Elysia chlorotica is a sea slug that acquires chloroplasts by consuming an algal food source, Vaucheria litorea. The ingested chloroplasts are sequestered in the sea slug's digestive epithelium, where they actively photosynthesize for months after ingestion. In the algae, the algal nuclear genome encodes more than 90% but not all of the proteins required for chloroplast metabolism. Thus it is suspected that the sea slug actively maintains ingested chloroplasts, supplying them with photosynthetic proteins encoded in the sea slug genome. How would you determine whether the sea slug has acquired photosynthetic genes by horizontal gene transfer from its algal food source? Discuss the steps required for heritable endosymbiosis to eventuate, and their plausibility.