lo 1 The discovery of Polycomb group proteins

[?] How were Polycomb group proteins initially discovered?

In 1947, Pamela Lewis discovered an induced dominant mutant called Polycomb (Pc) in Drosophila melanogaster. Around the same time, another mutant called extra sex combs (esc) was found. These mutations cause additional sex combs on the second and third legs of male flies, instead of only on the first leg where they normally belong.

[?] What did Edward Lewis study in the late 1940s that led to the discovery of homeotic genes?

Edward Lewis studied homeotic mutations in Drosophila melanogaster that caused bizarre rearrangements of body parts. He studied the Ultrabithorax (Ubx) gene mutation, which causes the 3rd thoracic segment to develop structures from the 2nd thoracic segment (legs and fully developed wings instead of legs and halteres).

[?] What is the Antennapedia (Antp) mutation and what does it cause?

The Antennapedia mutation causes legs to develop on the head of a fly instead of antennae, which is another famous example of a homeotic mutation.

[?] What did the Polycomb mutant phenotype reveal about Hox genes in the late 1970s and early 1980s?

The phenotypes were shown to be caused by loss of Scr Hox gene repression, indicating that Polycomb genes function to repress Hox genes.

[?] What are Trithorax (TrxG) genes and how do they relate to Polycomb?

Trithorax family genes were later identified and include MLL-Complex members involved in H3K4 mono and tri methylation that promote gene activation. They function antagonistically to Polycomb - trxG mutations can suppress PcG mutant phenotypes.

[?] What happens to Hox gene expression in Polycomb mutants?

In Pc mutants, there is a loss of Polycomb's repressive activity resulting in ectopic expression of Hox genes like AbdB in anterior body segments where they are normally silent