Reevaluating Emx gene phylogeny: homopolymeric amino acid tracts as a potential factor obscuring orthology signals in cyclostome genes.
BACKGROUND
Vertebrate Emx genes, retained as multiple copies, are expressed in a nested pattern in the early embryonic forebrain and required for its regionalization. This pattern seems to have originated in a vertebrate common ancestor; however, a previous analysis, reporting two lamprey Emx genes, claimed independent Emx gene duplications in both cyclostome (extant jawless fish) and gnathostome (jawed vertebrate) lineages after their divergence. This scenario is neither parsimonious nor consistent with the hypothesis that genome expansion occurred before the cyclostome-gnathostome split, which is supported by recent genome-wide analyses.


RESULTS
We isolated and sequenced cDNA of two hagfish Emx genes and performed intensive molecular phylogenetic analyses, including the hagfish and/or lamprey Emx genes. The lamprey genes tended to attract each other in inferred phylogenetic trees, an effect that tended to be relaxed on inclusion of the hagfish genes. The results of these analyses suggest that cyclostome EmxB is orthologous to gnathostome Emx2, which was also supported by conserved synteny. Homopolymeric amino acid (HPAA) tracts represent a remarkable feature of the lamprey Emx sequences, and a comparative genome-wide scan revealed that lamprey proteins exhibit a unique pattern of HPAA tract accumulation.


CONCLUSIONS
Our analysis, including hagfish Emx genes, suggests that gene duplications gave rise to Emx1, -2 and -3 before the cyclostome-gnathostome split. We propose that independent HPAA tract accumulations in multiple ancient duplicates, as identified in lamprey Emx gene products, may have led to erroneous identification of gene duplication in the lamprey lineage. Overall, our reanalysis favors the scenario that the nested Emx expression pattern in mouse and lamprey shares a common origin.
Genome Resource and Analysis Unit, RIKEN Center for Developmental Biology, 2-2-3 Minatojima-minami, Kobe, 650-0047, Japan. lepidosaurian@gmail.com.
Miyuki
M
Noro
Division of Biology, Hyogo College of Medicine, 1-1 Mukogawa-cho, Nishinomiya, 663-8501, Japan. fu-sugahara@hyo-med.ac.jp.|Evolutionary Morphology Laboratory, RIKEN, 2-2-3 Minatojima-minami, Kobe, 650-0047, Japan. fu-sugahara@hyo-med.ac.jp.
Fumiaki
F
Sugahara
Phyloinformatics Unit, RIKEN Center for Life Science Technologies, 2-2-3 Minatojima-minami, Kobe, 650-0047, Japan. shigehiro.kuraku@riken.jp.
Shigehiro
S
Kuraku
D000595:Amino Acid Sequence; D000818:Animals; D019143:Evolution, Molecular; D020440:Gene Duplication; D055106:Genome-Wide Association Study; D006196:Hagfishes / Q000235:genetics; D018398:Homeodomain Proteins / Q000737:chemistry / Q000235:genetics*; D007798:Lampreys / Q000235:genetics; D051379:Mice; D008969:Molecular Sequence Data; D010802:Phylogeny; D016415:Sequence Alignment; D026801:Synteny; D014157:Transcription Factors / Q000737:chemistry / Q000235:genetics*; D014714:Vertebrates / Q000235:genetics*; D000099260:Empty Spiracles Homeobox Proteins
D018398:Homeodomain Proteins; D014157:Transcription Factors; D000099260:Empty Spiracles Homeobox Proteins