Relatedness of distinct tissues

Read Also

Aside from grouping genes according to the similarity of gene expression patterns, our dataset can be used to define relatedness among tissues based on the similarity of the set of genes expressed in them. The analysis of tissue relatedness was restricted to terminally differentiated tissues that show the greatest diversity. We created a matrix that links tissues sharing expression of significant numbers of genes. The strength of a link is proportional to the number of commonly expressed genes. We used a force directed layout to produce the matrix (Materials and methods) and colored the terms in the resulting 2D network plot based on their assigned organ system

Most organ systems cluster tightly together as exemplified by the grouping together of terms of the same color. For examples, the muscle system, PNS and CNS form tight and relatively isolated groups. The Bolwig’s organ is isolated from the rest of the tissues highlighting its distinct set of expressed genes. Similarly, tissues such as germ cells and amnioserosa, ring gland, stomatogastric nervous system, malpighian tubule, midgut and garland cells share relatively few expressed genes with other tissues. In contrast, the genes expressed in the posterior spiracle, despite forming their own cluster ), appear to be components of many other patterns. As noted above, yolk nuclei, fat body and plasmatocytes share a number of genes. Interestingly, in this representation, this set of structures is weakly related to lymph gland, which in turn shares expressed genes with the circulatory system. Many of the genes expressed in the oenocyte appear to be pleiotropic and are also expressed in crystal cells, lymph gland, ring gland, midline, gonad and circulatory system. However, the oenocytes must have a number of expressed genes unique to itself judging from its relative isolation. The largest, most interconnected set of structures roughly corresponds to the epithelial pattern defined by clusters 5,6 and 7R. Notably the salivary gland duct is isolated from the salivary gland body despite their common organ system assignment reflecting their functional divergence and differential gene expression requirements. The salivary gland duct and trachea fall closely together reflecting their shared expression of genes related to cuticle deposition. Anal pads, in terms of gene expression, are more similar to hindgut then to other epidermal structures.

In terms of developmental origin, neural tissues appear to be more related to tissues derived from mesoderm than to other ectodermal tissues suggesting that specification of neuronal versus epidermal cell fate leads to profound genome-wide changes in transcriptional program. Components of the digestive system, midgut, hindgut and foregut have few genes in common despite their functional and spatial relationship reflecting their different developmental origin.