Whole-genome duplications (WGDs) have contributed to gene-repertoire enrichment 
in many eukaryotic lineages. However, most duplicated genes are eventually lost 
and it is still unclear why some duplicated genes are evolutionary successful 
whereas others quickly turn to pseudogenes. Here, we show that dosage 
constraints are major factors opposing post-WGD gene loss in several Paramecium 
species that share a common ancestral WGD. We propose a model where a majority 
of WGD-derived duplicates preserve their ancestral function and are retained to 
produce enough of the proteins performing this same ancestral function. Under 
this model, the expression level of individual duplicated genes can evolve 
neutrally as long as they maintain a roughly constant summed expression, and 
this allows random genetic drift toward uneven contributions of the two copies 
to total expression. Our analysis suggests that once a high level of imbalance 
is reached, which can require substantial lengths of time, the copy with the 
lowest expression level contributes a small enough fraction of the total 
expression that selection no longer opposes its loss. Extension of our analysis 
to yeast species sharing a common ancestral WGD yields similar results, 
suggesting that duplicated-gene retention for dosage constraints followed by 
divergence in expression level and eventual deterministic gene loss might be a 
universal feature of post-WGD evolution.