Meeting Abstract
The egg-yolk precursor vitellogenin (VTG) can be useful for studying evolutionary processes due to its crucial role in providing embryos of oviparous animals with energy and nutrients. Phosvitin, a segment of VTG, is characterized by many serine residues over a short span of amino acids. Little is known about the evolution of phosvitin. I performed analyses of the serine composition and gene codon usage of the phosvitin region of avian vitellogenins, using data from genome sequencing of over 30 bird species. Of the three avian VTGs, VTG1 has most serines, the major egg-yolk precursor VTG2 has fewer and VTG3 has the least. However, VTG2 shows the widest variation in serine number. Amino acid composition analysis revealed substantial variation in the number of charged residues among species, particularly lysine, and the number of charged residues was positively correlated with the number of serines. Codon usage analysis for serine revealed differences between vitellogenin and other proteins (e.g., ovalbumin and serum albumin) and among the VTG regions, with the phosvitin region favoring AGC and AGT. Most repetitive serine sequences in the phosvitin region used AGC for their codons. The downy woodpecker had the most serines in the phosvitin region of VTG2 and had a repeated sequence of SSSSSSK with a perfectly repeating codon sequence. There were 18 perfect repeats of this “mini-satellite”. VTGs of many other bird species contained this sequence, but not all. The presence of multiple serine repeats, and their selective codon usage, are consistent with characteristics of a disorganized protein region. Triplet expansion (codon slippage), mainly of AGC, is likely the mechanism for generating the serine repeats in the phosvitin region of avian vitellogenins.
