Genic Balance Theory of Sex Determination in Drosophila melanogaster

What is sex determination?

Living organisms, with a very few exceptions, are differentiated into male and female individuals based on their morphological, physiological and behavioral characteristics. Even though the mechanism greatly varies, the sexes of the individuals are genetically determined. The biological system that determines the development of sexual characteristics in an organism is called sex-determination. There are two different types of sex determination systems, they are:

(1). Chromosomal sex determination
(2). Non–genetic sex determination

In chromosomal sex determination, the individuals will process specialized chromosomes called sex chromosomes. In this case, the presence or absence of a particular sex chromosome or the relative ratio of these sex chromosomes determines the sex of the individuals.

Similar to human beings, Drosophila melanogaster (fruit flies) shows XX female and XY male sex chromosomal constitution. However, the mechanism of sex determination is quite different in Drosophila from human. In humans, the Y chromosome is the actual sex determining chromosome. Invariable to the number of X chromosomes, the presence of a single Y chromosome initiates the development of male sex in human embryos. Thus, in human XX and XY individuals will be always female and male respectively. In human XXY and XXXY individuals are also males because of the presence of Y chromosome.

What is Genic Balance System of Sex Determination?

Drosophila melanogaster (source wikipedia)

As we mentioned, the sex determination in Drosophila is quite different from humans. Drosophila has eight chromosomes (n = 4), three pairs of autosomes and one pair of sex chromosomes. Even though Drosophila possesses XX and XY sex chromosomal organization, unlike human beings, the Y chromosome does not have any role in determining the sex of individuals. The sex in Drosophila is determined by the ratio of number of X chromosomes to that of the number of sets of autosomes. In simpler terms, the sex determination is achieved by a balance of female determinants on the X chromosome (X) and male determinants on the autosomes (A). This type of sex determination is called Genic Balance System. The genic balance system of sex determination was proposed by Calvin Bridges in 1926. The genic balance system of sex determination also explains the reasons for the occurrence of sexual variants in the fruit fly population such as inter-sex, metamales and metafemales apart the normal male and female individuals. The sex of the fruit fly individuals may be of different types according to the ratio of X/A as follows:

Calvin Bridges

Ø  If X/A ratio is 1.00, the individuals will be female

Ø  If X/A ratio is 0.50, the individuals will be males

Ø  If X/A ratio is between 1.00 and 0.50, the individuals will be intersex (individuals with a mixture of male and female characteristics)

Ø  If X/A ratio is above 1.00, the individuals will be metafemale (metafemales have severe developmental problems, and they usually do not emerge from pupal stage)

Ø  If X/A ratio is below 0.50, the individuals will be metamales (metamales are sterile and weak)

In usual conditions, Drosophila has either one or two X chromosomes and two sets of autosomes. If there is two X chromosomes (2X) and two sets of autosomes (2A) as in usual females, the individuals will be females since 2/2 = 1.00. If there is one X chromosome (1X), one Y chromosome (1Y) and 2 sets of autosomes (2A) as in usual males, the ratio becomes 0.50 (1/2 = 0.50) and the individuals will be males. Apart from the normal males and females, Drosophila possesses other sex variants such as intersex, metafemales and metamales. Study the table (below) for a detailed understanding of the formation of these sexual variants. These observations also shows that, in Drosophila, the Y chromosome do not have any role in determining sex of individuals.

Different X/A ratios and the resulting sex in Drosophila melanogaster

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There are many suggestions regarding the exact molecular mechanism genic balance system of sex determination in Drosophila, the exact mechanism is still unknown. Studies shown that the X chromosome contains female determining factors and the autosomes contains male determining factors. Y chromosome in Drosophila does not have any role in sex determination.

Key concept: Sexual phenotype of Drosophila melanogaster is determined by the ratio of the number of X chromosomes to the number of sets of autosomal chromosomes (X/A ratio).

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