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Figure 9.11. The
TP gene in
Sterkiella nova
is interrupted by only three IESs
and in
Stylonychia mytilus
by only two IESs (Figure 9.11). None of the IESs
correspond in size, sequence, or position from one organism to another. For
example, IES-1 (32 bp) in
Sterkiella nova
is in the gene-coding region (just
after the ATG start codon). In
Stylonychia mytilus
IES 1 (20 bp) occurs in the
leader region of the gene just before the start of the coding region. In
Sterkiella
histriomuscorum
IES 1 (82 bp) is in the leader region but farther to the left
of the start of the coding region than in
Stylonychia mytilus
. This could be
interpreted to mean that IES 1 was inserted separately and independently in
the three species during their divergence from a common evolutionary ancestor
in which there was no IES. Alternatively, IES 1 may have been present in a
common evolutionary ancestor, but mutations caused it to migrate to different
positions and to change in sequence and size. Similarly, IES 2 in
Sterkiella
nova
and
Stylonychia mytilus
and one of the IESs (e.g., IES 5 or 6) in
Sterkiella
histriomuscorum
(see Figure 9.11) might have been derived from the same IES
in a common ancestor. Whether or not this speculation is correct, it is clear
that additional IESs have been inserted after the evolutionary divergence of
three species, particularly in
Sterkiella histriomuscorum
, which has six IESs
compared to two and three in the other two species.
β
Scrambling the Arrangement of MDSs by IES Recombination
IESs show yet another behavior during evolution that has a profound effect on
micronuclear gene structure: IESs within the DNA molecule of a gene are able to
recombine with one another. Such intramolecular recombination between IESs
consists of breakage of two IESs and rejoining of broken ends in a switched
configuration. For example, the micronuclear actin I gene of
Sterkiella nova
contains eight IESs and nine MDSs. Originally, eight IESs must have been
inserted into the actin I gene in an ancestor of
Sterkiella nova
, creating nine
MDSs probably in the orthodox order, 1-2-3-4-5-6-7-8-9. During evolution of
Sterkiella nova
from its ancestor, the nine MDSs have been rearranged into
the scrambled order 3-4-6-5-7-9-2-1-8 by recombinations between IESs. An
example of how IES recombination can generate MDS scrambling is shown in
Figure 9.16.
In Figure 9.16a the nine MDSs in the micronuclear actin I gene in a theoretical
ancestor of
Sterkiella nova
are arranged in their orthodox order. Figure 9.16b
shows two IES recombinations: one recombination between IESs 4 and 5 and a
second between IESs 4 and 6. For the recombinations to occur, the DNA must
form two loops to bring IESs 4, 5, and 6 into parallel alignment. Recombination
between IES 4 and 5 (indicated as an “x”) creates the composite IES 4-5 that
connects MDS 4 with MDS 6. A second recombination between IES 6 and IES 4
(indicated as an “x”) creates the composite IES 4-6 connecting MDS 6 to MDS
5 and a third composite IES 5-4-6 connecting MDS 5 to MDS 7. As a result
