Gene expression and evolution

The control of the spatial expression of gene products is one of the most important and complex processes contributing to the embryogenesis of metazoans. This control can be achieved through many levels of regulation. Transcription factors such as homeodomain proteins are important regulators of the phenotypic output of cells in which they are expressed. The Hox genes are a genomic cluster of homeodomain containing master control proteins, each of which is expressed in, and controls the morphology of a specific anteroposterior domain. While transcription factors have long been known to be an important type of regulatory molecule, recent discoveries have shown that there is always more out there for biology to teach us. The discovery of microRNAs has led to a whole new paradigm, in which the expression of many genes is controlled by the activity of tiny RNAs that were once invisible to researchers. The Hox complexes of bilaterian animals typically contain a few microRNAs, but nearly all of them have the mir-10 gene. This ancient regulatory molecule is conserved in its sequence, genomic position, and to some extent its spatial expression over evolutionary time scales. In Drosophila melanogaster, this gene produces two mature microRNA products, each of which has strong complementarity to conserved sequences in Hox genes. These microRNAs are also expressed in patterns that are complementary to these potential target genes. Despite these correlations, disruption of the normal expression of the mir-10 gene does not strongly effect the expression of these genes, but may instead function to canalize their expression. Appendages are structures common to a wide variety of animal body plans. When compared between distantly related animals there are no obvious structural characteristics which suggest they are homologous, yet they are patterned by similar transcription factors. Since these structures are not likely ancestral, their seemingly conserved patterning mechanism may have been acquired through independent co-option of a pre-existing network of genes for a similar purpose. The core genes responsible for patterning the proximodistal axis of appendages are also expressed in domains of the anterior neurectoderm that are conserved among distantly related animals and function to pattern the anteroposterior axis of the head. This ancient gene network was likely co-opted in order to pattern the axis of appendages during the early evolution of bilaterian animals. The advent of multicellularity has provided our planet with a rich variety of organisms. It is unknown what was necessary to allow single celled organisms to give rise eventually to the people that would research them, but there are many proteins that are assumed to be important for this process. Multicellular organisms are composed of many different types of cells, and homeodomain transcription factors are often involved in the specification of these cell types. The nearest relatives to animals are choanoflagellates, which are simple single celled organisms that sometimes exist in colonies. Although most animals have a specific set of homeodomain containing genes, most these are either evolutionary novelties specific to animals or have been secondarily lost in choanoflagellates. For this reason it is unknown what role homeodomain proteins played in the process of multicellularization that gave rise to animals, but further studies may provide exciting answers