The
evolutionary
and
functional
significance
of
an
intron
sequence
has
been
much
discussed.
One
of
the
most
appealing
hypotheses,
originally
put
forward
by
W.
Gilbert
of
Harvard
University
in
1981,
is
that
the
presence
of
introns
is
related
to
an
evolutionary
process
by
which
new
genes
arise
from
recombination
events
that
juxtapose
previously
existing
genes
or
portions
of
genes.
Given
the
ability
to
splice
transcripts
from
two
or
more
DNA
segments
into
a
single
mRNA, "compound" genes
may
form.The
number
of
introns
is
highly
variable
in
both
plant
and
animal
genes.
There
are
a
number
of
plant
genes
with
no
introns
at
all,
such
as
the
zein
storage
protein
genes
of
maize,
two
other
soybean
protein
genes,
one
rare
class
leaf
gene
and
most
cab
genes
(nuclear
genes
encoding
the
chlorophyll
a/b-binding
protein
of
the
photosystem).
Other
plant
genes
have
two
or
three
introns.
Nucleotide
sequence
data
for
the
French
bean
(Phaseolus)
genes,
soybean
leghemoglobin
genes,
a
soybean
actin
gene,
soybean
glycinin
gene,
a
rare
class
leaf
gene
and
genes
for
the
small
subunit
of
RuBP
carboxylase,
has
shown
that
all
these
plant
genes
have
two
to
three
introns.
