FERN GAMETOPHYTE DEVELOPMENT: CONTROLS OF DIMORPHISM IN CERATOPTERIS THALICTROIDES

Multispore cultures of Ceratopteris thalictroides (L.) Brongn. gametophytes contain plants of two distinct morphologies, hermaphrodite and male (Fig. 3). The male gametophyte has previously been shown to be induced by an antheridogen system. In the absence of the antheridogen all gametophytes become hermaphroditic. It is shown that the hermaphroditic gametophytes are first to develop in multispore cultures and are also the only source of antheridogen. Thus, since males are only produced in the presence of antheridogen, an explanation of the occurrence of two morphologies is evident. That is, the first gametophytes to develop do so in the absence of antheridogen, become hermaphrodites and produce antheridogen to which later developing gametophytes respond by becoming males. This does not explain why there is a range in development times for the gametophytes. Three possible controlling factors of development are investigated in multispore cultures: spore size, time of germination, and gametophyte growth. Data collected on these three factors are statistically analyzed. Analyses were done to determine the relative importance (singly or in combination) of these factors in predicting the developmental potential of a gametophyte. It was found that spore size most accurately indicated (ca 75 % of the time) future gametophyte development. GAMETOPHYTE CULTURES of the fern Ceratopteris thalictroides contain plants of two distinct morphologies (Kny, 1875). One is spatulate in outline, lacks a notch meristem and bears only antheridia. This plant (Fig. 1) is referred to as a male gametophyte. The other plant is cordate, possesses a notch meristem and produces both antheridia and archegonia. This plant (Fig. 3) is referred to as an hermaphrodite. The occurrence of two morphologically distinct gametophytes has been documented under a number of growth conditions (Kny, 1875; Javalgekar, 1960; Nayar and Kaur, 1969; Pal and Pal, 1963). Both morphologies exist in multispore cultures at the same time. They are derived from two distinct developmental pathways-the hermaphrodite involves the initiation of a lateral notch meristem, the male does not (Fig. 6). It has also been documented that the two distinct morphologies exist only in multispore culture, and that the male morphology is inducible by the presence of an antheridogen (Schedlbauer and Klekowski, 1972). If no antheridogen is present, only the hermaphroditic morphology is found. In addition, male morphol1 Received for publication 26 May 1975. Critical review of this manuscript by Dr. Edward J. Klekowski, is gratefully acknowledged. Appreciation is also expressed to Dr. R. Freund, Dr. L. Ringer and Mr. J. Russel of the Institute of Statistics, Texas AM Wilkie, 1963), Anemia phyllitidis and Onoclea sensibilis, at least initially, have bigametophyte systems (Voeller, 1966), and Lygodium japonicum has a bigametophyte system (Naf, 1960). It is also possible that bigametophyte systems exist in other homosporous ferns. The relationship between gametophyte morphology and an antheridogen system is based on monospore culture data showing induction of male morphology by continuous presence of antheridogen. Absence or removal of antheridogen results in production of cordate or hermaphroditic gametophytes. Wilkie (1963) has reported the anomalous situation in which Pteridium males removed from antheridogen continue to produce only antheridia. In the Pteridium bigametophyte reproductive system, antheridogen is produced by the cordate gametophytes (Ddpp, 1950, 1959) or their precursors prior to notch meristem formation (Voeller, 1966), but not by males. It has also been noted in multispore cultures of P. aquilinum