Webpage of Masafumi Nozawa

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Research

8. Evolution of sex chromosomes

  Sex chromosomes have independently evolved in many different groups of organisms. While sex chromosomes provide stable sex determination to organisms irrespective of their surrounding environments, Y chromosomes generally lose a lot of genes due to recombination suppression, which could potentially be deleterious to organisms. To solve this paradox, I have used the neo-sex chromosome systems in Drosophila species and investigated the early stages of sex chromosome evolution. 

 

Nozawa et al. 2014 MBE [pdf]

Nozawa et al. 2016b Nat Commun [pdf]

Nozawa et al. 2018 GBE [pdf]

   

7. Origins and evolution of microRNA genes

  MicroRNAs (miRs) regulate many genes at the posttranscriptional level. To understand their evolutonary origins, we have identified miR genes in the Drosophila and plant genomes by bioinformatics approaches. The results showed that many miR genes appear to have originated from random hairpin structures in Drosophila species, whereas many of them have been derived from duplication of preexisting miRNA genes. It is possible that the difference in target gene recognition explain this different evolutionary origins.

 

Nozawa et al. 2010a GBE [pdf]

Miura et al. 2011 GBE [pdf]

Nozawa et al. 2012 GBE [pdf]

Nozawa and Nei 2016 EEB [pdf]

Nozawa et al. 2016a GBE [pdf]

   

6. Reliabilities of statistical methods for detecting positive selection

  There are many statistical methods for identifying positive selection in protein-coding genes. However, the reliabilities of these methods are not well studied. We have therefore studied the reliabilities of some statistical methods which are commonly used by using a computer simulation and real data analysis. The results showed that the methods give significant results even when no positive selection is operating and often fail to identify the true positive selection. It is important not to be overenthusiastic about statistical signature of positive selection.

 

Nozawa et al. 2009a PNAS [pdf]

Nozawa et al. 2009b PNAS [pdf]

Nozawa et al. 2010b PNAS [pdf]

Nei et al. 2010 ARGHG (review) [pdf]

   

5. Copy number variation of chemosensory receptor genes

  There is a common belief that the physiological requirement for a species is the major factor for determining the numbers of chemosensory receptor genes. However, gene copy number can also change by random duplication and deletion (genomic drift). We have therefore investigated copy number variation (CNV) of chemosensory receptor genes. Our results suggest that genomic drift plays important role for generating intra- and interspecific CNVs of chemosensory receptor genes.

 

Nozawa et al. 2007 PNAS [pdf]

Nozawa and Nei 2008 CGR [pdf]

Nei et al. 2008 NRG (review) [pdf]

   

4. Evolutionary dynamics of olfactory receptor genes

  Olfactory receptor (OR) genes are important to detect various odorants in nature and form one of the largest multigene families in animals. How these large families have evolved and how dynamically the gene number has changed? We revealed the difference of evolutionary patterns of OR genes between Drosophila and mammals.

 

Nozawa and Nei 2007 PNAS [pdf]

Nei et al. 2008 NRG (review) [pdf]

   

3. Origin and amplification mechanisms of repeat sequences

pict6.jpg

  Repeat sequences occupy substantial proportion of genomes. How tandem and interspersed repeat sequences are accumulated and differentiated during the evolution? We have therefore studied the evolutionary mechanisms of a repeat sequence which was newly described in Drosophila ananassae subgroup. The results showed that the evolutionary mechanisms of a repeat sequence can potentially change during the evolution. [more details in Japanese]

 

Nozawa et al. 2006 MBE [pdf]

Nozawa 2006 PhD Thesis [pdf]

   

2. Functional differentiation of siren after gene duplication

pict4.jpg

  After gene duplication, one of the duplicated genes can acquire new function. I am interested in the evolutionary process after gene duplication. Here, I have investigated the functional differentiation of siren, a chimeric gene of CG11779 and Adh. The molecular evolutionary analyses indicated that the siren has acquired new function after duplication from these parental genes.

 

Nozawa 2006 PhD Thesis [pdf]

   

1. A new mechanism of gene generation by retroposition

pict5.jpg

  How new functional genes are generated is one of the central issues in evolution. We found a novel chimeric gene, siren in some species of Drosophila. This gene was generated by retrotransposons during the evolution. Analyzing this chimeric gene, we revealed a new potential of retroposition for generating promoter sequences which are essential for new genes to be functional.

[more details in Japanese]

 

Nozawa et al. 2005 Genetics [pdf]

Nozawa 2006 PhD Thesis [pdf]