[XeTeX] Changing Font Styles for a defined font

[Andy Black]

Hello.

I'm working on writing an XSLT that will take an XML-based document 
mark-up package and format it via XeLaTeX.

The XML document has language elements which include font information 
(such as font-family, font-style, font-size, and font-weight).  I've 
learned that by using the fontspec package, I can define fonts such as

    \font\LangTuzFont="Charis SIL/I" at 10pt

This works just fine to display some text within a table, say, such as this:

    &\LangTuzFont{Duu}&

What the XML mark-up allows, however, and what I have not been able to 
figure out how to do in XeLaTeX so far, is to modify some aspect of the 
font such as

    &\LangTuzFont{Duu-\bfseries{gu}}&

where I want the result (Duu-gu) to all be in the Charis SIL font with 
italic, with the "-gu" portion to also be bold (as well as Charis SIL 
italic).  (The idea is to draw the reader's attention to the bold part.)

What I am getting is the "Duu" part in Charis SIL italic, but the "-gu" 
part is bold in the default font of the document.

Could someone please tell me how to get what I want?

Thanks so much.,

--Andy