Once published, do writers have to stick to a particular genre or are there cross-genre writers out there?
It isn't necessary to stick to one genre, and a lot of writers in the past have written pieces belonging to different genres.
Nope! People are free to write whatever genre they want. =) However, you might have to find various publishing places just in case they're geared towards a single type. For example, the same place that publishes heavy fantasy stories probably wouldn't publish a modern-setting crime story.
you also may want to consider using a nom de plume for your erotica genre novels, for instance, if you also write for the YA market!
I've had short stories published in a variety of genres, from mystery and horror to inspirational and science fiction. My novels are fantasy, but I also write SF--and may something else. A pen name might be useful in instances such as Mammamaia suggested.
On a related note, I've noticed that a lot of young adult and children's agents won't represent any adult work. This is all hypothetical (I'm not submitting yet), but if someone was offered representation by one of them, what would be protocol for trying to publish any other novels for an older market? Just something I've been wondering recently. I guess that'd be something you'd bring up with the agent, wouldn't it?
I think that while certain authours may be more comfortable, or more recognised in a certain genre there is nothing to stop them writing and doing well in other genres. Stephen King is prominantley known for his horror writing, but he has also done well with his fantasy epics. Iain Banks writes generalised fiction and even better sci fi under Iain M Banks. Anne Rice has been all over the place with erotic works, and her more well known vampire novels. I think in all three cases tough pen names were used - or altered namestates as i cant remember what King used but i know for while he was writing under a different name to begin with, and Rice has a load of pen names. I personally write under a bunch of pen names, which are genre specific. OutlawedAngel.
You're right that some agents are more comfortable with certain genres. An agent who agrees to represent your mystery might not want to take on your SF thriller. Doesn't mean you don't write it. Write whatever you desire to write, worry about the rest later.
I write horror and SF. They are, however, related genres (both under the bracket "speculative fiction"). But I wouldn't stick to those genres to the exclusion of others if I had an idea outside of H&SF that I liked. That said, I'm very fond of mixing strains of other genres into my writing. I think Stephen King is a very interesting example. In my view, he started off writing horror, but nowadays writes much less of it. The thing is, he made his name as a horror writer, so even though he has since broadened and written examples of almost everything he is still known as a horror writer. But writing other genres doesn't seem to have done him any harm.
As far as I know most writers try different genres for some purpose; e. g. to enhance their ability, avoid their readers to loose interest and they got challenged to something new (at lest that’s what I believe). However they still have their signature genre, the real forte where they became famous.
I think authors who write one particular genre do so because that is what their audience expects, and it will increase their sales. Nobody can tell a crime writer that they can only write crime if they do change, they will have to aim it at another audience. I think it basically comes down to reputation and keeping audiences happy.
I think writers generally write what moves them. Crossing genres can work for some, like craigpay said about China Mieville. I think it also depends on if you are published or not.
dizzy... you can have more than one agent... if you have one for your children's/YA work, you can seek another for the adult books...