BACKGROUND: Selenoproteins are a diverse family of proteins notable for the 
presence of the 21st amino acid, selenocysteine. Until very recently, all 
metazoan genomes investigated encoded selenoproteins, and these proteins had 
therefore been believed to be essential for animal life. Challenging this 
assumption, recent comparative analyses of insect genomes have revealed that 
some insect genomes appear to have lost selenoprotein genes.
METHODOLOGY/PRINCIPAL FINDINGS: In this paper we investigate in detail the fate 
of selenoproteins, and that of selenoprotein factors, in all available arthropod 
genomes. We use a variety of in silico comparative genomics approaches to look 
for known selenoprotein genes and factors involved in selenoprotein 
biosynthesis. We have found that five insect species have completely lost the 
ability to encode selenoproteins and that selenoprotein loss in these species, 
although so far confined to the Endopterygota infraclass, cannot be attributed 
to a single evolutionary event, but rather to multiple, independent events. Loss 
of selenoproteins and selenoprotein factors is usually coupled to the deletion 
of the entire no-longer functional genomic region, rather than to sequence 
degradation and consequent pseudogenisation. Such dynamics of gene extinction 
are consistent with the high rate of genome rearrangements observed in 
Drosophila. We have also found that, while many selenoprotein factors are 
concomitantly lost with the selenoproteins, others are present and conserved in 
all investigated genomes, irrespective of whether they code for selenoproteins 
or not, suggesting that they are involved in additional, non-selenoprotein 
related functions.
CONCLUSIONS/SIGNIFICANCE: Selenoproteins have been independently lost in several 
insect species, possibly as a consequence of the relaxation in insects of the 
selective constraints acting across metazoans to maintain selenoproteins. The 
dispensability of selenoproteins in insects may be related to the fundamental 
differences in antioxidant defense between these animals and other metazoans.