Expansion of symmetric exon-bordering domains does not explain evolution of lineage specific genes in mammals

In order to examine the evolution of lineage specific genes, we analyzed intron phase distributions and exon-bordering domains in primate and rodent specific genes. We found that the expansion of symmetric exon-bordering domains could not explain the evolution of lineage specific genes. Rather internal intron loss of a domain can partially explain the excess of class 1-1 intron phases in the lineage specific genes. We suggest the event that led to excess of symmetric exons in lineage specific genes had little bearing on shaping the phenotypes specific to the individual lineage. Instead, Kruppel-associated box (KRAB) proteins associated with zinc finger C2H2 (zf-C2H2) type are likely to be responsible for the lineage specific function.