Taxonomic publications: past and future - Senckenberg Museum

Burckhardt & Mühlethaler (eds): 8 th GfBS Annual Conference Abstracts 16
Loss of phylogenetic information in histone genes because of gene conversion
and GC3 drive? A data analysis of mammals and land snails
G. F. J. Armbruster & M. Böhme
Replication-dependent histone genes occur in all eukaryotes and provide essential
proteins for cell division. Histone gene sequences are often used in phylogenetic
reconstructions of animal and plant taxa. We give an introduction on genomic
arrangement of histone genes, and put the focus on gene conversion and GC3 drive
mechanisms. GC3 drive, i.e., the enrichment of G or C nucleotides at the third,
“wobble” codon position in protein-coding regions, is a favorite explanation why
mammals (human, mouse) have high GC3 values in histone gene copies. This
process seems to be influenced by conversion of chromosomal neighbours of histone
genes of identical function (Galtier, 2003: Trends in Genetics, 19: 65-68). Because of
these mechanisms of neighbouring loci, histone gene families of mammals are
subjected to a loss of phylogenetic signals (i.e., loss of A or T nucleotides at the third
codon position). This loss of phylogenetic information at the third codon position
usually does not alter the amino acid sequence due to the degenerated code of
“wobble” bases. Hence, GC3 mechanisms are not correlated with a shift to another
amino acid sequence.
We then present data of the partial Histone(H3)-spacer-Histone(H4) gene cluster of
land snails. We did not find clues for strong G or C enrichment at wobble base
positions. Hence, phylogenetic information with A and T (and C and G) nucleotides
was still found in the H3-H4 sequence alignment. At least two hypotheses are
relevant for discussing putative histone gene differences of land snails and
mammals: 1) Land snails have another arrangement of histone gene copies on their
chromosomes and, hence, they might not have the typical “neighbouring” effect as
mammalian histone gene cluster; 2) gastropods are poikilotherm animals,
presumably with no strong constraint to enrich G or C nucleotides at the third codon
position. Thus, a stabilizing shift of A/TG/C could be more likely necessary in warmblooded
animals because of their higher body temperature. The latter hypothesis,
however, is controversially discussed (Galtier, 2003: p. 67). The data analysis shows
that phylogenetic information of protein-coding gene families can be influenced by
gene arrangement, chromosomal neighbourhood and gene conversion.
Org. Divers. Evol. 5, Electr. Suppl. 13 (2005)