Is the genomic hourglass supported in the genes?
May 13
Within developmental biology a distinct and intriguing pattern exists, the “Genomic Hourglass”.
This pattern describes how different species from the same phylum vary in similarity during development in a predictable way, such startlingly clear morphological similarities in such diverged species shocked early biologists.
Recognition of this pattern is nothing new, in 1828 a German biologist who had forgotten to label two of his remarked that;
“At present I am unable to determine the genus to which they belong. They may be lizards, small birds, or even mammals.”
Genomic Hourglass… Without genomic proof?
While the “Genomic Hourglass” morphological pattern has been recognised and academically discussed at length, research into gene-expression during development had not taken place. Gene-expression should logically be closely tied to developmental morphology, as it is the expression of the genes that allows biological processes to occur.
In order to measure gene-expression (the activiation and deactivation of genes – similar to opening a computer program to do work, then closing it when it’s done) during embryonic development, Kalinka et al. (2010) took six species of Drosophila (a commonly used research animal) and sampled 3000+ genes at 60 different points of their development. For each gene sampled, a gene-expression timeline was created, as shown via the example on the right.
Not every gene sampled had the same “variability” (likelihood to change). Genes that change rapidly are much more likely to be different between species, where genes that are stable are unlikely to have changed between the 6 different Drosophila species sampled from. This gene variability can be used to describe the “age” of the genes, as less variable genes are likely to be “older”, as they are less likely to change as compared to more variable and therefore “younger” genes.
The results showed a relationship between the percentage of similar genes activated at a particular point in development across the 6 species of Drosophila and the “age” of the genes. This revealed a “gene activation hourglass”, where newer genes were active at the beginning and end of development, with older genes active in the middle. This pattern correlates well with the ”morphological hourglass” pattern.
This research strongly suggests a link between gene age, gene activation and developmental morphological differentiation between similar species and that natural selection acts to convert patterns of gene expression during the middle of embryo development. However, further work on species with less similarity could give more strength to their findings.
Interestingly, researchers were able to determine gene functionality for each of the 3000+ gene’s sampled across the six species of Drosophila and noted that genes related to core developmental processes conformed strongly to the hourglass model, whereas genes involved with secondary metabolism, immunological process’ and responses to wounding stress conformed the least.
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