b$.(0\mathrm{.}5pt)$ You also found that some of these isolates encode the $\frac{?}{ytH}$ gene, which is a gene important for making cell walls in Gram$-$positive organisms $($but not Gram$-$negative organisms$).$ You found that $lytH$ is encoded in the genomes of isolates $4$ and $5$ ; but not in $1,3,$ or $2.$ How do you think this gene was acquired $($horizontal transfer, vertical transmission, or novel mutation$)$ and why?