Do you think authors need to stick with one genre to build readership? Do you think readers will read other works by an author in a different genre? Should authors use pen names if they jump around to different genres?
This is one of those "it depends" questions. There are pros and cons to both sides of the argument.
In the beginning of a career, I think it's wise to stick with one genre, or perhaps two closely related genres (like suspense/horror; sci-fi/fantasy). It helps build readership. But sometimes authors get bored with that; or they have way too many ideas and want to branch out. There's nothing inherently wrong with that, but it needs to be handled carefully. You & your publisher or agent should make the decision on pen names together, after weighing all the pros and cons.
Many authors write in several different genres successfully. However, most of them will have a different pen name for each genre—at least in the beginning. (Ex: Nora Roberts/J.D. Robb; Elizabeth Peters/Barbara Michaels; Janette Rallison /Sierra St. James; Obert Skye/??-is he still keeping this a secret?)
I think this use of pen names is wise because yes, readers will read other genres by their favorite authors. Use of a pen name keeps the successful author's name "safe" while they're experimenting in other areas. For example, let's say an author* is a hugely famous best-seller in sci-fi/fantasy. Then they decide to write a few romances and they stink, at least compared to the sff. So here's what happens. Rabid fans of the sff read the romance and hate it. Next sff title comes out. The bad taste of the romance is lodged there in their memory. They may still buy that new title, but maybe they won't wait in line on release day. And from the other direction, readers who like both romance and sff may stumble across the sub-par romance novels first, then be unwilling to give the sff a chance, thinking the quality of the writing will be the same.
Or let's say someone is a well-known writer of sweet middle grade readers. Kids love them; parents trust them. Then they decide to write a racy YA novel. Past fans read it because of the author's name. The little kids are shocked. Parents are outraged. Suddenly all books by this author are suspect and sales of the middle grade readers drop.
Now, if the second genre is as big a hit as their first, publishers will let it leak out that the two authors are one and the same, which then boosts sales of both genres as fans of each will try the other.
In the middle grade/racy YA example, I'd probably try to keep that a secret for as long as I could, although it didn't seem to hurt Shel Silverstein (or is that an urban myth?).
From another marketing point of view, writing under a pen name is like starting out as a brand new author. You don't have a following yet. The publisher is going to have to invest more in marketing the new book. That can be a pain. The upside is that the new book under the pen name is likely to be better than the author's original first book, simply because of the experience the author has gained as a published writer. But the downside is some authors think they can write in a second (or third) genre, and they really can't.
As a publisher, I don't mind if an author writes in multiple genres, as long as they keep producing books in their stronger genre on a reasonable time frame. As a reader, I hate it when the next novel in a series is slow coming out because an author is off playing under another pen name. (cough-Robert-cough-Jordan)
Does this make sense?
*10 points to the first commenter who knows who I'm talking about.