In a special entry in his Xenobiology series, James Ashworth explains why it’s entirely biologically feasible for a Time Lord to change gender. First published in The Tides of Time number 41, Trinity 2018

YOU MAY HAVE NOTICED THAT WHEN JODIE WHITTAKER WAS ANNOUNCED AS THE THIRTEENTH DOCTOR THAT THE REACTION WAS NOT ENTIRELY POSITIVE. Similar, though less strong reactions, had also greeted the regenerations of the Master into Missy and the General (recently named Kenossium by the DWM comic strip), with the idea that an individual could change gender as part of their life cycle being something that sparked hostility from some sections of fandom. Even here on earth, there’s plenty of scientific basis for this idea, forming a pantheon of species to which the Time Lords are just the latest addition.

The pinnacle of this ability surely lies with fungi. Fungi don’t have genders in the strictest sense, having multiple mating types instead. Many just have two, a and α, but occasionally more. For example, Schizophyllum commune is a fungus that has over 10,000 mating types! These mating types allow different fungi of compatible types to mate, making them heterothallic, but some fungi, the homothallic ones, may also do it by themselves, fusing two compatible nuclei produced from within itself. What allows it to produce nuclei is the ability of fungi to change mating types. In the case of a typical fungus genome with a and α mating types, there is a mating locus, or MAT, which determines the mating type. It separates two silent loci, Hidden MAT left (HML) which codes for a and Hidden MAT right (HMR) for α. Upon beginning division, the HO gene is activated, which produces an endonuclease, which is an enzyme able to cut the bonds within the DNA chain. It causes a double strand break, and this break is repaired using either the HML or HMR. To ensure that a mating switch occurs, a specialised enhancer, which affects how easily DNA can be replicated, is used to ensure the opposite locus is selected. This can happen regularly, once every division, if the HO gene is dominant, or once every 10,000 divisions if it is recessive. In S.cerevisiae, or yeast, this produces a mother and daughter cell of opposite mating types, as the daughter gets an original copy of the mother’s genome as well as an mRNA transcript that codes for a HO inhibitor. The mother, meanwhile, can switch as her HO gene has already been activated during the cell cycle. Hypothetically, this could be a valid explanation for a regeneration changing sex, it would just require the absence of the switching enhancer so that each HO activation randomly selected a mating locus.

Another possibility encompasses the process found in vertebrates, which makes use of change on a hormonal rather than genetic level. The two key hormones are testosterone, resulting in mascularisation, and oestradiol which results in feminisation. Clownfish are protandrous, which means that the loss of the dominant female leads to the dominant male changing to replace them. In this situation, the aromatase enzyme converts testosterone into oestradiol, leading to the restructuring of the testes into ovaries. Conversely, the blue-streak cleaner wrasse is protogynous, with the dominant female becoming male in response to inhibition of aromatase. The reason for these changes is thought to be due to increasing the fitness of an organism by allowing it to have more offspring. In the wrasse, males maintain a harem of females, so becoming male enables the mass production of sperm to have more offspring with multiple individuals. Those that change also have an advantage over other males to begin with, having larger testes to increase their competitiveness. In the clownfish, only a pair of individuals reproduce at one time, so being female means that an individual can select for the male with the best attributes and have offspring that are fitter. Other species, such as the bluebanded goby, can change sex multiple times depending on their social position. This explanation perhaps ties in better with some of the Twelfth Doctor’s statements, such as his hope that the future is ‘all girl’, and the possibility that his conscious/subconscious thoughts brought about this change through the facultative production of the aromatase enzyme.

The final mechanism that can induce a change in sex is the temperature of the environment. This can occur in the eggs of particularly reptiles, but also some birds under certain conditions. Both sets of organisms use the ZW sex determination system, where females are the heterogametic sex (ZW) while males are homogametic (ZZ), in opposition to our XY system. The sex chromosomes can be overridden, with certain temperatures inducing the expression of the opposing sex to that indicated genetically. In an experiment using the Australian bearded dragon, ZZ females that had changed in this way were mated with ZZ males at low temperatures (28˚C) during incubation. Their offspring were completely ZZ male. From 28-32˚C, the sex ratios were equal, and above 32˚C, the offspring were almost entirely male, despite their sex chromosomes. In the case that the period of regeneration and incubation are analogous, you could argue that the energy released during the repeated destruction/crashing of the TARDIS in recent regenerations could be a valid reason for this switch. Of course, the destruction of the TARDIS during this regeneration occurred after the regeneration had completed, while the destructions during the Tenth and Eleventh Doctor’s regenerations did not cause the Doctor to become female earlier, so this can be discounted as a possibility.

I’ll admit that when the concept of a female Doctor was first broached, I wasn’t overly keen. However, I came to the conclusion that the Doctor, as a Time Lord, and an alien, is probably unlikely to share our concepts of gender and reproduction, something that brought me around to the idea. While this article has been an attempt to scientifically explain the process of sex change during regeneration, it doesn’t change the fact that the Doctor is now female. At the end of the day, Doctor Who should be something that brings us together, and I look forward to see how Series Eleven develops the ongoing saga of the Doctor.

Tides 41 is, at time of publication, available to buy through this link