Abstract
Multicellular life is based on the ability of cells to divide, differentiate, cooperate and die in a controlled and organised manner, generating and maintaining an organism. The temporal distribution of division, differentiation and death determines the cellular composition of the organism at any particular point in time. Like these ontogenetic events, phylogenetic development takes place with the changes in total cell numbers, the allocation of these cells to different tissues and the disappearance of certain tissues. Fractal properties of complex networks, a result of growth, can be estimated by box counting, whereby the topological properties of the network are mapped by changing the resolution of examination, that is changing the size of the boxes used to identify and group network components. Here we develop the concept of cellular box-counting, referring to the fact that cells can be grouped on various levels of hierarchy and these various levels can be interpreted as boxes of different linear sizes. We apply the method to data representing distinct stages and groups of evolution of life and interpret the network properties of brown algae, green plants and animals. The results are in agreement with previously established values of degree exponent of biological networks and provide clues to the differences in the network organization of multicellular life.
Highlights
Molecular pathways responsible for cellular complexity in a given multicellular organism are those that arose and have been selected during evolution leading to that organism
By recording the relationships and cellularity of these stages a biological network (Albert, 2005) representing the evolutionary development of multicellular organisms could be drawn. In this fractal network a node represents a cell belonging to a given tissue in the examined organism, the tissue being the virtual descendent of a single cell in an earlier organism, and the cell being the precursor of a tissue developing in an organism to appear later (Figure 1A)
The different relationships between fractal dimension and degree distribution exponent in three independently evolved multicellular groups should correspond to structural differences of cellular and molecular networks in these phylogenetic groups
Summary
Molecular pathways responsible for cellular complexity in a given multicellular organism are those that arose and have been selected during evolution leading to that organism. By going backwards in time, or alternatively by taking contemporary representative organisms of those backward steps, ancestor cells and tissues, of every organ and organ system of the examined organism can be traced These tissues are the result of different expression patterns, different branches of molecular pathways. By recording the relationships (lineage) and cellularity of these stages a biological network (Albert, 2005) representing the evolutionary development of multicellular organisms could be drawn In this fractal network a node represents a cell belonging to a given tissue in the examined organism, the tissue being the virtual descendent of a single cell in an earlier organism, and the cell being the precursor of a tissue developing in an organism to appear later (Figure 1A).
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