Control Points in the Generation Cycle of Ferns

Abstract

Summary The homosporous ferns, having an independent gametophytic phase, are well suited for the study of the factors controlling the life cycle. The sites of phase change can be identified as gametogenesis and sporogenesis respectively. The aspect of gametogenesis significant in relation to the change of growth appears to be oogenesis and the generation of the complex cytoplasm of the egg cell, to which the spermatozoid adds little more than a complement of Mendelian genes. The archegonium, the site of oogenesis, is surrounded by a thickened wall lacking plasmodesmata, probably impermeable to large molecules. Autoradiographs of maturing archegonia following feeding with tritiated nucleosides show that the products of lysis of the cells in the upper part of the canal move down to the egg cell, which becomes in consequence heavily labelled. In microbial cells the existence of «global regulatory networks» coming into play in response to stressful changes in the cell's environment and controlling gene activation are well established (see, for example, Slater, J. H. et al.; 1983). It would be strange if eukaryotic cells were not similarly equipped. In the particular instance of the egg cell, subjected to the assault of complex macromolecules in the intimate confines of the archegonium, we may legitimately suppose that amongst the genes activated by the regulatory network are those responsible for sporophytic growth. The presence of sporophytic messengers already present in the mature gamete is indicated by its failure to produce stable embryos when its RNA is modified during maturation by the incorporation of uridine analogues. The deactivation of the sporophytic genes in sporogenesis is envisaged as resulting from the isolation of the spore mother cells from the correlating influences of the parent sporophyte. Although there is substantial loss of cytoplasmic RNA during sporogenesis in ferns, recent work with gymnosperms (particularly Taxus) has revealed that this is not a universal phenomenon in meiotic prophase. The extent of the depolymerization may be related, possibly causally, more with the time spent in meiosis than with change of phase. Experiments with apospory have revealed isolation from the parent plant to be the common element in all instances of transition from sporophyte to gametophyte. In sporangia this is achieved by the thickened outer boundary of the tapetum and the massive and often callosed wall of the spore mother cell, in both instances conspicuously free of plasmodesmata. In the case of excised sporophytic tissue in vitro gametophytic outgrowths may appear within one or two days, but spores, lying at maturity in dry containers, must await liberation and hydration. Megasporogenesis in the heterosporous ferns takes on features of oogenesis in the homosporous. The surviving megaspore matures in a particularly rich environment to which the lysis of the abortive spores contributes. This may initiate a precocious reactivation of the sporophytic genes, and account for the simpler archegonium and egg cell in these ferns. On this view heterosporous cycles lack a pure gametophytic phase on the female side. This interpretation also holds for the heterosporous seed plants. The remarkable similarity between a linear tetrad of megaspores and an archegonium may extend beyond morphology to function, the haploid genome of the surviving megaspore being subjected to an environmental stress similar to that experienced by the genome of the egg cell of a homosporous fern. [The experimental work on which this abstract is based will be found reviewed in Sheffield, E. and Bell, P. R. Current studies of the pteridophyte life cycle. Bot. Rev. (53, 442-490 (1987)].