Biologists agree that the ancestor of mitochondria was an alpha-proteobacterium. 
But there is no consensus as to what constitutes an alpha-proteobacterial gene. 
Is it a gene found in all or several alpha-proteobacteria, or in only one? Here, 
we examine the proportion of alpha-proteobacterial genes in 
alpha-proteobacterial genomes by means of sequence comparisons. We find that 
each alpha-proteobacterium harbours a particular collection of genes and that, 
depending upon the lineage examined, between 97 and 33% are 
alpha-proteobacterial by the nearest-neighbour criterion. Our findings bear upon 
attempts to reconstruct the mitochondrial ancestor and upon inferences 
concerning the collection of genes that the mitochondrial ancestor possessed at 
the time that it became an endosymbiont.