05/11/07 - Ciclo de Palestras Pós Graduação em Estatística UFRJ

["Alexandra M. Schmidt" <alex@im.ufrj.br>]

Dando continuidade ao Ciclo de Palestras do Programa de Pós-Graduação em
Estatística do IM-UFRJ teremos, nesta 2a feira (excepcionalmente) , 05/11, as
15:30h o

Palestrante:  Hedibert F. Lopes (Chicago)

Titulo: "Título: Sex chromosome evolution and gene expression in Drosophila
spermatogenesis (Resumo) "

Local: Sala C-116, Centro de Tecnologia, Cidade Universitária.

Resumo:
Background: The Drosophila genome is constituted of a group of genes that are
distributed in diferent chromosomes: three pairs of autosomes and the sex
chromosomes (X and Y). The study of sex chromosome evolution main purpose is
to understand which genes are localized in and out of the sex chromosomes. In
addition, evolutionary biologists are interested in understanding the
mechanisms and selective forces driving such distribution. In the last 
decade,
one important observation regarding the evolution of sex chromosomes is the
unidirectional movement of genes out of the X chromosome to the autosomes.
Interestingly, those genes are especially expressed (active) in the male
reproductive tract, the testis. One explanation for the unidirectional
movement of genes is the possible X inactivation during spermatogenesis
(spermatozoa formation and development). In other words, if X is inactivated
then spermatogenic expressed genes that move out of the X chromosome to
autosomes are expected to be selected during evolution. However, the
spermatogenic X-inactivation has not yet been proved in Drosophila. There are
only indirect evidences that strongly suggest that the X chromosome ceases 
its
expression during spermatogenic meiotic stage. Moreover, even if the X 
becomes
inactive, the unidirectional movement of genes out of X chromosome is not
necessarily caused by the decrease of expression level. Results: Here we
investigate meiotic X-inactivation in Drosophila melanogaster by monitoring
X-linked and autosome gene expression during developmental phases of
spermatogenesis. Cells from two di erent phases were collected:
spermatogonias, the mitotic cells; and spermatocytes, the cells in meiotic
division. Genes that are putatively over or under expressed in meiosis were
selected by quantifying and comparing the mRNA level in the two di erent
phases using Microarray analysis (Genechip arrays from A ymetrix). Multiple
hypothesis testing and inference were conducted by Bayesian hierarchical
modeling (BHM) in order to classify 18,000 genes (3,000 X-linked genes and
15,000 autosome genes) as over, under, equally expressed. The main motivation
for using this modeling structure is to avoid inconsistencies of standard
frequentist multiple hypothesis testing commonly encountered in the analysis
of gene expression. In agreement to Xinactivation hypothesis, we observed
depletion and enrichment of X-linked genes over and under expressed in
meiosis, respectively. In other words, the expression of X-linked genes is
under represented in meiosis compared to autosome genes. In order to
investigate if X-inactivation is one of the selective forces driving the
unidirectional movement of genes out the X chromosomes, we compared gene
expression of a specific set of genes that moved between chromosomes during
Drosophila genus evolution. We show that, based on an extension of the
previous BHM, genes moving out of the X-chromosome have a higher posterior
probability of being under-expressed in meiosis than genes moving out of the
autosomes. Conclusion: By properly accounting for the uncertainties regarding
the gene expression measurements and interactions, as well as gene biology,
our modeling approach explicitly and probabilistically quantifies X-linked
gene expression, pointing in favor of the X-inactivation hypothesis. In
addition, pioneer evidence of meiotic X chromosome under-expression as 
driving
force for gene movement out of the X chromosome is provided.

   Atenciosamente,

Secretaria de Pós-Graduação do IM / UFRJ 
hp: www.pg.im.ufrj.br 
e-mail: posgrad@pg.im.ufrj.br 
Tel. / Fax: (21)2562-7374
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Alexandra Mello Schmidt, PhD
Professora Adjunta
Instituto de Matemática - UFRJ
Departamento de Métodos Estatísticos
Caixa Postal 68530 Rio de Janeiro - RJ 
CEP:21.945-970 Brasil
Tel: 0055 21 2562 7505 Ramal (Extension) 204
Fax: 0055 21 2562 7374

http://www.dme.ufrj.br/~alex
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There is no time to lose. Cash your dreams before they slip away. 
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