Here's a mystery with teeth -- black ones

Lucy and Ethel.

Laverne and Shirley.

Hazel and Alison.

The latest entry into the pantheon of dynamic duos, local business partners Hazel Perez and Alison K. Boston have achieved near-cult status across the city due to the odd accessory that accompanies their BostonRealtors.com ads on the back of MBTA buses.

Seemingly part of a cruel cosmic joke, someone, or some group -- or some thing -- has taken to blackening Alison's teeth on a gaggle of buses as they bound across the Hub, from Dot. Ave. to Mass. Ave.

Though several of Hazel's images have been defaced with Dali-esque mustaches, it is Alison's mouth that has been the main target of the graffiti guerrilla -- and mostly her left lateral incisor at that.

The tarnished tooth has become so lodged in the city's consciousness -- Alison believes it appears on more than half of the 70 buses she says display her ads -- that it has spawned speculation about the disfigurer's identity:

Is it a deranged dentist?

A crew of jealous gap-toothed hockey players who are into high jinks? A chain of miscreant copycats? A vandal with a personal vendetta?

''I certainly don't live the sort of life that would suggest someone is retaliating against me," says Alison, a 38-year-old realtor living in the South End.

Rather, Alison suspects it is someone with access to an MBTA bus garage. For even as her ad campaign, which she pegs at tens of thousands of dollars, went through several iterations to incorporate changes in fonts, and her switchover from a redhead to a blonde, the black tooth mysteriously reappeared -- the very mornings the new photos were trotted out, she says. And the same ads on taxis haven't been touched, she says.

T spokeswoman Lydia Rivera says she seriously doubts the perpetrator is an MBTA employee. But if so, she says, that person will be punished.

''It's highly unlikely," she says. ''If it is, he or she will be suspended."

Alison has mixed feelings about her defacement, which she says can't be erased and is too costly to replace. On the one hand, she says, she feels like she's been given a black eye. ''It's a tremendous amount of money," she says. ''It's hurtful. . . . I don't like seeing my teeth blackened out."

On the other hand, she realizes she has gotten more bounce from her ads than she would flashing plain old pearly whites. And she's become a mini-celebrity around town, hearing from all kinds of people, save her dentist -- who's out of state.

Students at a real-estate class she teaches have greeted her with: ''Oh, you're that lady with the tooth. Smile for me!"

Friends have suggested she go out for Halloween -- as her dark-toothed self.

An ad guy pitched her on the virtues of changing mediums, suggesting, ''No one can black out Alison's tooth on a TV commercial."

Alison says that if she found the prankster, first she'd go off on him. ''I'd like that person to stop," she says. ''I'm proud of my teeth."

Then she'd hug him. ''We get so many people who notice the graffiti," she says. ''I don't know if they'd see the ad otherwise."

Now, Alison may not have to put up with her graphic stalker much longer. Last week, she received word that, as the result of her ongoing contract dispute with the company now handling the T's vehicle advertising -- one she believes has nothing to do with the mystery of the distorted denticle -- her ubiquitous image may be coming off the buses soon.

Alison K. Boston says she would dearly miss her visage -- snaggletooth and all.