Abstract
Dcc is the key receptor that mediates attractive responses of axonal growth cones to netrins, a family of axon guidance cues used throughout evolution. However, a Dcc homolog has not yet been identified in the chicken genome, raising the possibility that Dcc is not present in avians. Here we show that the closely related family member neogenin may functionally substitute for Dcc in the developing chicken spinal cord. The expression pattern of chicken neogenin in the developing spinal cord is a composite of the distribution patterns of both rodent Dcc and neogenin. Moreover, whereas the loss of mouse neogenin has no effect on the trajectory of commissural axons, removing chicken neogenin by RNA interference results in a phenotype similar to the functional inactivation of Dcc in mouse. Taken together, these data suggest that the chick neogenin is functionally equivalent to rodent Dcc.
Highlights
The chicken genome is significantly more compact than mammalian genomes [1]
We present evidence for such a substitution, chicken neogenin may function in place of the Deleted in Colorectal Cancer (Dcc) protein, since the Dcc gene does not appear to be present in the chicken genome
To assess whether chicken neogenin has an activity that compensates for the loss of Dcc, we have examined the role of neogenin in the developing spinal cord where the function of Dcc has been well described [10,36]
Summary
The chicken genome is significantly more compact than mammalian genomes [1]. The human genome is currently thought to be 3.28 Gb whereas the mouse genome is 3.42 Gb (Ensembl). The total number of known or novel protein coding genes identified to date in the chicken genome is significantly smaller than the number of equivalent genes in mammalian genomes. There are estimated to be 16,680 such genes in the chicken genome compared to 21,550 genes in human and 22,670 genes in mouse genomes (Ensembl estimates). It is perhaps not surprising that this threefold difference in size between the chicken and mammalian genomes has resulted in a number of genes, and even gene families, being apparently missing from the chicken genome. The International Chicken Genome Sequencing Consortium previously identified that genes encoding the vomeronasal receptors, casein milk proteins and salivaryassociated proteins seem to be absent from the chicken genome as well as proteins containing the SCAN domain, a dimerization motif often found in zinc-finger transcription factors [1,3]
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