In article , P van Rijckevorsel
writes
http://www.botany2002.org/section11/abstracts/4.shtml
http://www.jaknouse.athens.oh.us/ferns/g_azol.html

It's not clear whether Aa. microphylla and mexicana represent one or two
hybridisation events. (Or even zero, depending on whether you accept the
position given in the first link,)

+ + +
I am not much on ferns, but I don't see anything on hybrids in these links?

From the first link

"we suggest the Azolla microphylla - mexicana lineage arose through
hybridization and homoploid speciation involving A. caroliniana and A.
filiculoides."
--
Stewart Robert Hinsley