Gene conversion and evolution of daphniid hemoglobins (Crustacea, cladocera).
The extracellular hemoglobins of cladocerans derive from the aggregation of 12 two-domain globin subunits that are apparently encoded by four genes. This study establishes that at least some of these genes occur as a tandem array in both Daphnia magna and Daphnia exilis. The genes share a uniform structure; a bridge intron separates two globin domains which each include three exons and two introns. Introns are small, averaging just 77 bp, but a longer sequence (2.2-3.2 kb) separates adjacent globin genes. A survey of structural diversity in globin genes from other daphniids revealed three independent cases of intron loss, but exon lengths were identical, excepting a 3-bp insertion in exon 5 of Simocephalus. Heterogeneity in the extent of nucleotide divergence was marked among exons, largely as a result of the pronounced diversification of the terminal exon. This variation reflected, in part, varying exposure to concerted evolution. Conversion events were frequent in exons 1-4 but were absent from exons 5 and 6. Because of this difference, the results of phylogenetic analyses were strongly affected by the sequences employed in this construction. Phylogenies based on total nucleotide divergence in exons 1-4 revealed affinities among all genes isolated from a single species, reflecting the impact of gene conversion events. In contrast, phylogenies based on total nucleotide divergence in exons 5 and 6 revealed affinities among orthologous genes from different taxa.
Hebert
P D
PD
Department of Zoology, University of Guelph, Guelph, Ontario N1G 2W1, Canada. phebert@uoguelph.ca
Um
Y M
YM
Prokopowich
C D
CD
Taylor
D J
DJ
D019943:Amino Acid Substitution / Q000235:genetics; D000818:Animals; D001483:Base Sequence; D003062:Codon / Q000235:genetics; D003621:Daphnia / Q000145:classification / Q000235:genetics*; D019143:Evolution, Molecular; D005091:Exons / Q000235:genetics*; D005785:Gene Conversion / Q000235:genetics*; D014644:Genetic Variation / Q000235:genetics; D006454:Hemoglobins / Q000235:genetics*; D007438:Introns / Q000235:genetics; D008969:Molecular Sequence Data; D005810:Multigene Family / Q000235:genetics; D010802:Phylogeny; D016415:Sequence Alignment
D003062:Codon; D006454:Hemoglobins