We’ve studied in high school that ontogeny recapitulates phylogeny. Translating it into simple words, we say that embryonic development replays the entire evolutionary history of an organism. Instead, scientists have paved a new way to figure out how animals evolved. They compared regulatory networks that control gene expression patterns, particularly embryonic ones, across species. The research appeared in the journal of Nature explains the concept. The research was conducted by researchers at the Stowers Institute for Medical Research.
In the study, the investigators showed that the sea lamprey exhibits a pattern of gene expression that is evocative of its jawed cousins, who evolved much, much later. Note that sea lamprey or Petromyzon marinus is a survivor of ancient jawless vertebrates.
The Hox genes function as a molecular ruler and determine where along the anterior-posterior (AP) axis an animal will place a particular feature or appendage. This respective, new research means that genetic program used by jawed vertebrates, like fish, mice, and us, was up and running ages before a vertebrate ever possessed an identifiable face.
According to the researchers, Hox genes regulate the character or shape of the tissues that control head or facial features. Previous researches addressed how factors make unique structures. For example, what makes an arm different from a leg? And this research reveals the common role that similar sets of genes play in creating a basic structural plan.
The research team focused on sea lamprey because of its fossil record. It showed that its ancestors emerged from Cambrian silt approximately 500 million years ago. This is around 100 million years before jawed fish ever swam onto the scene. The question was could the gene regulatory network in the hindbrain, that constructs the “modern” vertebrate head, have originated in animals that lack those structures?
What did the researchers do?
The researchers created so-called “reporter” genes from stretches of regulatory DNA flanking a specific Hox gene in zebrafish or mice. These stretches were linked to fluorescent tags and inserted into an experimental animal. The tagged reporters, upon entry into the cell, glow in tissues where the gene is expressed or activated. The researchers chose this particular series of Hox reporters because of their fluorescent activity in embryos of a jawed fish. They fluoresce in adjacent rainbow stripes up and down the embryonic hindbrain.
The researchers inserted the very same reporters into lamprey embryos. The lamprey embryos exhibited the same rainbow pattern of Hox reporters as the jawed fish. Interestingly, the pattern had exactly the same order along the AP axis of the hindbrain.
The research team was surprised to see any reporter expression in lamprey. These findings led the researcher to conclude that the gene regulatory network, governing segmental patterning of the hindbrain, likely evolved prior to the divergence of jawed vertebrates.
In mouse and zebrafish, short stretches of DNA in one Hox reporter (Hoxb3) formed a landing pad known by a DNA-binding protein, flipping on the gene. Moreover, the reporter mutant in those sequences didn’t glow in the hindbrain. This reveals that a control switch has been around for a very long time.
The results of the study suggest that regulatory circuits controlling hindbrain patterning were likely fixed in place in ancient vertebrates. However, embryos of lamprey and other vertebrates show many striking similarities, so it makes sense that there are features common to all.