that i) sex ratios vary among different families; ii) parental genotypes have a major effect on the sex ratio; and iii) one of the two sexes can be depleted through systematic selection in a few generations. Moreover, PCR-based screens and aCGH performed by a custom-designed tiling array were both unable to find general differences between the genome of the two sexes in two different zebrafish strains. The above data all point towards a genetic mechanism of sex determination and the lack of a chromosomal sex determination system in the zebrafish. We, therefore, propose that zebrafish sex determination is polygenic.
Using classical breeding experiments, we found that sex ratios across families were wide ranging (4.8% to 97.3% males). On the other hand, repeated single pair crossings produced broods of very similar sex ratios, indicating that parental genotypes have a role in the sex ratio of the offspring. Variation among family sex ratios was reduced after selection for breeding pairs with predominantly male or female offspring, another indication that zebrafish sex is regulated genetically. Further examinations by a PCR-based “blind assay" and array comparative genomic hybridization both failed to find universal sex-linked differences between the male and female genomes. Together with the ability to increase the sex bias of lines by selective breeding, these data suggest that zebrafish is unlikely to utilize a chromosomal sex determination (CSD) system.
Although zebrafish has become one of the prime vertebrate models for developmental biology, its sex determination mechanism is still unknown. Therefore, the primary aim of this project was to find out more about the sex determination of this species. The first question we asked was: does zebrafish use a chromosomal sex determination system?
Since molecular and breeding studies failed to identify heteromorphic sex chromosomes or their effect, we next sought to find out if genetic factors are involved in zebrafish sex determination. We performed repeated single pair mating in which 19 randomly selected breeding pairs were bred twice. The environmental factors such as ambient temperature, amount of food given and rearing density were not tightly controlled. Even so, broods derived from the same breeding pair did not exhibit major sex ratio differences between repeated crossings of 18 out of 19 breeding pairs tested (). This indicates that the wide-ranging sex ratios normally observed are most likely due to the parental genotypes. In addition, we showed that sex ratio variation decreases substantially under selective pressure, a strong indication that sex is a genetic trait. Another interesting phenomenon we observed was that after three generations of selection we were able to obtain two all-male families while attempts to produce all-female families were unsuccessful. We do not have an explanation for this difference and we propose that further investigations are needed to elucidate the underlying reasons. Nevertheless, our data show that zebrafish uses primarily genetic sex determination system. As we have also demonstrated that CSD is not likely the mode of sex determination in zebrafish, we propose that a PGSD is in place. Based on our data and the recent aforementioned association study , we propose that the number of genes contributing to the sex determination process might be far more than just a handful.
Research on sex determination (the differentiation of the embryonic bipotential gonad into a testis or an ovary) traditionally focused on testis development. Andrew Sinclair’s 1990 Nature paper famously identified a Y-chromosome gene as the Sex-Determining Region Y (SRY). Female sexual development, by contrast, was thought to proceed as a "default" in the absence of Sry. In the case of sex determination, "default" became the prevailing concept for female pathways—i.e., an ovary results in the absence of other action. The active processes controlling ovarian development remained a blind spot. The notion of a "passive" female fit with current scientific theories and gender assumptions in the broader society.
Swain, A., Narvaez, V., Burgoyne, P., Camerino, G., & Lovell-Badge, R. (1998). Dax1 Antagonizes Sry Action in Mammalian Sex Determination. Nature, 391 (6669), 761-767.
Despite the popularity of zebrafish as a research model, its sex determination (SD) mechanism is still unknown. Most cytogenetic studies failed to find dimorphic sex chromosomes and no primary sex determining switch has been identified even though the assembly of zebrafish genome sequence is near to completion and a high resolution genetic map is available. Recent publications suggest that environmental factors within the natural range have minimal impact on sex ratios of zebrafish populations. The primary aim of this study is to find out more about how sex is determined in zebrafish.
These observations led to a search to find a sex determining gene on the Y-chromosome. In a 1990 Nature paper, Andrew Sinclair and colleagues identified a Y-chromosome gene as the Sex-Determining Region Y (SRY), while acknowledging that it is likely that many different genes are required for both male and female sex determination (Sinclair et al., 1990). Subsequent research confirmed that XX mice develop testes if injected with Sry-bearing DNA fragments during embryonic development (Koopman et al., 1991).
Temperature is the most commonly studied environmental cue for sex determination. It is utilized by many reptile species , , , and some fish species , . In animals with temperature-based sex determination (TSD), substantial fluctuations in the environmental temperature will likely cause significant changes in the offspring sex ratio . Two papers reported that the temperature at natural habitat of zebrafish ranges from 26 to 38°C , . However, it is believed that 26 to 29°C is the temperature range for normal zebrafish development and rearing them within this range did not result in significant sex ratio changes . It was also observed that exposure to increased temperature (35–37°C) either during early development (5–48 hpf) or between 17–27 dpf resulted in male-biased sex ratio. On the other hand, at our laboratories we observed high mortality if zebrafish larvae were grown at 37°C from the beginning. Therefore, temperature is unlikely to be the primary signal for zebrafish sex determination, but might exert secondary effects on its sexual development.
What is uniquely valuable about these books that would make it worth owning, or at least reading, both? Those with a strong interest in that area of developmental biology pertaining to the evolution, genetics, endocrinology, development, and clinical aspects of sex determination and sex differentiation will find that, in spite of considerable overlap, these volumes complement each other in may ways, each bringing exceptionally valuable contributions. The Wiley book is, in all respects, a later and more up-to-date treatment.
The study builds on a classic theory first proposed in a 1973 paper by scientists Robert Trivers and Dan Willard, founders of the field of evolutionary sociobiology. They challenged the conventional wisdom that sex determination in mammals is random, with parents investing equally in their offspring to generate a 50-50 sex ratio in the population. Instead, they hypothesized that mammals are selfish creatures, manipulating the sex of their offspring in order to maximize their own reproductive success. Thus, parents in good condition, based on health, size, dominance or other traits, would invest more in producing sons, whose inherited strength and bulk could help them better compete in the mating market and give them greater opportunities to produce more offspring. Conversely, mothers in poor condition would likely play it safe, producing more daughters, whose productivity is physiologically limited. Other hypotheses make similar predictions -- that females who choose mates with particularly "good genes" (e.g. for attractiveness) should produce so called "sexy sons" as a result, Garner said.