Genetic mechanisms of coral metamorphosis identified

Date:
March 10, 2022
Source:
Tohoku University
Summary:
Researchers discovered the gene expression regulation mechanisms
that drive coral transition from a floating larvae to one that
sits sedentary in reefs.



FULL STORY ========================================================================== Researchers have identified the gene expression regulation mechanisms
that drive the metamorphosis of coral from larvae that float freely in
the ocean to sedentary adult reef builder adults.


==========================================================================
A study describing the researchers' findings appeared in the journal
Zoological Letters on January 25, 2022.

The moment that corals metamorphose from free-swimming (planktonic)
larvae to sedentary polyps is perhaps the momentous event in this animal's life, producing a radical and abrupt change in both body structure and behavior. When larvae encounter certain cues or stimuli from an underlying surface (substratum), they stop swimming and rest on the substratum on
one end and take on a round shape. In the case of Acropora, these cues
come from a buildup (biofilm) of algae. If the larvae receive sufficient stimuli from this substratum, they take the next step and undergo a
radical and irreversible differentiation of cells, including a stable attachment to the substratum.

However, the molecular processes at the genetic and cellular level of
this metamorphosis have long been something of a mystery.

Other researchers have in the past managed to identify a critical period
of time when the cells of the coral genus of Acropora (whose more than
150 species include some of the main species responsible for the building
of reefs) are committed to this transformation. From this point on,
these corals are unable to revert back to their pre-metamorphosis state
-- what coral scientists call a "point of no return." But quite what
happens during the point of no return at a deeper level has until now
remained unknown.

To investigate further, the authors of the study analysed the changes in
the transcriptome -- the complete set of messenger RNA (mRNA) molecules produced (expressed) by an organism or cell -- before and after the
point of no return in an Acropora species. Messenger RNA molecules play
the key role of copying ("transcribing") the genetic "recipe" encoded in
DNA for making and taking it to the protein factories in the cell known
as ribosomes. Analysis of the coral transcriptome at the moments before
and after metamorphosis thus should tell scientists what the difference
is in genetic recipes that are being activated ("gene expression")
over this period.



==========================================================================
The researchers collected samples from colonies of the species
Acropora tenuis from Sesoko Island in Japan's Okinawa Prefecture. The
corals were then kept in tank conditions designed to approximate their
natural setting. Metamorphosis was induced using the application of a neuropeptide known to prompt this process and further samples intended
for RNA extraction were taken at multiple different times before and
after the point of no return.

The RNA was then sequenced, offering an analysis of gene expression
changes at a fine time resolution that showed a series of molecular
mechanisms that are likely involved in the irreversibility of
metamorphosis.

The researchers found that alterations in signal perception occurs through
a change in expression of a group of genes that regulate G-protein-couple receptors (GPCRs), in particular the GABA receptor proteins and "Frizzled" proteins, at the early stage of metamorphosis, as well as those driving
the breakdown of certain proteins.

GPCRs are one of the largest groups of receptors on the surface of
cells and act as a sort of mailbox, receiving messages from proteins,
sugars, fats and other entities. These messages tell the cell something
about their environment or pass on information from other cells, and
activate other messenger molecules within the cell, directing some sort
of response. Frizzled receptors play a key role in embryo development,
while GABA receptors respond to the neurotransmitter GABA, a compound
that works to inhibit particular activities and to prompt cells to differentiate (take different forms).

In this case, the receptor-mediated cellular response to stimuli
involves cell differentiation and the breakdown of certain proteins that
in combination result in drastic changes that make the metamorphosis
process irreversible.

The researchers also suggest that the GABA receptors may play a role in
the larvae's search for an appropriate substratum before metamorphosis, ensuring a proper environment for the adult form. This ability is lost
as metamorphosis proceeds, as such searching is no longer needed.

"An intriguing unexpected finding was gene expression of green
fluorescence proteins after the point of no return," said Shinichiro
Maruyama an evolutionary biologist at the Tohoku University Graduate
School of Life Sciences and co-author of the study, "implying there is
some sort of key physiological and ecological role of fluorescence going
on at early developmental stages that is completely understudied, for
example, studying the attraction of symbiotic algae." Having identified
the candidates for genes involved in metamorphosis, the scientists want
to see whether modification of these same genes can alter the point
of no return. In addition, they stress that their findings are limited
to corals in a controlled laboratory setting, and want to explore how
things change in a real-world setting.


========================================================================== Story Source: Materials provided by Tohoku_University. Note: Content
may be edited for style and length.


========================================================================== Related Multimedia:
* A_model_reef-building_coral_Acropora_tenuis ========================================================================== Journal Reference:
1. Yuu Ishii, Masayuki Hatta, Ryusaku Deguchi, Masakado Kawata,
Shinichiro
Maruyama. Gene expression alterations from reversible to
irreversible stages during coral metamorphosis. Zoological Letters,
2022; 8 (1) DOI: 10.1186/s40851-022-00187-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220310115148.htm

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