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,)

P van Rijckevorsel writes
I am not much on ferns, but I don't see anything on hybrids in these
links?

Stewart Robert Hinsley schreef in
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

+ + +
Ah yes, I must have fallen prey to the modern disease of being unable to
read text unless this is accompanied by a phylogenetic tree ;-)
PvR