Fresh pesto is always a better choice than pre-made; here the basil flavor is intensified.
In a large pot of boiling salted water, cook whole-wheat spaghetti until al dente, according to package instructions. Reserve 1/2 cup pasta water; drain pasta, and return to pot.
Toss pasta with lighter pesto, adding reserved pasta water a little at a time to create a thin sauce that coats spaghetti (you may not need all the water). Season with salt and pepper. Serve, garnished with shredded basil, if desired.