This event provides an opportunity to track the fate of the duplicated protein-coding genes. How many of the original duplicates are left and what happened to them?
There were able to get reliable data on 9,040 of the original genes in the ancestral genome. (That's about one third of the estimated 31,000 genes in the genome of the original species.) Of those 9,040 genes, 4,728 (52%) are now single copy genes because one of the duplicated genes has been lost. Many of these original genes are still detectable as pseudogenes at the right position in the genome.
By combining these results with studies of more ancient genome duplications in the vertebrate lineage, it looks like the average rate of gene loss is about 170 genes per million years (Berthelot et al., 2004). It's likely that in the majority of cases one of the duplicates will eventually become inactivated by mutation and that allele will become fixed in the genome by random genetic drift. (Some early inactivation events may be selected.)
4,312 (42%) of the original duplications have been retained in the trout genome as a small family consisting of two paralogues. In some cases the two paralogues have diverged and in some cases they are expressed in different tissues or at different stages of development. This suggests that the two copies have evolved different functions.
However, most of the duplicated genes seem to be performing similar functions and it's likely that there is no selective pressure to retain two copies. There just hasn't been enough time to inactivate one copy.
The trout genome contains 241 ancient microRNA genes and 233 of them still have two copies, one from each of the duplicated genomes. The authors suggest that this is significant and it indicates that multiple copies on these microRNA genes are needed. I'm not sure if this is true since these genes are quite a bit smaller than the average protein-coding gene so they will take longer to inactivate by mutation.
In any case, the big picture provides us with lots of data on the birth of new genes by duplication and death of genes by pseudogene formation.
Berthelot, C., Brunet, F., Chalopin, D., Juanchich, A., Bernard, M., Noël, B., Bento, P., Da Silva, C., Labadie, K., and Alberti, A. (2014) The rainbow trout genome provides novel insights into evolution after whole-genome duplication in vertebrates. Nature communications, 5:3657 April 22, 2014 [doi:10.1038/ncomms4657]