A Novel Emx Gene in Pelromyzon marinus Offers Insight into the Timing and Consequences of Gene Duplication in the Chordate Lineage
Dekker, Robert G., II
As the most primitive extant vertebrates, lampreys occupy an important phylogenetic position in the study of evolution from non-vertebrate chordates to jawed vertebrates (gnathostomes). The Emx gene family is widespread and originally thought to only be involved in forebrain development, but expression seen in other areas suggests a more widespread role. Two copies of Emx exist in amphioxus and three copies exist in some gnathostomes. The study of Emx genes, their evolution and expression, can provide valuable information into the timing and consequences of gene duplication events in chordates. In the lamprey P. marin us it was shown that only one Emx gene existed. In this study we isolated a second Emx gene from lamprey that is sister to PmEmxA and orthologous to all other gnathostome Emx genes. Phylogenetic analysis of this novel Emx gene is not consistent with the 2R hypothesis, which states that the early vertebrate lineage underwent one or more complete genome duplications. Rather, this new gene suggests independent Emx duplication in lamprey. Analysis of this novel gene is compatible with the duplication-degeneration-complementation (DOC) model as our study suggests that duplicates of an ancestral Emx gene suffered regulatory element innovation allowing for complementary Emx subfunctionalization. In-situ hybridization reveals that this new Emx gene displays an expression pattern that differs both spatially and temporally from its homolog. PmEmxA appears to nest within PmEmxB expression in the pallial divisions of the forebrain, but exhibit complementary expression in the hypothalamus. These unique expression patterns suggests that innovations in regulatory elements following an EmxA/B ancestral gene duplication allowed for the apparent subfunctionalization of PmEmxB, thus supporting the DDC model.
vi, 33 p.
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