Meeting Abstract
The egg-yolk precursor protein vitellogenin (Vtg) can be useful for studying evolutionary processes, due to its vital role in providing embryos of oviparous animals with energy and nutrients. Studies of Vtg evolution have focused on invertebrates and fishes and there is limited knowledge of this topic in tetrapods. Recent genome projects have provided Vtg sequences for a number of tetrapods, particularly birds. This study evaluates evolution of Vtgs in several sauropsid groups. The sauropsids have three Vtgs (1, 2, and 3), corresponding to VtgAB1, VtgAB2, and VtgAB3, using the terminology of the ‘3R hypothesis’ of Vtg evolution. Vtg2 and Vtg3 share greater amino acid identity and cluster more closely together than to Vtg1, as determined by neighbor-joining analysis. Each Vtg has four distinct regions identified by conserved marker sequences: an N-terminal lipovitellin heavy chain, a phosvitin region containing many serine residues, a lipovitellin light chain and a C-terminal B’-CT region. For all species, Vtg1 is the largest and Vtg3 is smallest, with the differences due to the size of the phosvitin regions. Variation in the phosvitin region is largely due to the number of serine residues. Phosvitin serines have an altered codon usage compared to the other Vtg regions and to other proteins, favoring AGC and AGT. These data, and an analysis of serine codon repeats, are consistent with codon slippage as the means of altering serine numbers in the phosvitin region. Vtg1 and Vtg2 have more variation in serine number than does Vtg3. Codon slippage appears to provide a rapid mechanism for altering phosvitin composition. The functional significance of phosvitin serine variation remains to be determined. One hypothesis is that it may be related to the capacity of the phosphorylated serine residues to bind calcium ions.
