Poultry Evolution: A Concentration on NAG, CPSI, and the Urea Cycle

Abstract

This study concentrates on the urea cycle of poultry and investigates the evolutionary process behind the absence of urea production in poultry. Previous genomic mapping studies of the genes encoding the urea cycle enzymes have detected all urea cycle genes except N-acetyl glutamate synthase (NAGS) and polymerase chain reaction (PCR) experiments revealed low levels of expression of the remaining enzymes. So, while this study began with the hypothesis that poultry did not contain the enzymes necessary to produce urea, genomic studies and sequence analysis shows that there is only one absent enzyme, NAGS, as well as one possibly deficient enzyme, carbamoyl phosphate synthase I (CPSI). It has been hypothesized that, by selecting against N-acetyl glutamate (NAG) production and thus eliminating synthesis of urea, poultry enabled the laying of land-based and hard-shelled eggs. CPSI, while found in the poultry genome, appears to be nonfunctional due to sequence analysis showing lack of a mitochondrial transit peptide. Why, then, is CPSI conserved in the chicken genome? This is potentially due to interactions between CPSI and Raf, a promoter of cell growth. While CPSI remains enzymatically nonfunctional in relation to urea cycle studies, it has been shown to interact with Raf (1). Phylogenetic tree analysis performed in this experiment also revealed that most of the urea cycle enzyme genes are in evolutionary agreement, suggesting that the genes are all intact, potentially serving some other purpose in the poultry genome. Finally, use of the Genomicus v66.01 gene search helped depict conserved syntanic relationships of CPSI with its surrounding enzymes across similar species. Meanwhile gene searching of NAGS revealed much less conservation across the genomes of various organisms, with a notable absence of any NAGS locus in the anole. Closer analysis showing the deletion of the locus of NAGS in both lizards and birds makes it appear as though the deletion must have occurred basal to the divergence of these genuses, resulting in elimination of the urea cycle.