Evolution of internal eliminated segments and scrambling in the micronuclear gene encoding DNA polymerase alpha in two Oxytricha species

To learn about the evolution of internal eliminated segments (IESs) and gene scrambling in hypotrichous ciliates we determined the structure of the micronuclear (germline) gene encoding DNA polymerasealpha(DNA polalpha) in Oxytricha trifallax and compared it to the previously published structure of the germline DNA polalphagene in Oxytricha nova . The DNA polalphagene of O.trifallax contains 51 macronuclear-destined segments (MDSs) separated by 50 IESs, compared to 45 MDSs and 44 IESs in the O.nova gene. This means that IESs and MDSs have been gained and/or lost during evolutionary divergence of the two species. Most of the MDSs are highly scrambled in a similar non-random pattern in the two species. We present a model to explain how IESs, non-scrambled MDSs and scrambled MDSs may be added and/or eliminated during evolution. Corresponding IESs in the two species differ totally in sequence, and junctions between MDSs and IESs are shifted by 1-18 bp in O.trifallax compared to the O.nova gene. In both species a short region of the gene is distantly separated from the main part of the gene. Comparison of the gene in the two species shows that IESs and scrambling are highly malleable over evolutionary time.