Circles importing as crossed boxes into Adobe Illustrator

Question

I am reading into Illustrator a PDF output from R (grid graphics I believe) in which the circles show as boxes with crosses inside them. These pictures illustrate:

In OSX Preview:

In Illustrator:

Any idea why this is happening, and how to work around it?

The following Illustrator message pops up when opening the file, which may be significant.

Answer 1

You're missing the AdobePi font, specifically, that glyph. That's exactly what that message states... and the [X] is an indicator of a missing glyph.

Select the [X]s with the text tool, and choose a different font. For example Zapf Dingbats has a Circle glyph.