'The endosymbionts with the smallest genomes have lost so many genes that they lack the ability to do much at all on their own and start to resemble mitochondria and chloroplasts more than typical bacteria. How these hosts and endosymbionts integrate their biochemical and cell biological processes is largely unknown.'

https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002577&rev=2