UNSW Embryology

Sex Determination
Molecular Development

Background

¥ Mammalian sex determination is regulated by chromosomes.

¥ Females have two X chromosomes. (XX)

¥ Males have a single X and a small Y. (XY)

¥ The X and Y chromosome are morphologically and functionally different from each other.

¥ Evolutionary studies have shown that the Y was once the homologous pair for X.

¥ It is only in the last 5 years that we have some idea about how these two types of chromosomes may be regulated and genes of inportance located upon them.

¥ In females the main scientific problem was that of gene dosage, only one copy of X chromosome is needed to be active.

¥ In males the main scientific problem was what on the Y chromosome determined "maleness", and how did it do it.

Y-Chromosome

¥ The Y chromosome is male determining.

¥ Evolutionary studies have shown that the Y was once the homologous pair for X.

¥ It has been progressively degraded consisting mainly of degraded copies of X linked genes and large regions of repeated sequences.

¥ Somatic cell phenotype is regulated by testicular hormones.

¥ Therefore testis determination is the inital step in sex determination.

¥ Some factor on the Y chromosome must initiate the formation of testis, the "testis determining factor" (TDF).

¥ The breakthrough was the discovery of the SRY gene which turned out to be the TDF.

¥ This gene was discovered by 2 groups separately in 1990.

Berta et al., (1990) Nature 348 448-45

Jager et al., (1990) Nature 348 452- 53

¥ It was found by studying a human XY female. It turned out that she had a deletion in the Y chromosome that did not allow testis development. Mapping of the deletion allowed isolation and characterization of the TDF gene.

¥ Activation of the SRY gene leads to the development of male testis.

¥ SRY is only expressed:

- when testes begin to form

- in gonadal tissue

- does not require the presence of germ cells

¥ SRY encodes a zinc-finger transcription factor. That is it is a protein that binds to specific sites of DNA and regulates the transcription of other genes.

¥ It is still not fully known what genes SRY regulates.

X- Chromosome

¥ Contains about 5% of the haploid genome.

¥ Genes encode house-keeping and specialized functions.

¥ Completely conserved in gene content between species.

¥ It does not encode sex determination or differentiation.

¥ In females one of the X-chromosomes is inactivated in each and every cell. [known since 1961]

¥ This inactivation occurs during embryogenesis.

¥ X Inactivation appears to be random in somatic cells. (mosaic pattern)

¥ The process starts at the "X inactivation centre" and spreads along the chromosome.

¥ The mechanism of inactivation was unknown.

X-Chromosome Inactivation

¥ The breakthrough was the discovery of the X inactive specific transcript (XIST). Brown et. al., (1991) Nature 349, 372-373.

¥ This gene is located within the "X inactivation centre" and only expressed by the inactive X chromosome.

¥ unlike other genes that encode protein XIST contained no "open reading frames" (ie no codons to encode amino acids).

¥ XIST is transcribed but not translated.

¥ XIST appears to act as RNA. Current thinking is that it binds to the X Chromosome and is involved in it's translocation to the nuclear periphery.

¥It now appears that XIST appears to initiate X inactivation and it is the methylation of the inactive X genes that maintains inactivity.