In Many Animals, SiRNAs Can Only Function Within the Cell

In many animals, siRNAs can only function within the cell in which the siRNA is introduced. In contrast, the worm Caenorhabditis elegans can be fed with bacteria that synthesize double-stranded RNAs, and the RNAi spreads throughout the animal. A genetic screen to identify genes involved in systemic RNAi led to the discovery of Sid-1, a transmembrane protein expressed in most tissues in the adult worm. Loss-of-function mutations in the gene encoding Sid-1 restrict the RNAi activity to the cells surrounding the digestive tract after siRNA feeding. Ectopic expression of Sid-1 in Drosophila melanogaster cells that normally lack systemic RNAi and are unable to take up siRNAs from the medium, enables these cells to passively take up siRNA . These findings suggest that …

A) Sid-1 is necessary and sufficient for systemic RNAi and siRNA uptake, respectively.
B) Sid-1 is necessary but not sufficient for systemic RNAi and siRNA uptake, respectively.
C) Sid-1 is not necessary but is sufficient for systemic RNAi and siRNA uptake, respectively.
D) Sid-1 is not necessary and not sufficient for systemic RNAi and siRNA uptake, respectively.