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Figure 9.17 A model explaining how internal eliminated segment (IES) recombi-
nation brought about inversion of macronuclear destined segment (MDS) 2 in the
actin I gene of Sterkiella nova during evolution. (a) Original arrangement of MDSs
9, 2, and 1. (b) DNA folded into a hairpin, and recombination between IES 6 and 7.
(c) Inversion of MDS 2 as a result of the IES recombination.
hairpin allows the flanking IESs 6 and 7 to align with each other (Figure 9.17b).
Recombination between the two IESs inverts MDS 2 so that it now reads right
to left (Figure 9.17c; i.e., it is inverted).
Evolution of IES Additions and MDS Scrambling in the Actin I Gene
Examining the actin I gene in a series of nine stichotrich species shows how
IESs have been added to the micronuclear actin I gene and how the MDSs
became scrambled during evolution. First, however, we can arrange these nine
organisms in an evolutionary tree based on the degree of similarity of the base
sequences in their genes encoding ribosomal RNA (called the rDNA gene)
(Figure 9.18).
The longer ago any two organisms diverged from each other in evolution,
the more time has been available for mutations to accumulate and the less their
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