gnus-summary-dummy-line-format

gnus-summary-dummy-line-format is a variable defined in `gnus-sum.el'.
Its value is

"   %(:                             :%) %S
"

Documentation:
*The format specification for the dummy roots in the summary buffer.
It works along the same lines as a normal formatting string,
with some simple extensions.

%S The subject

General format specifiers can also be used.
See `(gnus)Formatting Variables'.

You can customize this variable.