Adam Wexler (fuller@u.washington.edu)
Fri, 24 Jun 1994 12:18:51 -0700 (PDT)

Many interspecific crosses fail not because the embryo is deffective but
because the endosperm is aborted. Endosperm formation is governed by
the EBN or Endosperm Balance Number, this is an arbitrary factor used by
plant breeders to make succesful crosses. It is possible that in some
CPs the EBN is very plastic (i.e. D. eneabba) because the chromosome
number and ploidy vary so widely in one "morphological species". You
can beat the EBN; a technique known as embryo rescue is used alot in the
production of new crops. It requires axenic conditions and a steady
hand but if you REALLY want to make some crosses you can. For
intergeneric crosses the fusion of protplasts, cells without walls, also
requires axenic conditions. So if you are already setup for tissue
culture or know someone who is, these techniques aren't too far out
there. But as for what a species is, that's what I work on, I have no
simple answer.