A certain sea slug, Elysia chlorotica, �steals� chloroplasts from algae it normally feeds upon, and then uses the chloroplasts to conduct photosynthesis during times of low food availability. Scientists found that the slug carries a chloroplast-maintenance gene in its genome, a gene identical to one found in the algae. The process by which the slug first acquired the gene is called what?

E.chlorotica incorporates to its DNA the algae genes, through a process known as Horizontal Gene Transference.

Explanation:

Elysia chlorotica is a marine gasteropod capable of photosynthesizing. It normally feeds on algae and, and while doing it, the gasteropod incorporates chloroplasts to its tissues, becoming green with time.  The species is capable of photosynthesizing during scarce food seasons, and this is what lets it keep alive for a long time without any other food source than solar light and CO².

During digestion, algae photosynthetic organelles are incorporated in the gasteropod´s cells.  Incorporated chloroplast locates facing the body surface. E.chlorotica incorporates to its DNA the algae genes, through a process known as Horizontal Gene Transference. This process refers to the acquisition of genetic material in other ways than the reproductive one. These genes allow the gasteropod to synthesize its chlorophyll, and they are transmitted to progeny.