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Genetics. 2018 Nov;210(3):757-772. doi: 10.1534/genetics.118.301146.

The Drosophila Dot Chromosome: Where Genes Flourish Amidst Repeats.

Author information

1
Department of Biology, The University of Alabama at Birmingham, Alabama 35294 riddlenc@uab.edu.
2
Department of Biology, Washington University in St. Louis, Missouri 63130.

Abstract

The F element of the Drosophila karyotype (the fourth chromosome in Drosophila melanogaster) is often referred to as the "dot chromosome" because of its appearance in a metaphase chromosome spread. This chromosome is distinct from other Drosophila autosomes in possessing both a high level of repetitious sequences (in particular, remnants of transposable elements) and a gene density similar to that found in the other chromosome arms, ∼80 genes distributed throughout its 1.3-Mb "long arm." The dot chromosome is notorious for its lack of recombination and is often neglected as a consequence. This and other features suggest that the F element is packaged as heterochromatin throughout. F element genes have distinct characteristics (e.g, low codon bias, and larger size due both to larger introns and an increased number of exons), but exhibit expression levels comparable to genes found in euchromatin. Mapping experiments show the presence of appropriate chromatin modifications for the formation of DNaseI hypersensitive sites and transcript initiation at the 5' ends of active genes, but, in most cases, high levels of heterochromatin proteins are observed over the body of these genes. These various features raise many interesting questions about the relationships of chromatin structures with gene and chromosome function. The apparent evolution of the F element as an autosome from an ancestral sex chromosome also raises intriguing questions. The findings argue that the F element is a unique chromosome that occupies its own space in the nucleus. Further study of the F element should provide new insights into chromosome structure and function.

KEYWORDS:

F element; FlyBook; HP1a; chromosome evolution; fourth chromosome; heterochromatin

PMID:
30401762
PMCID:
PMC6218221
DOI:
10.1534/genetics.118.301146
[Indexed for MEDLINE]
Free PMC Article

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