The origin of mitochondria in light of a fluid prokaryotic chromosome model

Abstract

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.

Figure 1

Distribution across taxonomic groups for (a) α-proteobacterial nearest neighbours by BBH, (b) prokaryotic genes, (c) α-proteobacterial nearest neighbours by NJ and (d) mitochondrial nearest neighbours by BBH. The number of query proteins used for the analysis is shown to the left of the species name (y-axis).

Figure 2

Neighbour-Net (Huson & Bryant 2006) of proteobacterial 16S rRNA. The bootstrap support for the split of Magnetococcus with α-proteobacteria (highlighted in red and with arrow) is 73% using neighbour-joining, 61% using Neighbour-Net and 45% using maximum likelihood (see electronic supplementary material).