There exists a seeming
paradox among the many species that constitute vertebrates – animals with
backbones. The fact is that although so
many of these species share similar families of protein-coding genes, they are
very distinct in terms of their physical characteristics – their phenotypes. There is no clear explanation of how this phenomenon
can be defined on the molecular level.
What has been well
established is the fact that over the evolutionary time scale for these various
species – approximately 350 million years – significant changes have occurred
in their respective transciptomes. A
transcriptome represents the entire set of all RNA molecules within a cell
including Messenger RNA (mRNA), Ribosomal RNA (rRNA) and Transfer RNA (tRNA)
and the so-called “non-coding” RNA. It
can either be used to represent the complete set of transcripts for an entire
organism or for a particular cell type – for a particular organ, for
example. The transcriptome, in essence,
is a quantitative measure of the genes that are actually being expressed at any
given time and as such can very dependent upon the particular set of
environmental conditions that the organism is subjected to.
From the accumulated
data, it seems unlikely that changes in gene expression can account for the diversity
of characteristics found among members of the vertebrate family. It seems more likely that changes in
alternative splicing (AS) may be responsible for the species-specific
differences found in nature. AS is a
process in which a single gene can, in fact, result in the production of more
than one protein. This is a regulated
process that determines what exons – that portion of a gene that that has
coding information for the final protein product - will be included or excluded
in the final m-RNA. It is this mechanism
that accounts for the fact that far more proteins are produced from the ~
20,000 protein-coding genes found in the human genome than would ordinarily be
expected.
Given these assumptions
arrived at from well-supported evidence, Dr. Nuno L. Barbosa-Morais and
colleagues, from the Banting and Best Department at the Donnelly Centre of
Medical Research at the University of Toronto, Canada, conducted a genome-wide
investigation of AS differences among equivalent organs from vertebrate
organisms encompassing all vertebrates higher on the evolutionary scale than
fishes – tetrapods. The organs that they
used for the investigation were whole brain, forebrain cortex, cerebellum,
heart, skeletal muscle, liver, kidney, and testis. This approach engendered a significant amount
of precise and demanding work
From this exhaustive
study, they found significant differences in the complexity of AS between the
various lineages studied with the highest complexity found within
primates. The species examined spanned ~
350 million years in the evolutionary time scale. Furthermore, within 6 million years the
splicing patterns of the individual organs studied diverged to the extent that
they were more closely correlated with the identity of the species than to the
organ type.
These results add
significantly to the understanding of the underlying genetic mechanisms that
account for the diversity of characteristics found within the vertebrate family
of organisms.
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