As extensively detailed elsewhere in this book, lipocalins exibit three characteristic features, which include: (i) an unusually low amino acid sequence similarity (typically 15-25% between paralogs) (ii) a highly conserved protein tertiary structure, and (iii) a similar arrangement of exons and introns in the coding sequence of their genes. These shared protein and gene features are overwhelming arguments for the existence of a single lipocalin ancestral gene that once extended into a family. The ancestral gene appears to have arisen in a group of bacteria, and possibly was inherited by eukaryotes as a result of genome fusion (see Chapter 4). Given this hypothetical beginning, lipocalins are expected to be found in all descendants of the eukaryotic common ancestor. Currently, and aside of prokaryotes, bona fide lipocalin have been recovered from a protoctist, a fungus, several plants, a nematode, several arthropods, a tunicate, a cephalochordate, and many examples of chordates. This review will first focus on the structure of lipocalin genes in eukaryotes, and then on our current view of the evolutionary hostory of this family.
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