Characterization of histone H3/H4 gene region and phylogenetic affinity ofIchthyophthirius multifiliis based on H4 DNA sequence variation

We sequenced the amino-terminal third of the histone H3 and H4 genes and th e intergenic region from Ichthyophthirius multifiliis. Fourteen recombinant clones of 646 bp were sequenced and the level of sequence variation detect ed among these clones was similar to that reported among closely related sp ecies of Tetrahymena and to levels of sequence variation detected within ot her ciliates. The intergenic region is 417 bp and approximately 92% AT rich , making it the longest and most AT-rich ciliate H3/H4 intergenic region ye t identified. Similar to Tetrahymena, the intergenic region of Ichthyophthi rius contains two CCAAT regions arranged in a complementary orientation. A neighbor-joining tree was constructed based on nucleotide sequence variatio n among H4 genes to evaluate evolutionary relationships within and among si x classes of Ciliophora. The single shortest neighbor-joining tree depicted a sister-group relationship of Ichthyophthirius with taxa of Tetrahymenina , thereby supporting monophyly of Oligohymenophorea. (C) 2000 Academic Pres s.