4.1 Adaptive mechanism of hearing related genes
Rhinolophids arguably possess the most sophisticated echolocation
systems, and can emit relatively long calls adapted to detect and
classify the wing beats of insects. They are heavily reliant on hearing
for a variety of ecologically important roles. Previous studies have
documented that hearing-related genes are predominantly evolutionarily
conserved in mammals (Kirwan et al.,
2013). Comparative analysis of auditory perception can help to
elucidate the molecular basis that underpins different auditory
capabilities (Dong et al., 2016). Dong et
al. (2016) revealed some hearing-related genes undergone natural
selection associated with the evolution of specialized constant
frequency echolocation, probably resulting from the extreme selectivity
used in the auditory processing by these bats. We performed comparative
transcriptome for species with two echolocation modes, and tested some
genes under selective pressure for the species within themacrotis group.
We selected some specific amino acid changes in six PSGs (CRYM ,FOXM1 , MAP6 , PYCARD , SLC35A2, andWRB ) related to hearing. CRYM was tested in R.
marshalli , and the previous functional study found mutations inCRYM may cause hearing loss and the expression of CRYM is
essential for maintaining cochlear cells and preserving normal hearing
(Abe et al., 2003;
Hosoya et al., 2016). We testedFOXM1 , MAP6 , and WRB in R. episcopus .
Previous studies showed that FOXM1 activities are modulated in
the mouse cochlea, and FOXM1 is associated with cell cycle
control and essential for the transcriptional response during DNA
damage/checkpoint signaling
(Sanchez-Calderon et al., 2010).MAP6 is involved in molecular transport, nervous system
development, and function, and is related to reflect auditory fear
conditioning (Hong et al., 2013). The
absence of WRB from inner hair cells results in significantly
reduced intracellular levels of otoferlin, thus causing hair cell
synaptic disruption and hearing impairment
(Pangrsic and Vogl, 2018). PYCARDand SLC52A2 were tested in R. siamensis . Thereinto,PYCARD was related to hearing-loss and tinnitus
(Wei et al., 2010). Mutation inSLC52A2 was associated with spinocerebellar ataxia with blindness
and deafness type2 (O’Callaghan et al.,
2019). We tested different hearing related genes in different
specieslow, and this may imply these species adapted to
the environment through the different evolutionary mechanisms.
Particularly, we found one PSG SPRY2 in R. osgoodi , who
emit echolocation with normal frequency, differ from other species
within the macrotis group. SPRY2 is related to hearing
loss and plays an important role in the regulation of endochondral bone
formation, which may influence early inner ear development
(Joo et al., 2016;
Wright et al., 2015;
Yousaf et al., 2018). The positive
selection sites detected in SPRY2 showed similar amino acid
changes with R. pusillus , but different with other species. This
result indicated that SPRY2 plays an important role in the
echolocation development, and may experience similar acoustic evolution
in R. pusillus , thus promote R. osgoodi and R.
pusillus evolved similar acoustic characteristics.
Although the function of specific amino acid variants in these genes is
still unclear, our results still indicate the potentially important
function in acoustic development. These genes may regulate the neural
activity or the formation process of inner ear structure, and then
affect the sensitivity of specific frequency, that is, affect the
echolocation signal for these bats.