I'm very skeptical of gray wolf blood in coyotes. Wolves and coyotes get along about as well as cats and dogs. Where their ranges overlap wolves are much more likely to kill coyotes than to breed with them. I do know coyotes will interbreed with red wolves; but, red wolves are so rare that this is insignificant.
However, in his book Mammals of Illinois, a U of I professor observed that 11% of coyote specimens he studies were coydogs (coyote-dog hybrids). This determination was based on bone & skull measurements and not DNA testing. I've also hear statements that all eastern coyotes have some dog blood in them. Don't know how true this is.
Don't put too much stock in the Mississippi as a barrier to wildlife. Coyotes can swim and also walk across bridges. Here in central MO, if armadillos can make it across the Missouri River, coyotes can get across the Mississippi.
Other posters are correct about northern coyotes being larger than southern 'yotes. In general, within a given species, the further north you go, the larger the individuals of that species. This applies to deer, raccoons, bear, beaver, etc. as well as coyotes.
Coyotes may be filling a niche left vacant by extirpated wolves, but wolves are gradually reclaiming that niche. In the midwest, wolves used to be restricted to northern Minnesota and Isle Royale, MI. The've expanded their range in Minnesota and are found across parts of Michigan and Wisconsin too. Dispersing males have been found in western Illinois and northern Missouri in the last 5 years. I believe there was also a roadkilled wolf in Lake County, Illinois, north of Chicago a few years ago. They're not the shy, wilderness creatures we were led to believe, but can get along quite well near humans in highly modified landscapes. As this occurs, I think it more likely wolves will try to kill coyotes rather than breed with them. But 'yotes are survivors. If they can survive bounties, shooting and poisoning campaigns, they can persist sympatric with wolves, albeit at reduced densities.
