Abstract
BackgroundGlobin isoforms with variant properties and functions have been found in the pseudocoel, body wall and cuticle of various nematode species and even in the eyespots of the insect-parasite Mermis nigrescens. In fact, much higher levels of complexity exist, as shown by recent whole genome analysis studies. In silico analysis of the genome of Caenorhabditis elegans revealed an unexpectedly high number of globin genes featuring a remarkable diversity in gene structure, amino acid sequence and expression profiles.ResultsIn the present study we have analyzed whole genomic data from C. briggsae, C. remanei, Pristionchus pacificus and Brugia malayi and EST data from several other nematode species to study the evolutionary history of the nematode globin gene family. We find a high level of conservation of the C. elegans globin complement, with even distantly related nematodes harboring orthologs to many Caenorhabditis globins. Bayesian phylogenetic analysis resolves all nematode globins into two distinct globin classes. Analysis of the globin intron-exon structures suggests extensive loss of ancestral introns and gain of new positions in deep nematode ancestors, and mainly loss in the Caenorhabditis lineage. We also show that the Caenorhabditis globin genes are expressed in distinct, mostly non-overlapping, sets of cells and that they are all under strong purifying selection.ConclusionOur results enable reconstruction of the evolutionary history of the globin gene family in the nematode phylum. A duplication of an ancestral globin gene occurred before the divergence of the Platyhelminthes and the Nematoda and one of the duplicated genes radiated further in the nematode phylum before the split of the Spirurina and Rhabditina and was followed by further radiation in the lineage leading to Caenorhabditis. The resulting globin genes were subject to processes of subfunctionalization and diversification leading to cell-specific expression patterns. Strong purifying selection subsequently dampened further evolution and facilitated fixation of the duplicated genes in the genome.
Highlights
Globin isoforms with variant properties and functions have been found in the pseudocoel, body wall and cuticle of various nematode species and even in the eyespots of the insect-parasite Mermis nigrescens
Occurrence of globins in the Nematoda We identified globin gene sequences and exon-structures from full genomic sequences of five nematodes: the Caenorhabditis species C. elegans, C. briggsae and C. remanei (Rhabditina, Rhabditidae) and the distantly related nematodes Pristionchus pacificus (Rhabditina, Diplogasteromorpha) and Brugia malayi (Spirurina, Spiruromorpha) (Additional file 1)
We identified clear orthologs of all 33 C. elegans genes in C. briggsae and C. remanei, but reciprocal blast searches with the TBLASTN algorithm [8] and the C. elegans globin amino acid sequences identified only 24 and 13 globins in the genomes of Pristionchus pacificus and Brugia malayi, respectively, reducing the number of orthologs shared by all 5 species to 10
Summary
Globin isoforms with variant properties and functions have been found in the pseudocoel, body wall and cuticle of various nematode species and even in the eyespots of the insect-parasite Mermis nigrescens. In silico analysis of the genome of Caenorhabditis elegans revealed an unexpectedly high number of globin genes featuring a remarkable diversity in gene structure, amino acid sequence and expression profiles. In silico analysis of the C. elegans genome identified 33 putative globin genes in this species using a robust alignment procedure based on conserved structural features of the classical globin fold. These globins feature a wide diversity in gene structure, amino acid sequence and expression profiles. Despite this remarkable variety some of them display significant sequence similarity to vertebrate myoglobin, neuroglobin and cytoglobin [6,7]
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