Although sex chromosomes have evolved independently in many taxa, they are overall very similar, suggesting broadly similar evolutionary pathways. In particular, the acquisition of a male-determining gene on the proto-Y chromosome (or a female-determining gene on the proto W, in the case of female heterogamety) is quickly followed by the loss of recombination and consequent degeneration of genes on this newly formed Y/W chromosome. In XY systems, this is coupled with the acquisition of dosage compensation mechanisms that increase the expression of X-linked genes in males, to compensate for the missing (or non-functional) Y-linked copy. Given the similarities between the degenerated Y and W chromosomes, one would expect to find such dosage compensation mechanisms in ZW females, as W-linked copies of genes are also often missing or non-functional. However, the two independently evolved ZW systems that have so far been studied in detail, birds and Lepidoptera, lack dosage compensation. Although absence of dosage compensation may be a general feature of female-heterogametic species, it is also possible that the absence of DC mechanisms in these two clades is a mere coincidence, and the analysis of other independently evolved ZW systems is needed to confirm the pattern. We are analysing an independently evolved ZW system to test for this: Tephritids are dipteran insects closely related to Drosophila of which one genus, Tephritis, has a conserved ZW system. We are using Illumina sequencing to sequence the transcriptome of Tephritis species caught in the wild, in order to compare Z-linked and autosomal expression in males and females. If dosage compensation mechanisms are absent in these species, we expect ZW females, but not ZZ males, to show decreased Z-linked expression relative to the autosomal expression.

Beatriz Vicoso

University of Edinburgh, UK – PhD in Evolutionary Biology, 2008
Federal University of Rio de Janeiro, Brazil – Bachelor Degree in Genetics, 2003

Current/Past Research
Dosage compensation in Female-heterogametic species We are sequencing the transcriptome of males and females of heterogametic species to test for the presence of dosage compensation in these ZW systems.
X-chromosome evolution in Drosophila (2004-2008) PhD research project, supervised by Prof. B. Charlesworth and Dr. P. Haddrill: We compared coding sequences from different species of Drosophila to test if the X chromosome has higher rates of adaptive evolution than the autosomes (Faster-X), and computed theoretically which conditions can lead to faster-X evolution. The accumulation of sex-biased genes on the D. melanogaster X chromosome was also investigated theoretically and using published expression data.
The birth of new genes on the D. melanogaster Y chromosome (2003) Undergraduate Research project, supervised by Prof. A.B. Carvalho: By blasting a set of proteins against the unassembled fraction of the D. melanogaster genome, potential Y-linked genes were identified, and experimentally tested for sex-linkage.

Vicoso, B. and Bachtrog, D. (2009) Progress and prospects toward our understanding of the evolution of dosage compensation. Chromosome Research 17(5).
Vicoso, B
. and Charlesworth, B. (2009) Effective population size and the Faster-X Effect: an Extended Model. Evolution 63(9):2413-26.
Mank, J., Vicoso, B., Berlin, S. and Charlesworth, B. Effective population size and the Faster-X Effect: Empirical Results and their Interpretation. Evolution PMID: 19796145
Vicoso, B. Charlesworth, B. (2009) The deficit of male-biased genes on the D. melanogaster X is expression-dependent: a consequence of dosage compensation? J Mol Evol. 68(5):576-83.
Vicoso, B.,
Charlesworth, B. (2009) Recombination may affect polymorphism levels on the X and the autosomes in African populations of Drosophila. Genetics 181(4):1699-701.
Vicoso, B., Haddrill, P.R., Charlesworth, B. (2008) A multi-species approach for comparing sequence evolution of X-linked and autosomal sites in Drosophila. Genetics Research, Genetics Research 90:421-431.
Vicoso B, Charlesworth B (2006) Evolution on the X chromosome: unusual patterns and processes. Nat Rev Genet 7: 645–653.