Figure 4.

The fate of composite elements such as introns plus IEPs (I + H+). The composite element invades an empty site and from here it could spread into other sites (only site B shown for simplicity) and related IEPs could still interact with their ancestral intron version possibly facilitating splicing or mobility thus enhancing the chances of the ancestral intron to persist. This form of mutualism could even complement situations where the ancestral intron ORF has started to accumulate deleterious mutations (H−). Other composite elements may be strictly subject to drift and first the ORF is degenerating and eventually the intron is lost from the genome and possibly from the population. There might be situations where the composite elements have been co-opted as maturases or regulatory elements or as platforms for expressing essential genes (rps3) and these introns would be subject to adaptive selection and thus could become fixed in the genome and the population (Figure adapted from Gogarten and Hilario⁸³; licensed under a CCBY 2.0 license, (https://creativecommons.org/licenses/by/2.0/). This Figure adds a new component to the standard homing endonucleases life cycle^27,83 that suggests that some HEGs can avoid extinction due to mutualistic interactions that can complement for the accumulation of mutations.