Question

Why in case of lizard, turtle, bee, the sex is determined by environmental factors ?

Answer 1

Environmental Sex Determination

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Temperature-dependent sex determination in reptiles

While the sex of most snakes and most lizards is determined by sex chromosomes at the time of fertilization, the sex of most turtles and all species of crocodilians is determined by the environment after fertilization. In these reptiles, the temperature of the eggs during a certain period of development is the deciding factor in determining sex, and small changes in temperature can cause dramatic changes in the sex ratio (Bull 1980). Often, eggs incubated at low temperatures (22–27°C) produce one sex, whereas eggs incubated at higher temperatures (30°C and above) produce the other. There is only a small range of temperatures that permits both males and females to hatch from the same brood of eggs. Figure 17.20 shows the abrupt temperature-induced change in sex ratios for the red-eared slider turtle. If eggs are incubated below 28°C, all the turtles hatching from them will be male. Above 31°C, every egg gives rise to a female. At temperatures in between, the broods will give rise to individuals of both sexes. Variations on this theme also exist. The eggs of the snapping turtle Macroclemys, for instance, become female at either cool (22°C or lower) or hot (28°C or above) temperatures. Between these extremes, males predominate.

Temperature-dependent sex determination in three reptile species: the American alligator (Alligator mississippiensis), the red-eared slider turtle (Trachemys scripta elegans), and the alligator snapping turtle (Macroclemys temminckii).

Temperature-dependent sex determination in three reptile species: the American alligator (Alligator mississippiensis), the red-eared slider turtle (Trachemys scripta elegans), and the alligator snapping turtle (Macroclemys temminckii). (After Crain and (more...)

One of the best-studied reptiles is the European pond turtle, Emys obicularis. In laboratory studies, incubating Emys eggs at temperatures above 30°C produces all females, while temperatures below 25°C produce all-male broods. The threshold temperature (at which the sex ratio is even) is 28.5°C (Pieau et al. 1994). The developmental period during which sex determination occurs can be discovered by incubating eggs at the male-producing temperature for a certain amount of time and then shifting the eggs to an incubator at the female-producing temperature (and vice versa). In Emys, the last third of development appears to be the most critical for sex determination. It is not thought that turtles can reverse their sex after this period.

The pathways toward maleness and femaleness in reptiles are just being delineated. Unlike the situation in mammals, sex determination in reptiles (and birds) is hormone-dependent. In birds and reptiles, estrogen is essential for ovarian development. Estrogen can override temperature and induce ovarian differentiation even at masculinizing temperatures. Similarly, injecting eggs with inhibitors of estrogen synthesis will produce male offspring, even if the eggs are incubated at temperatures that usually produce females (Dorizzi et al. 1994; Rhen and Lang 1994). Moreover, the sensitive time for the effects of estrogens and their inhibitors coincides with the time when sex determination usually occurs (Bull et al. 1988; Gutzke and Chymiy 1988).

It appears that the enzyme aromatase (which can convert testosterone into estrogen) is important in temperaturedependent sex determination. The estrogen synthesis inhibitors used in the experiments mentioned above worked by blocking the aromatase enzyme, showing that experimentally low aromatase conditions yield male offspring. This correlation is seen to hold under natural conditions as well. The aromatase activity of Emys is very low at the male-promoting temperature of 25°C. At the female-promoting temperature of 30°C, aromatase activity increases dramatically during the critical period for sex determination (Desvages et al. 1993; Pieau et al. 1994). Temperature-dependent aromatase activity is also seen in diamondback terrapins, and its inhibition masculinizes their gonads (Jeyasuria et al. 1994). One remarkable finding is that the injection of an aromatase inhibitor into the eggs of an all-female parthenogenetic species of lizards causes the formation of males (Wibbels and Crews 1994).

Answer 2

In temperature-dependent sex determination, however, it is the environmental temperature during a critical period of embryonic development that determines whether an egg develops as male or female. This thermosensitive period occurs after the egg has been laid, so sex determination in these reptiles is at the mercy of the ambient conditions affecting egg clutches in nests. For example, in many turtle species, eggs from cooler nests hatch as all males, and eggs from warmer nests hatch as all females. In crocodilian species—the most studied of which is the American alligator—both low and high temperatures result in females and intermediate temperatures select for males.

A widely held view is that temperature-dependent and genotypic sex determination are mutually exclusive, incompatible mechanisms—in other words, a reptile's sex is never under the influence of both sex chromosomes and environmental temperature. This model indicates that there is no genetic predisposition for the embryo of a temperature-sensitive reptile to develop as either male or female, so the early embryo does not have a "sex" until it enters the thermosensitive period of its development.

This paradigm, though, has been recently challenged, with new evidence now emerging that there may indeed be both sex chromosomes and temperature involved in the sex determination of some reptile species. Apparently, in animals where both occur, certain incubation temperatures can "reverse" the genotypic sex of an embryo. For example, there is an Australian skink lizard that is genotypically governed by X and Y sex chromosomes. A low incubation temperature during the development of this lizard's egg reverses some genotypic females (XX) into "phenotypic" males—so that they have only functioning male reproductive organs. Therefore, in this species, there are both XX and XY males, but females are always XX. A slightly different example of this temperature-induced sex reversal is found in an Australian dragon lizard, which has the ZW system of sex chromosomes. In this species, high incubation temperature during egg development reverses genotypic males (ZZ) into phenotypic females; so females can be ZZ or ZW, but males are always ZZ.

Reptiles in which both incubation temperature and sex chromosomes interact to determine sex may represent "transitional" evolutionary states between two end points: complete GSD and complete TSD. It is quite possible that there are other species of reptiles with more complicated scenarios of temperature reversal of chromosomal sex. There are certainly many known examples of fish and amphibians with GSD, in which both high and low incubation temperatures can cause sex reversal. In these cases, all genotypes (from ZZ and ZW to XX and XY) are susceptible to reversal by extremes of incubation temperature.