Change the DirectoryIndex based on a domain/sub-domain in .htaccess

I have a shared hosting with one domain and one sub-domain (for mobile and clients). Each domain and sub-domains has different default index pages. The hosting company told me to put everything in my .htaccess file since I don't have access to the httpd.conf.

What I want to do is this:

1. If a user goes to domain1.com the DirectoryIndex should be: index.html
2. If a user goes to mobile.domain1.com the DirectoryIndex should be: mobile-index.html
3. If a user goes to post.domain1.com the DirectoryIndex should be: post.php
4. If a user goes to vote.domain1.com the DirectoryIndex should be: vote.php

Edit: In addition, if I go to domain1.com/page/ the DirectoryIndex should be: index.html. If I go to mobile.domain1.com/page/ the DirectoryIndex should be: mobile-index.html

What can I put in my .htaccess file in order to change the DirectoryIndex for each sub-domain?

Thank You very mich