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point. The pathway continues, leading to the remaining stichotrichs in Figure
9.19. Addition of another IES [7 in Figure 9.19] and further IES recombinations
[8, 9, and 10 in Figure 9.19] produced the MDS pattern, 3-4-6-5-7-9-
2
-1-8. The
pathway then split. In one branch the pattern was inherited by
Sterkiella nova
,
which, in turn, gave rise to
Sterkiella
sp. (Aspen) and, by addition of another IES,
to
Sterkiella histriomuscorum
. The other branch evolved first into
Stylonychia
pustulata
and, finally, by addition of three more IESs to the 3-4-6-5-7-9-
2
-1-8
pattern into
Oxytricha
sp. (Misty) with 12 MDSs, 3-4-5-6-7-9-8-10-12-
2
-1-
11. IES additions and MDS scrambling in Figure 9.19 not only conform to the
pathway defined by rDNA sequences, but it is also the most parsimonious series
of steps explaining the evolution of actin I gene structure. This evolutionary
scheme in Figure 9.19 may be further refined when the structure of the mi-
cronuclear actin I gene is determined in additional stichotrichs. For example,
some of the theoretical intermediates might be verified, but it is unlikely that
the overall evolutionary pattern will require any major rearrangements. Thus,
we have compelling evidence that IESs are added sequentially in evolution and
that these IES additions are intermixed with sporadic IES recombinations that
scramble MDSs.
ASSEMBLY OF MACRONUCLEAR GENES
We noted earlier that the pair of repeat sequences in the ends of MDSs imme-
diately flanking an IES in the micronuclear
TP gene offers a clue about how
a stichotrich ciliate removes the IES during macronuclear development (see
Figure 9.13). Recombination between the repeats excises the IES, along with
one copy of the repeat, and ligates the two MDSs into a composite MDS. In
some micronuclear genes, recombination between IESs during evolution has
rearranged MDSs into a scrambled order, in some instances inverting MDSs.
In this case, too, the pairs of repeat sequences are crucial for excision of IESs
as well as for ligation of MDSs into the orthodox order, including reinversion
of MDSs into their 5'
β
3' orthodox polarity. We present now three molec-
ular operations that accomplish the assembly into its macronuclear form of
any micronuclear gene composed of nonscrambled or scrambled MDSs or a
combination of nonscrambled and scrambled MDSs.
First, we refer to the pairs of repeats in the ends of MDSs as a pair of pointers
that points one MDS to join with another MDS in the appropriate order during
gene assembly. An MDS has an incoming pointer and an outgoing pointer, as
diagrammed in Figure 9.20 for the four MDSs in the micronuclear
→
TP gene
of
Engelmanniella mobilis
. For example, MDS 2 has an incoming pointer (P2)
in its 5' end (left end) and an outgoing pointer (P3) in its 3' end (right end).
A sequence can act as a pointer only if it is at the boundary between an MDS
and IES.
β