• Use the following slides in conjunction with your notes to learn about Temperaturedependent sex determination and Sex determination in Drosophila.

Sex determination in Drosophila Melanogaster
Drosophila have been studied extensively as a model organism and they show a very similar inheritance pattern with regard to X and
Y sex-linked genes.
Unlike mammals though, the sex determination involves the autosomal chromosomes as well.
In mammals the presence of the Y chromosome determines maleness. In drosophila it is the ratio of X chromosomes present related to the autosomes present.
Normally flies have either 1 or 2 X chromosomes and two sets of autosomes. If there is one X in a diploid cell (1X:2A) the fly is male. If there are two X chromosomes in a diploid cell (2X:2A) the fly is female. clip

Tutorial on sex determination in drosophila

Sex determination by non-genetic means
Instead of using a genetic basis for sexdetermination, many species use environmental triggers.
Some of the best triggers are characterised in reptiles where temperature is the common trigger.
Using a specific trigger can allow individuals to respond to conditions and determine the ratio of the sexes in offspring. e.g. changing nesting behaviours according to food availability.

During the incubation of alligator and turtle eggs, there is a thermosensitive period of a few days long where the sex of the organism is determined. After this period, the sex cannot be changed.

The sex of turtles can be determined in one of two scenarios; either that at high temps, sex will be female, or at very low or very high temps, the sex will be female.

But in crocodiles and alligators, the opposite is true, where low temperatures result in females and high temps results in males.
How is this possible?
It has been proposed that temperature acts on genes coding for enzymes that catalyse steroid hormone production, which would lead to sex determination, but the whole mechanism is poorly understood. News clip

In certain populations of spotted skink, a small lizard in Tasmania, it is advantageous to have females early in the season.
The warmth early in the season ensures femalebiased broods that then have more time to grow and reach maturity and possibly reproduce before they experience their first winter, thereby increasing fitness of the individual.

It is also thought that this type of sex determination comes from ancient reptile ancestors that benefitted from it, benefits that we don’t understand due to the deep passage of time.
Even if TSD bestows no evolutionary advantage today, perhaps it does not also have any costs in terms of fitness, and therefore remains.

In many reptiles, e.g. turtles and many lizards, sex is determined by X and Y chromosomes as in humans. Females are XX and males are XY.

Some species of snake, like birds, do not have X and Y sex chromosomes, but instead have W and Z sex chromosomes.
Males are ZZ and females are ZW.
Reference here
Macaw genotype 20+ZW

Hermaphrodites
Many species are hermaphroditic for part or all of their life cycle. This means that they are either capable of forming both female and male structures e.g. many flowering plants, snails. Or that they can move between female and male forms.

Earthworms are an example of a simultaneous hermaphrodite – with both male and female reproductive organs.
Although they possess ovaries and testes, they have a protective mechanism against self-fertilization.
Sexual reproduction occurs when two worms meet and exchange gametes, copulating on damp nights during warm seasons. Fertilized eggs are protected by a cocoon, which is buried on or near the surface of the ground.

• Animals that can change between one sex and the other during their lifetime are called
sequential hermaphrodites, e.g. teleost (rayfinned) fish and jellyfish.
• Clownfish are all born male and the largest male becomes female and mates with the next largest male.

How do humans become hermaphrodites and display both sets of genitalia?
One possible pathophysiologic explanation of this rare phenomenon is a parthenogenetic division of a haploid ovum into two haploid ova.
Upon fertilization of the two ova by two sperm cells (one carrying an X and the other carrying a Y chromosome), the two fertilized ova are then fused together resulting in a person having dual genitalia.

• Some sequential hermaphrodites are female to begin with and become male later in life e.g. many bony fish including wrasse and parrot fish.

Snails have both sets of male and female reproductive organs.
• Some snails produce sperm and fertilise their own eggs (fairly rare).
• Some snails produce sperm that mature at a different time from their eggs, so they fertilise another snail’s eggs.

Some hermaphrodite species can change back and forth between male and female throughout their life cycle.
Some coral reef fish change sex according to their social status, regardless of the initial sex, based on a simple principle: if the fish expresses subordinate behaviour then it changes its sex to female, and if the fish expresses dominant or not subordinate behavior then the fish changes its sex to male

There are many different triggers used to produce any such change in sex.
Parasitic infection
Snails were found to actively avoid mating as females with other snails who had a parasitic infection. They therefore only mated with snails who were resistant to disease, increasing their reproductive success. Link .

Complete the ‘What you should know’ passage on p19 of your sheets