BACKGROUND: The selenocysteine (Sec) containing proteins, selenoproteins, are an 
important group of proteins present throughout all 3 kingdoms of life. With the 
rapid progression of selenoprotein research in the post-genomic era, application 
of bioinformatics methods to the identification of selenoproteins in newly 
sequenced species has become increasingly important. Although selenoproteins in 
human and other vertebrates have been investigated, studies of primitive 
invertebrate selenoproteomes are rarely reported outside of insects and 
nematodes.
RESULT: A more integrated view of selenoprotein evolution was constructed using 
several representative species from different evolutionary eras. Using a 
SelGenAmic-based selenoprotein identification method, 178 selenoprotein genes 
were identified in 6 invertebrates: Amphimedon queenslandica, Trichoplax 
adhaerens, Nematostella vectensis, Lottia gigantean, Capitella teleta, and 
Branchiostoma floridae. Amphioxus was found to have the most abundant and 
variant selenoproteins of any animal currently characterized, including a 
special selenoprotein P (SelP) possessing 3 repeated Trx-like domains and Sec 
residues in the N-terminal and 2 Sec residues in the C-terminal. This gene 
structure suggests the existence of two different strategies for extension of 
Sec numbers in SelP for the preservation and transportation of selenium. In 
addition, novel eukaryotic AphC-like selenoproteins were identified in sponges.
CONCLUSION: Comparison of various animal species suggests that even the most 
primitive animals possess a selenoproteome range and variety similar to humans. 
During evolutionary history, only a few new selenoproteins have emerged and few 
were lost. Furthermore, the massive loss of selenoproteins in nematodes and 
insects likely occurred independently in isolated partial evolutionary branches.