title: A multilevel layout algorithm for visualizing physical and genetic interaction networks, with emphasis on their modular organization.
abstract: BACKGROUND
  Graph drawing is an integral part of many systems biology studies, enabling visual exploration and mining of large-scale biological networks. While a number of layout algorithms are available in popular network analysis platforms, such as Cytoscape, it remains poorly understood how well their solutions reflect the underlying biological processes that give rise to the network connectivity structure. Moreover, visualizations obtained using conventional layout algorithms, such as those based on the force-directed drawing approach, may become uninformative when applied to larger networks with dense or clustered connectivity structure.
  
  
  METHODS
  We implemented a modified layout plug-in, named Multilevel Layout, which applies the conventional layout algorithms within a multilevel optimization framework to better capture the hierarchical modularity of many biological networks. Using a wide variety of real life biological networks, we carried out a systematic evaluation of the method in comparison with other layout algorithms in Cytoscape.
  
  
  RESULTS
  The multilevel approach provided both biologically relevant and visually pleasant layout solutions in most network types, hence complementing the layout options available in Cytoscape. In particular, it could improve drawing of large-scale networks of yeast genetic interactions and human physical interactions. In more general terms, the biological evaluation framework developed here enables one to assess the layout solutions from any existing or future graph drawing algorithm as well as to optimize their performance for a given network type or structure.
  
  
  CONCLUSIONS
  By making use of the multilevel modular organization when visualizing biological networks, together with the biological evaluation of the layout solutions, one can generate convenient visualizations for many network biology applications.
authors.affiliation: Department of Mathematics, FI-20014 University of Turku, Turku, Finland. tero.aittokallio@fimm.fi.
authors.forename: Johannes
authors.initials: J
authors.lastname: Tuikkala
authors.forename: Heidi
authors.initials: H
authors.lastname: Vähämaa
authors.forename: Pekka
authors.initials: P
authors.lastname: Salmela
authors.forename: Olli S
authors.initials: OS
authors.lastname: Nevalainen
authors.forename: Tero
authors.initials: T
authors.lastname: Aittokallio