Kleptoplasty (from the Greek kleptes, meaning “thief”) is a phenomenon whereby host organisms ingest a chlorophyll-utilizing species, typically algae, and use the energy-producing organelles called chloroplasts contained within the consumed species to help meet their own metabolic needs. In this way, kleptoplasty is an example of symbiosis, a close relationship between two different species. Further, it is an endosymbiosis, such that one of the species resides completely within another. Unlike the example of mitochondria, thought to have once been fully separate bacteria that came to live within animal cells and perform a mutualistic metabolic function, the algae are only partially utilized; most of the organism is digested and discarded, leaving only the chloroplasts to be retained by the host. Most kleptoplastic species are unicellular ciliates or dinoflagellates. The only known members of the animal kingdom that practice kleptoplasty are several species of sarcoglossan sea slugs. These “solar-powered” sea slugs incorporate whole chloroplasts into their body cells, where the stolen plastids can convert sunlight into useful energy for as long as ten months in some species.

Chloroplasts produce energy by using sunlight to power a series of reactions that result in sugars that can be used as a food source for the host organism. Algae have genes that encode proteins that act as enzymes that support this process. PRK, for example, is an enzyme that is responsible for the regeneration of ribulose-1,5-bisphosphate, an organic molecule used in the reductive pentose phosphate pathway (RPPP) of photosynthesis. Sea slugs lack the PRK gene, so for many kleptoplastic species, once the raw materials within the chloroplasts are exhausted, photosynthesis ceases and new chloroplasts must be obtained. As a result, until recently it remained a mystery how some sarcoglossan sea slug species were able to sustain chloroplast function for many months.

Genome sequencing revealed the answer. Polymerase chain reaction (PCR) analysis of the genome of individual sarcoglossan sea slugs of species Elysia chlorotica that had been exposed to chlorophyll-utilizing algae revealed that these individuals did in fact have the PRK gene, whereas individuals that had not been exposed to algae lacked the gene. Radioactive labeling confirmed the surprising result: E. chlorotica incorporates genes from the algae into its own genome. This process, known as horizontal gene transfer, is common in bacteria and unicellular eukaryotes but is rare to find in more complex species.

According to the passage, the main difference between the sarcoglossan sea slug species discussed in paragraph 1 and other sea slug species is that ​

A​ they can incorporate the PRK gene into their genome, whereas other sea slugs cannot ​
B​ they obtain energy primarily by digesting only certain components of algae, whereas other sea slugs obtain their energy from digesting the entire organism ​
C​ they obtain at least some of their metabolic energy from chloroplasts, whereas other sea slugs do not
D​ they lack the enzyme to regenerate compounds necessary for photosynthesis, whereas other sea slugs produce this enzyme
​E​ they lack mitochondria to perform metabolic functions, whereas other sea slugs utilize mitochondria as their primary source of energy





Which of the following best expresses the main point of the passage? ​

A​ Until recently, scientists found it difficult to understand the exact mechanisms of kleptoplasty due to misconceptions about the relationship between sea slugs and algae.
​B​ Elysia chlorotica incorporates algal genes into its own genome, solving a problem that would leave it unable to survive in the wild. ​
C​ Ribulose-1,5-bisphosphate is an organic compound used in photosynthesis that kleptoplastic species lack the ability to produce on their own.
​D​ Kleptoplasty is unusual compared to other forms of symbiotic relationships in that the species that practice it derive some benefit from horizontal gene transfer. ​
E​ Kleptoplastic species not only use the biochemical machinery of another species to meet their own metabolic needs but can also incorporate genetic material to facilitate this process.





The passage provides support for which of the following statements about E. chlorotica?

​A​ In the absence of algae, E. chlorotica typically lacks the ability to synthesize ribulose-1,5-bisphosphate. ​
B​ Some of the characteristics of E. chlorotica are not commonly found within the animal kingdom. ​
C​ E. chlorotica is the only sarcoglossan sea-slug species that incorporates the PRK gene into its genome.