The Genetics of Ear Evolution in Mammals

Mammals have vast capabilities when it comes to hearing. Bats have high-frequency echolocation calls, while whales produce low frequency sounds. Dogs have a range of hearing that is twice as wide as humans. A group of science researchers discovered two new genes that are connected to hearing.
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Hearing Genes and Adaptations

It is understood that hearing adaptations are based on genetics. A group of scientists directed by Lucia Franchini, from the National Council of Scientific and Technological Research (CONICET), located in Buenos Aires, Argentina have pursued an objective to analyze the genetic foundations of the evolution in a mammal’s inner ear. Their latest discoveries identified two new hearing genes.  

The primary process of hearing in different mammals is identical. A mammal’s auditory system is represented by a middle ear comprised of three ossicles:

1. The incus (A.K.A. the anvil)
2. The malleus (A.K.A. the hammer)
3. The stapes (A.K.A. the stirrup). This part channels sound into the ear.

The researchers concentrated on the inner ear. The section transforms the shifts in sound intensity into electrical signals which is processed by the brain. The inner ear has a snail-shaped cochlea which changes sound waves into nerve impulses, including an auditory organ that has inner hair cells (IHCs) and outer hair cells (OHCs).

IHCs collect and deliver sound information. They are fundamental transducers, which discharge glutamate to energize the sensory fibers in the cochlear nerve. The outer hair cells intensify sound information. 
  
Study Conducted on Hearing Genes

The functional and form/structural changes in a mammal’s inner ear is what gives it unique hearing abilities. Approximately 13%, or 165 inner ear genes have been chosen for adaptation. An analysis of hearing genes and how they operate in mice was conducted. They concentrated on two formerly unidentified genes: STRIP2 (Striatin Interacting Protein 2) and ABLIM2 (Actin Binding LIM domain 2).  

There were auditory functional studies conducted with STRIP2 and ABLIM2, on mice by using two techniques that show different conclusions of OHC versus IHC/neuronal dysfunction in the cochlea. To assess how trustworthy the hearing system was, the researchers recorded ABRs (Auditory Brainstem Responses) that are “sound-evoked potentials” created by neuronal circuits in auditory pathways. 

They found that STRIP2 probably has a practical duty in the first synapse between IHCs and nerve fibers. When they are at the cochlear sensory epithelium, they found a considerable reduction of auditory-nerve synapses.   
 
The researchers determined that STRIP2 had a strong positive selection in the mammalian genealogy, and has an influential function of the inner ear. Based on the evolution and functional studies, the changes that this gene underwent showed that it was able to adapt. The study also shows evidence that evolutionary approaches, combined with research, can help researchers discover essential elements in how organs and tissues function.   

If you, or a loved one, are in need of a hearing test or new hearing aids, contact us at Pure Sound Hearing Aids to schedule an appointment with one of our hearing instrument specialists.  

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