Conceived and designed the experiments: FP. Performed the experiments: NO. Analyzed the data: NO FP. Wrote the paper: FP.
Current address: Instituto de Investigaciones Marinas, CSIC, Vigo, Spain
The authors have declared that no competing interests exist.
In gonochoristic vertebrates, sex determination mechanisms can be classified as genotypic (GSD) or temperature-dependent (TSD). Some cases of TSD in fish have been questioned, but the prevalent view is that TSD is very common in this group of animals, with three different response patterns to temperature.
We analyzed field and laboratory data for the 59 fish species where TSD has been explicitly or implicitly claimed so far. For each species, we compiled data on the presence or absence of sex chromosomes and determined if the sex ratio response was obtained within temperatures that the species experiences in the wild. If so, we studied whether this response was statistically significant. We found evidence that many cases of observed sex ratio shifts in response to temperature reveal thermal alterations of an otherwise predominately GSD mechanism rather than the presence of TSD. We also show that in those fish species that actually have TSD, sex ratio response to increasing temperatures invariably results in highly male-biased sex ratios, and that even small changes of just 1–2°C can significantly alter the sex ratio from 1∶1 (males∶females) up to 3∶1 in both freshwater and marine species.
We demonstrate that TSD in fish is far less widespread than currently believed, suggesting that TSD is clearly the exception in fish sex determination. Further, species with TSD exhibit only one general sex ratio response pattern to temperature. However, the viability of some fish populations with TSD can be compromised through alterations in their sex ratios as a response to temperature fluctuations of the magnitude predicted by climate change.
Sex determination mechanisms produce the sex ratio, a key demographic parameter crucial for population viability. In gonochoristic vertebrates, sex determining mechanisms can broadly be classified as genotypic (GSD) or temperature-dependent (TSD)
So far, predicted effects of climate change on fish populations include distribution shifts
In fish, the first evidence of TSD was obtained in field and laboratory studies carried out in the Atlantic silverside,
They are defined according to the sex ratio produced as a function of temperature during the thermosensitive period. A, Pattern 1, low temperatures produce female-biased sex ratios and high temperatures produce male-biased sex ratios. B, Pattern 2, low temperatures produce male-biased sex ratios and high temperatures produce female-biased sex ratios. C, Pattern 3, male-biased sex ratios are produced at low and high temperatures, while balanced sex ratios are produced at intermediate temperatures. In some cases, the response may be partial (dashed line in A). The present study demonstrates that fish species with TSD only exhibit pattern 1.
Criteria used here | Confirmation by statistical analyses | |||||||||||
SPECIES | Pattern of TSD previously assigned* | Evidence for the presence of sex chromosomes [Reference] | Sex ratio shift within the RTD (see Suppl. |
Diagnosis | Lineal regression/F-test | New pattern of TSD proposed here* | ||||||
n | Intercept | Slope | r2 | F | DFn/DFd | |||||||
(1) | Yes |
No | GSD+TE | - | - | - | - | - | - | - | 0 | |
1 | Yes |
No | GSD+TE | - | - | - | - | - | - | - | 0 | |
(1) | Yes |
No | GSD+TE | - | - | - | - | - | - | - | 0 | |
1 | Yes |
Yes | GSD+TE | - | - | - | - | - | - | - | 0 | |
1 | Yes |
Yes | GSD+TE | - | - | - | - | - | - | - | 0 | |
2 | Yes |
No | GSD+TE | - | - | - | - | - | - | - | 0 | |
1 | No | Yes | TSD | 16 | 2.53 | 2.26 | 0.40 | 9.30 | 1/14 | 0.009 | 1 | |
2 | Yes |
No | GSD+TE | - | - | - | - | - | - | - | 0 | |
1 | No | Yes | TSD | 10 | 113.79 | −995.21 | −0.90 | 70.21 | 1/8 | <0.0001 | 1 | |
1 | No | Yes | TSD | 20 | −27.81 | 2.74 | 0.58 | 24.61 | 1/18 | 0.0001 | 1 | |
1 | No | Yes | TSD | 9 | −55.62 | 3.91 | 0.67 | 14.42 | 1/7 | 0.0067 | 1 | |
1 | No | Yes | TSD | 6 | −182.40 | 9.39 | 0.98 | 242.90 | 1/4 | <0.0001 | 1 | |
1 | No | No | GSD+TE | - | - | - | - | - | - | - | 0 | |
(1) | Yes |
No | GSD+TE | - | - | - | - | - | - | - | 0 | |
1 | No | Yes | TSD | 9 | −22.15 | 2.60 | 0.69 | 15.29 | 1/7 | 0.0058 | 1 | |
1 | No | Yes | TSD | 21 | −139.50 | 7.21 | 0.76 | 60.91 | 1/19 | <0.0001 | 1 | |
(1) | Yes |
Yes | GSD+TE | - | - | - | - | - | - | - | 0 | |
1 | No | No | GSD+TE | - | - | - | - | - | - | - | 0 | |
1/2 | No | No | GSD+TE | - | - | - | - | - | - | - | 0 | |
1×33 | No | Yes | TSD | 93 | −75.93 | 4.78 | 0.75 | 283.60 | 1/91 | <0.0001 | 1×33 | |
1 | Yes |
Yes | GSD+TE | - | - | - | - | - | - | - | 0 | |
1/2 | Yes |
Yes | GSD+TE | - | - | - | - | - | - | - | 0 | |
1 | Yes |
Yes | GSD+TE | - | - | - | - | - | - | - | 0 | |
3 | Yes |
Yes | GSD+TE | - | - | - | - | - | - | - | 0 | |
3 | No | No | GSD+TE | - | - | - | - | - | - | - | 0 | |
1 | Yes |
No | GSD+TE | - | - | - | - | - | - | - | 0 | |
1 | No | No | GSD+TE | - | - | - | - | - | - | - | 0 |
Abbreviations: TSD, temperature-dependent sex determination; GSD+TE, genotypic sex determination plus temperature effects; RTD, range of temperature during development under natural conditions; n, number of sex ratio datapoints (see
GSD and TSD can be regarded as two discrete processes that give rise to a continuous pattern of sex determination mechanisms
The objective of this study was to assess the prevalence of TSD in fish by taking the species where this type of sex determining mechanism has been claimed and applying a series of proposed criteria to discern true cases of TSD from cases of GSD+TE. These included checking for the presence of sex chromosomes and determining whether the temperature used to elicit a change in sex ratios was ecologically relevant, i.e., a temperature that the species usually experiences in nature during the thermosensitive period. We found that TSD is far less widespread that currently thought. We also found that species who actually have TSD exhibit only one single response pattern, not three, producing highly male-biased sex ratios in response to even small increases in temperature. Thus, in one hand, by defining the species that actually have TSD, this study contributes to our understanding of the evolution of sex determining mechanisms. On the other hand, it reports previously unaccounted possible effects of global warming on fish sex ratios.
The 59 species analyzed in this study include all those gonochoristic fishes for which TSD has been explicitly or implicitly assumed as reported in published reviews on the subject
For each species analyzed, field data, including the range of natural temperature in which the species can live (RNT), the range of temperature during development in the wild (RTD) as well as the lethal temperature (LT), when available, were obtained from ad hoc reviews, e.g.,
To determine the actual prevalence of TSD in fish and to furnish robust patterns of sex ratio response to temperature, we have used a comparative analysis consisting of the application of two independent criteria to identify the presence of TSD (
This algorithm is based on the criteria of Valenzuela et al. (2003), and incorporates a modification of the criteria of Conover (2004). See text in the
A, Examples of authentic cases of TSD following pattern 1, more males with increasing temperatures. Sex ratio shifts occur within the range of temperature (shaded areas) normally experienced by fish in the wild. B, Examples of false cases of TSD. Sex ratio shifts only occur at extreme temperatures, and thus represent thermal effects on GSD (a, b). Formerly proposed pattern 2 (c), fewer males at high temperature, is not supported by re-analysis of data (see also Supplementary
Sex ratio deviations from 1∶1 in
Sex ratio data originally obtained from monosex (all-female) populations exposed to different temperatures were transformed to make them comparable with data obtained with mixed-sex populations of the same species by applying the following formula: Percent males in a 1∶1 (male∶female) population = 50+(percent males in the all-female population/2). Thus, for example, an all-female population that at 20°C the percent of males was 0% and at 28°C was 66% (indicating that two thirds of the females were masculinized) would be equivalent to an 1∶1 population that at 20°C the percent of males was 50% and at 28°C was 50+(66/2) = 83%. Notice that the possibility of producing all-female stocks is indicative that the species in question has a chromosomal system of sex determination, usually of the XX/XY type, thus suggesting the presence of GSD rather than of TSD, as is demonstrated.
The presence of a significant sex ratio response to temperature within the RTD and the verification of the presence of TSD in species diagnosed as having such mechanism of sex determination after applying the criteria explained above was carried out as follows: First, we tested if there was a statistically significant relationship between sex ratio produced and temperature by using the Spearman rank correlation coefficient method. If so, then we compared the slope with the F-test
In a few instances, more than one intermediate temperature has been tested. In these cases, for economy of space in the
In all cases, sex ratio data expressed as percentages (i.e., 100·p, where p is the proportion of males) were arcsin transformed (arcsin of the square root of p) prior to statistical analysis
Our results show that of the 53–55 species (depending on the authors) previously assigned to pattern 1, the 33 cichlid species of the genus
In all cases, higher temperatures imply a higher number of males produced. Key: 1,
Regarding pattern 2, analysis of the original data
Regarding pattern 3, the two flatfishes previously assigned to this pattern (
Based on the relationship between temperature and sex ratio produced as shown in
Species | Pivotal temp. (°C) | Percent of sexes (♂∶♀) at pivotal temp.+1.5°C | Percent of sexes (♂∶♀) at pivotal temp.+4°C |
25.3 | 62 ∶ 38 | 81 ∶ 19 | |
18.8 | 56 ∶ 44 | 65 ∶ 35 | |
25.8 | 57 ∶ 43 | 68 ∶ 32 | |
14.5 | 61 ∶ 39 | 75 ∶ 25 | |
26.6 | 57 ∶ 43 | 69 ∶ 31 | |
25.7 | 60 ∶ 40 | 76 ∶ 24 | |
24.2 | 73 ∶ 27 | 98 ∶ 2 | |
25.6 | 68 ∶ 32 | 92 ∶ 8 | |
Pivotal temp. (mean±S.E.M.) | 23.3±1.5 | - | - |
Percent males (mean±S.E.M.) | - | 61.7±2.1 | 78.0±4.1 |
Average of the 33 species shown in
In reptiles, where TSD was first discovered in vertebrates, this mechanism of sex determination is now well established (see the book by Valenzuela and Lance
The analysis of sex ratio response to temperature, considering the scope of such response as well as the presence or not of sex chromosomes, carried out in the present study indicated that many species where TSD had been claimed before are in fact GSD species affected by temperature, i.e., cases of GSD+TE. In GSD+TE species, temperature rather than being the external environmental factor controlling sex determination is capable of affecting the process of gonadal sex differentiation under some circumstances. This distinction is not trivial nor semantic since, according to the canonical definition
Our results support the presence of pattern 1 of sex ratio response to temperature (more males with increasing temperature) but the number of species with TSD is much lower than previously considered and concern mainly species of the families Cichlidae followed by species of the family Atherinopsidae. In addition, we have demonstrated that pattern 2 of sex ratio response to temperature does not exist in fish.
Regarding pattern 3, we propose that this pattern is the result of two independent effects unrelated to TSD (
The results of the present study have implications for our understanding of the evolution of vertebrate sex determining mechanisms. They still agree with the view that TSD has evolved independently many times
Orders, families and species with TSD are marked in color. Teleost phylogeny based on Nelson
What is the reliability of the original data used to assign TSD in the different species that survived our analysis? In the species of the F. Atherinopsidae (silversides) the evidence seems robust
The criteria used here allow the identification of the presence of TSD in a given species. However, this does not exclude the possibility that these species may also have populations with GSD. Therefore, populations with GSD and TSD may co-exist in a single species
The tilapias (genus
How species with TSD will respond to current rapid climate change is a timely question
The species identified as having TSD in this study constitute a heterogeneous group since they include both freshwater and marine species living also both in low and high latitudes. Some of them are typically eurithermal while others are stenothermal and, further, they exhibit different reproductive strategies.
Similarly, global warming is not a heterogeneous process, since it affects different parts of the Earth differently. Globally, however, mean temperatures of water bodies are projected to increase by up to ∼4°C by the end of this century according to plausible global change scenarios
Thus, the number of females in species with TSD, some of which are of economic or recreational importance, could decrease. One such species is the Argentinean silverside (
Potential temperature effects on sex ratios could be difficult to quantify if they are mitigated by other global warming-induced effects, including species distribution shifts
It should be noted that the impact of temperature on sex ratios could also affect species with identifiable sex chromosomes (by causing sex reversal) provided that those effects occur at temperatures within the natural range, or the new shifted range. However, at this point there is insufficient information to determine if, by virtue of their possible higher sensitivity to temperature, species with TSD are better indicators of the impacts of climate change on sex ratios than GSD+TE species.
In this study, we performed an analysis of field and laboratory data related to fish species for which TSD was assumed. By applying a series of criteria accepted to ascertain the actual presence of TSD, we can reasonably affirm that, excluding the species of the genus
Temperature-Dependent Sex Determination in Fish. Prevalence, Existence of a Single Sex Ratio Response Pattern, and Possible Effects of Climate Change.
(0.35 MB DOC)
We would like to thank the following persons: M. Blázquez, J. Cerdà, N. Mrosovsky, S. Sarre, M. Schartl and J. Viñas for helpful comments; F. Mayou for providing advice on statistical analyses; and J.I. Fernandino for providing information on