X chromosome evolution in Drosophila
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Date
2008Author
Vicoso, Beatriz
Metadata
Abstract
Although the X chromosome is usually similar to the autosomes in size, gene density
and cytogenetic appearance, theoretical models predict that its hemizygosity in males
may cause unusual patterns of evolution. The sequencing of several genomes has indeed
revealed differences between the X chromosome and the autosomes in the rates of gene
divergence, patterns of gene expression and rates of gene movement between
chromosomes. In this thesis, I have attempted to investigate some of these patterns and
their possible causes.
The first two chapters consist of theoretical and empirical work intended to analyse the
rates of evolution of coding sequences of X-linked and autosomal loci, with particular
emphasis on faster-X evolution, the theory that more effective selection on the X can lead
to higher rates of adaptive evolution on this chromosome. By analyzing X-linked and
autosomal coding sequence in several species of Drosophila, we found some evidence for
more effective selection on the X, particularly evident in the higher levels of codon usage
bias detected at X-linked loci. We argue that this could be due to higher levels of
recombination on the X chromosome increasing its effective population size (NeX) relative
to the autosomal effective population size (NeA). To further investigate this hypothesis,
we have modeled the effect of increased NeX/NeA on rates of evolution and confirmed that
this can contribute to faster-X evolution.
The last two chapters deal with the evolution of sex-biased genes and the possible
causes for their differential accumulation on the X. We used EST data to create
expression profiles for D. melanogaster male-, female- and unbiased genes. Our results
suggest that the expression levels of sex-biased genes are incompatible with the accepted
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model of sex-biased gene evolution. We also show that the deficit of testis-expressed
genes that is observed in Drosophila seems to be stronger for highly expressed genes. In
fact, for very lowly expressed genes, we observe a small excess of testis-expressed genes
on the X. We attempt to discuss this pattern in view of what is currently known about the
evolution of sex-biased gene expression.