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MBE Advance Access published online on August 29, 2003
Molecular Biology and Evolution, doi:10.1093/molbev/msg213
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2003; all rights reserved
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Accepted July 7, 2003
© 2003 Society for Molecular Biology and Evolution

Original Articles

Intervening Sequences in Paralogous Genes: A Comparative Genomic Approach to Study the Evolution of X Chromosome Introns

Barbara Cardazzo 1, Luca Bargelloni 2, Luisa Toffolatti 1, and Tomaso Patarnello 3*

1 Dipartimento di Biologia, Università di Padova, 35121 Padova
2 Facoltà di Medicina Veterinaria "Agripolis", Università di Padova, 35121 Padova
3 Dipartimento di Biologia, Università di Padova, 35121 Padova; Facoltà di Medicina Veterinaria "Agripolis", Università di Padova, 35121 Padova

* To whom correspondence should be addressed. E-mail: patarnel{at}civ.bio.unipd.it.


  Abstract

The enlargement of the genome size and the decrease in genome compactness with increase in the number and size of introns is a general pattern during the evolution of Eucaryotes. Among the possible mechanisms for modifying intron size, it has been suggested that the insertion of transposable elements might have an important role in driving intron evolution. The analysis of large portions of the human genome demonstrated that a relatively recent (50-100 million years ago) accumulation of transposable elements appears to be biased, favouring a preferential insertion of LINE 1 transposons into sex chromosomes rather than into autosomes.

In the present work the effect of chromosomal location on the increase in size of introns was evaluated with a comparative analysis performed on pairs of human paralogues genes, one located on the X chromosome and the second on an autosome. A phylogenetic analysis was also performed on the X encoded proteins and their paralogous in order to confirm orthology-paralogy and to approximately estimate the time of gene duplication. Statistical analysis of total intron length for each pair of paralogous genes provided no evidence for a larger size of introns in the gene copies located on the X chromosome. On the opposite, introns of autosomal genes were found to be significantly longer than introns of their X-linked paralogues. Likewise, LINE1 elements were not significantly more frequent in X chromosome introns, while the frequency of SINE elements showed a marginally significant bias toward autosomal introns.

Key Words: intron size, X chromosome, genome evolution, repeat elements


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