notypes of double and triple mutants, a model for the control of sex determination in Ceratopterishas
been proposed [77,81,82,88], shown in Figure 7.
Unfortunately, a lack of modern molecular tools (e.g., transformation) available to Ceratopterisre-
searchers has thus far prevented the cloning of the preceding genes. However, progress is being made at
the molecular level, as witnessed by the recent cloning of a gene whose expression is induced by ACE[88].
The gene, called ani1, is also constitutively expressed in a tramutant, and, interestingly, is induced even
in the absence of ACEby the protein synthesis inhibitor cycloheximide. Also, homologues of the MADS-
box genes that control higher plant flowering [89,90] and of homeodomain-leucine zipper genes [91] are
being examined in Ceratopteris, and they are certainly candidates for involvement in the sex determina-
tion process.
Perhaps the designation of Ceratopterisas “a model plant for the 90s” [11], was a bit premature, but
surely it will catch up to the others soon, and the plant research community at large will become more
aware of its unique advantages.
V. OUTLOOK
The great excitement of discovery in plant molecular genetics that occurred in the 1980s due to the ad-
vent of the new tools of molecular cloning, sequencing, and transformation originally left lower plants a
bit by the wayside. Most likely this was largely due to the relative paucity of groups working with lower
plants when compared with angiosperms, so that the techniques were slower to develop. However, now
that most of the molecular techniques are developed for use in these model systems, their unique advan-
tages are once again becoming apparent: the ease of genetic analysis because of haploidy and rapid gen-
eration time, coupled with the relative simplicity of their development, make lower plants excellent model
systems both for some practical aspects of crop plant development and for the study of very fundamental
biological processes. It is well worth noting, in this era of bioinformatics and comparative genomics, that
“lower” animals such as C.elegans,Drosophila, and zebrafish are being found to be of ever-increasing
relevance to research in mammals in general and to medical research specifically. Almost certainly the
same will be found true of lower plants.
816 WALLACEFigure 7 Diagram of the genetic interactions proposed to control sex determination in Ceratopteris
richardii. In the presence of the antheridiogen ACE, the signal transduction cascade composed of the hergenes
results in activation of the femgenes and repression of the tragenes. The gametophyte develops as a male. In
the absence of ACE, the tragenes are active: they inhibit the femgenes, and a hermaphrodite develops. Active
genes or processes are represented in bold, inactive ones in italics. (Modified from Ref. 82.)