Fig. 4

Burst-upon-drift (BUD) model: Small population sizes and high mitochondrial mutation rates can lead to fixation of slightly deleterious mitochondria-to-nucleus gene transfers. Path 1→ 2 represents mitochondria to nucleus transfer by adaptive mechanisms. Path 1 → 3 → 4 → 5 represents neutral transfers via the BUD model. (1) A mitogene is transferred to the nucleus (nu) and is transcribed, translated, and effectively targeted to the mitochondria (red cell in between orange cells). (2) The newly nuclear mitogene (nu) is beneficial and sweeps to fixation in a population due to natural selection, while the mitochondrial mitogene (mito) is lost because of bioenergetic benefit. New adaptations (nu*) will evolve in response to the new genomic location of the previous mitogene. (3) If the newly nuclear mitogene is neutral or mildly detrimental, the transfer can be fixed in the population by drift. In this situation, it is possible that the (mito)gene acquires moderate mutations leading to the sub-functionalization of the gene duplicates and their subsequent retention. (4) Loss of the mitogene may be fixed by drift if the mitochondrial mutation rate is high in a small population. In certain situations, this can occur even though there is a fitness cost caused by retaining only the nuclear mitogene. In these cases, several genes may transfer in quick succession leading to many fewer genes being encoded in the mitochondrial genome (black cells). (5) After the recovery from the population bottleneck, new adaptations (nu*) will evolve in response to the new genomic location of the previous mitogene. Ovals depict individual cells; colour changes of contours reflect changed cells (when compared with cells from a previous step). The colour code is consistent with the lineages in Fig. 1