In chickens, a key gene involved in sex determination has...

In chickens, a key gene involved in sex determination has recently been identified. Called $D M R T 1$, it is located on the $Z$ chromosome and is absent on the W chromosome. Like SRY in humans, it is male determining. Unlike $S R Y$ in humans, however, female chickens (ZW) have a single copy while males (ZZ) have two copies of the gene. Nevertheless, it is transcribed only in the developing testis. Working in the laboratory of Andrew sinclair (a co-discoverer of the human $S R Y$ gene), Craig Smith and colleagues were able to "knock down" expression of $D M R T 1$ in $Z Z$ embryos using RNA interference techniques (see Chapter 16 ). In such cases, the developing gonads look more like ovaries than testes [Nature 461: 267 (2009)]. What conclusions can you draw about the role that the DMRT1 gene plays in chickens in contrast to the role the SRY gene plays in humans?

Key Concepts

Sex Determination Systems

Sex determination systems refer to the genetic and developmental mechanisms by which organisms develop into males or females. In vertebrates, different systems can be found such as the XY system in mammals and the ZW system in birds. These systems employ specific genes on sex chromosomes that trigger the developmental pathways leading to the formation of testes or ovaries.

Gene Dosage Effect

The gene dosage effect is a concept where the number of gene copies influences the amount of gene product produced. In the context of sex determination, the difference in the number of DMRT1 gene copies in male and female birds (with males having two copies and females one) suggests that a threshold level of this gene product is necessary to trigger the development of testes, making gene dosage a key factor in sex determination in chickens.

Comparative Role of Sex-Determining Genes

This concept involves analyzing how different organisms use distinct genetic cues for initiating sex differentiation. For example, while the SRY gene in humans acts as a dominant, switch-like factor found on the Y chromosome, the DMRT1 gene in chickens appears to function via a dosage mechanism on the Z chromosome, with its expression level being critical for testis development. This comparison underscores the evolutionary diversity in sex determination pathways among vertebrates.

RNA Interference and Gene Knockdown

RNA interference (RNAi) is a powerful molecular biology technique used to decrease or silence the expression of specific genes. By ‘knocking down’ a gene, researchers can observe the resulting phenotypic changes, which helps to elucidate the gene's function. In studies of sex determination, RNAi has been utilized to specifically reduce DMRT1 levels in chicken embryos, demonstrating its essential role in directing testis development.

Transcript

00:01 This question is asking us in part a about the y chromosome and basically about asking us to speculate on the origin of the y chromosome.

00:12 And again, the idea is the y chromosome was once like the x chromosome.

00:16 It was a full -size chromosome with lots of genes on it.

00:20 And then over time, it has changed to become a very small chromosome with just a few genes on it.

00:27 And then lots of junk, lots of mutations and dysfunctional things on them.

00:32 And the question is what's happened.

00:33 And of course, what's happened is that there's no recombination, right? there's no crossing over between the y chromosome and another chromosome.

00:43 Now the x chromosome can cross over in females in mammals.

00:48 And so that has allowed the x chromosome to remain full size with a full complement of genes.

00:55 But there's no recombination.

00:57 And what that means is that it's hard to prevent the accumulation of deleterious alleles.

01:04 You can't recombine good alleles together on a chromosome because that's not possible.

01:12 So there has been a loss of alleles, functioning alleles, and accumulation of mutations because there's really no way to get rid of them.

01:27 Right.

01:28 So there's this accumulation of mutations.

01:31 There's also been genetic sweeps where you do have a good functioning allele that sort of brings crummy stuff along with it...

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