Sex Determination and Lizards

Sex determination is a pretty hot topic in molecular biology. Trying to elucidate the underlying mechanisms that determine whether a fetus is male or female has been a productive field. Many species rely on a hemizygous¹ method of sex determination, and we humans are no exception (but rather, we are the rule). Males are XY and females are XX, and this is the way it works in all mammals² (even in monotremes like the platypus, though monotremes go to the extreme and can have as many as ten sex chromosomes; a male platypus is XYXYXYXYXY, for instance). Birds rely on a similar ZW system, but in this case, the females are heterogametic; females are ZW and males are ZZ. Some insects use an entirely different system, the XO system, where there is only one sex chromosome - X - and females have two (XX) whereas makes only have one (XO)³.

Despite the system used to determine sex, all the above examples have something in common: they all rely on genotypic sex determination (GSD). In these cases, it is the sex chromosomes (the number, presence or absence) which determines the sex of the resulting fetus. GSD, however, is not the only mode of sex determination. External environmental factors may also influence sex determination. Crocodiles, for example, have no sex chromosomes whatsoever. It is the temperature of the eggs which determines the sex; eggs laid in a warm nest become male and those in a cooler nest become female. Environmental sex determination (ESD) and GSD are not mutually exclusive, of course, and it has been known that many species of reptiles rely on both GSD and ESD. The interaction between ESD and GSD in these species was not thought to be a complex one; eggs at moderate temperatures use GSD, but the sex-chromosome method is bypassed and a temperature-dependant method is used if the eggs are at more extreme temperatures.

A new paper in Current Biology, however, shows that things might not be so simple.

Rajkumar S. Radder, David A. Pike, Alexander E. Quinn, and Richard Shine looked at sex determination in the eggs of the lizard Bassiana duperreyi⁴. They were examining how temperature effected the sex of the hatchlings when they noticed a correlation between the size of the eggs and the resulting sex: those eggs that were larger had female hatchlings and the smaller eggs had male offspring.

Of course, a simple correlation like this does little to prove an actual relationship and may simply be coincidence. So Shine and colleagues decided to try adding or removing yolk from the eggs during their development. What they found was pretty astounding. When they added extra yolk to the eggs, the hatchlings came out female, even if the sex chromosomes had already determined the sex to be male. And those eggs that had yolk removed switched to male even when the sex chromosomes had been set to female. This finding would suggest that sex determination in B. duperreyi is determined by a complex interaction of a minimum of three factors: sex chromosomes, temperature and egg size.

This also suggests that sex determination in any species may not be as simple as once thought.

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1. Hemizygous may not be a term you learned in Genetics 101. Whereas homozygous and heterozygous refer to having one or two different alleles, respectively, hemizygous refers to having only one of a set of two chromosomes. Female humans have two X chromosomes (XX), whereas males are XY; they are hemizygous with respect to the X chromosome. You could also refer to females as homogametic and males as heterogametic.

2. To be more specific, sex determination in mammals relies on a gene called sry (Sex-determining Region Y) located on the Y-chromosome. The gene encodes for a transcription factor called TDF (testis determining factor). When TDF is expressed, it influences the undeveloped gonad to develop into testes instead of the default state of developing into ovaries. If the SRY region of the Y chromosome is deleted or mutated, then the resulting child will be phenotypically female but genotypically male. Likewise, a translocation of SRY onto an X chromosome can lead to children who are phenotypically male but genotypically female. Interestingly, TDF does not begin to work, and consequently sex is not determined, until after the nipples have developed, which is why men have nipples that are pointless (beyond giving purple nurples, of course).

3. One interesting downside of this system is that it can lead to bilateral hermaphroditism. It is possible for the X chromosome to form a "ring chromosome" where the ends of the chromosome fuse together to make a ring. This ring chromosome is easily lost during cell division. If an embryo begins as XX (female), and very early on in development (at the 4 cell stage, say) a ring chromosome is formed and lost in one cell, the embryo will become split right down the middle, one side being male and the other side being female. This is rare, but not uncommon, in Drosophlia. Such flies are called gynandromorphs.

4. Rajkumar S. Radder, David A. Pike, Alexander E. Quinn, and Richard Shine. Offspring Sex in a Lizard Depends on Egg Size. Current Biology, 2009; DOI: 10.1016/j.cub.2009.05.027