The evolutionary origin of a complex scrambled gene.

Some species of ciliates undergo massive DNA elimination and genome rearrangement to construct gene-sized "chromosomes" in their somatic nucleus. An example is the extensively scrambled DNA polymerase alpha gene that is broken into 48 pieces and distributed over two unlinked loci in Stylonychia. To understand the emergence of this complex phenomenon during evolution, we examined DNA polymerase alpha genes in several earlier diverging species, representing evolutionary intermediates. Mapping these data onto an evolutionary tree suggests that this gene became extensively fragmented and scrambled over evolutionary time through a series of steps, each leading to greater complexity. Our results also suggest a possible mechanism for intron loss by deletion of intron sequences as DNA during development of the somatic nucleus.