IV. FERNS
A. General Considerations
Fern development, especially of the gametophyte, has been a favorite object of study for many years [76].
When compared with those of VolvoxandPhyscomitrella, however, genetic analysis of fern development
is relatively new. Studies with the fern Ceratopteris richardii, however, are showing promise for the un-
derstanding of some fundamental features of its developmental genetics [10,11,77,78]. Like mosses, ferns
have a free-living, independent gametophyte stage that greatly aids mutant isolation and analysis. Diploid
gametophytes can be generated via apospory for assessment of dominance relationships [79]. Like
Physcomitrella,Ceratopterisis self-fertile, so homozygous sporophytes can easily be obtained, and its life
cycle can be completed in about 3 months. The highly developed vascular system and the independence of
the sporophyte, however, clearly make the study of ferns more relevant to higher plants than that of mosses.
Fern gametophyte development has been reviewed [77]. Like that of mosses, it begins with spore
germination producing a protonema. Protonemal development in ferns, however, is one dimensional and
very limited: after only a few cell divisions have resulted in a linear filament, the apical cell commences
two-dimensional divisions and becomes the meristem of the flat, heart-shaped prothallus. The sexual
archegonia and antheridia eventually develop on the prothallus (see later). After fertilization is effected,
the sporophyte rapidly outgrows the gametophyte and assumes an independent existence.
As already mentioned, the genetic study of ferns is still young, so relatively few mutants have been
isolated and characterized. A number of mutants resistant to metabolic inhibitors and herbicides, very use-
ful for traditional genetic analysis, have been isolated by Hickok and colleagues (reviewed in Ref. 10). In
addition, salt-tolerant mutants [78], mutants in photomorphogenesis of germinating spores [80], and mu-
tants with altered sex determination [81,82] have all been described. As the last subject is especially rel-
evant to the scope of this chapter, it will be described further.
B. Sex Determination
Unlike most higher plants, Ceratopterisis homosporous; i.e., it produces only one type of spore. Based
on environmental and other signals, the resulting gametophyte then develops into a male that produces
antheridia or a hermaphrodite that produces both antheridia and archegonia. By contrast, in higher plants
the decision as to what type of gametophyte will be produced is determined in the sporophyte, i.e.,
whether a stamen that produces microspores or a pistil that produces megaspores develops. There have
been some advances in the genetics of sex determination in the fern sporophyte (reviewed in Refs. 77, 81,
and 82), which are summarized in the following.
Male and hermaphrodite fern gametophytes differ by more than the types of gametes they produce.
The hermaphrodite consists of a heart-shaped leaflike prothallus about 2 mm in diameter that contains a
notched meristem. Archegonia develop below the notch and antheridia above it. The male gametophyte
lacks a meristem and is consequently only about one quarter the size of the hermaphrodite, which aids in
the screening for developmentally altered mutants. Essentially all cells of the male develop into antheridia.
If no outside signals are received, the default pathway for gametophyte development is the
hermaphrodite. Once the meristem develops, a pheromone (antheridiogen Ceratopterisor ACE) is pro-
duced, and this induces younger gametophytes to follow the male developmental pathway. Thus, in dense
populations of Ceratopteristhere is a high concentration of ACE, and essentially all gametophytes develop
as males, encouraging outcrossing and discouraging overcrowding. Abscisic acid acts as an antagonist to
ACE[83], which is not too surprising because ACEis very likely a gibberellin [84]. In hermaphrodites it
seems that there is nearly simultaneous development of (1) commitment to the hermaphrodite develop-
mental pathway, (2) development of the meristem, (3) production and secretion of ACE, and (4) loss of
sensitivity to the pheromone. One suspects that these traits are all controlled by the same “master gene,”
perhaps the tragene described in the following.
Several mutants have been described that affect the normal pathway [85–87]. Hermutants, in at least
five loci, develop as hermaphrodites regardless of the presence of ACEand are thus likely to be impaired
in the perception and/or signal transduction of the pheromonal signal. Tramutants (at least two loci) al-
ways develop as males, and manmutants produce supernormal numbers of antheridia in hermaphrodites.
Interestingly, some traandmanmutants also display an altered sporophyte phenotype. Finally, femmu-
tants always develop as females, which resemble hermaphrodites but lack antheridia. Based on the phe-
DEVELOPMENTAL GENETICS IN LOWER PLANTS 815