The origins of heterospory: a comparative study of sexual behaviour in the fern Platyzoma microphyllum R.Br. and the horsetail Equisetum giganteum L.

Abstract

DUCKETT, J. G. & PANG, W. C., 1984. The origins of heterospory: a comparative study of sexual behaviour in the fern Platyzoma microphyllum R.Br. and Equisetum giganteurn L. ‘The heterosporous fern Platyzoma microphyllum has exosporic photosynthetic gametophytes. Those from the small spores are filamentous, with few rhizoids and produce antheridia alone. The gametophytes from the large spores comprise a massive base bearing numerous rhizoids and an upright spathulate lobe of tissue one cell thick. They initially produce archegonia near the base but subsequently develop antheridia on the lobe. Adventitious outgrowths are produced after prolonged culture. These secondary individuals vary from branched filaments bearing numerous antheridia to spathulate lobes with archegonia near the base. Gametophyte sexuality determined during sporogenesis can be modified by subculturing gametophyte fragments. When isolated, portions of gametophytes from both kinds of spores produce adventitious individuals bearing antheridia or archegonia. The proportions of these secondary male and female outgrowths vary according to the nature of the fragment rather than the kind of spore from which the material was derived. The sexual behaviour of Equiseturn giganteurn follows the same pattern as in other species of Equisetum subgenus Hippochaete. Male and female secondary gametophytes may be derived from initially male or female individuals as a result of lamellar proliferation both on intact gametophytes and on subcultures derived from excised lamellae. The antheridia of E. giganteurn are indistinguishable from those of other species of the subgenus but the larnellae and archegonia are distinct for each speries. Detectable genetir load is absent in both Equisetum and Plafyzomo. On the basis of the striking similarities in the sexual behaviour of Equisetum and Plafyzoma it is suggested that a crucial step in the development of heterospory is an intimate association between gametophyte morphogenesis and sex organ formation. Gametophyte dioecism and dimorphism resulting from this association may have preceded heterospory in evolution.

Heterospory: The Most Iterative Key Innovation in the Evolutionary History of the Plant Kingdom

“…Duckett, 197oa, b, , 1977. Duckett, 197oa, b, , 1979aDuckett & Duckett, 1980;Duckett & Pang, 1984). Sporne (1964Sporne ( , 1975 argued that individual prothalli experienced a brief period of bisexuality and thus facultative autogamy in culture, but such temporally restricted gametophytic bisexuality appears to be rare in nature (Duckett & Duckett, 1980).…”

“…I 6 larger spores that consistently generate sequentially monoicous gametophytes (Fig. 66;Tryon, 1964;Tryon & Vida, 1967;Duckett & Pang, 1984). The difference in spore number reflects one less mitotic division of the archesporial cells rather than abortion of meiotic products (Haig & Westoby, 1988a). In the case of Platyzoma", dioicy is detectable by heterospory (albeit subtly expressed).…”

“…However, with a few exceptions (e.g. Lyon, 1904;Pincher, 1935;Manton, 1950;Sussex, 1966;Andrews, Gensel & Forbes, 1974;Bell, 1979Bell, , 1989Duckett & Pang, 1984; Turnau & Karczewska, 1 9 8 7 )~ reviewers have tended to view heterospory primarily as a precursor to the seed habit rather than a profound innovation in itself (e.g. Coulter, 1898; Thomson, 1927Thomson, , 1934Walton, 1953;Andrews, 1963;Meeuse, 1963;Smith, 1964;Long, 1966;Pettitt, 1970;Jonker, 1977; Chaloner & Sheerin, 1 9 8 1 ; Steeves, 1983;Crane, 1985a, b ;Doyle & Donoghue, 1986;Chaloner & Pettitt, 1987;Stein & Beck, 1987;Beck & Wight, 1988;Rothwell & Scheckler, 1988; "...bearing spores of distinctly different types".…”

“…Bell required instead a more stringent physiological criterion to define heterospory sensu stricto : the degeneration of spore mother cells and/or members of post-meiotic spore tetrads. Bell's decision was criticized by Duckett & Pang (1984), who argued that anisospory should be confined to morphologically bimodal spore populations occupying single sporangia (e. g. Vitt,I 968) and that size segregation in different sporangia was sufficient to define heterospory.…”

“…Chaloner & Pettitt, 1987;Pennell, 1988). Together, these heterosporic phenomena have engendered an unusually large number of biological controversies (cf. Bell, 1979 ;Duckett & Pang, 1984 smaller viable spore morph megaspore : cf. isospore : monomorphic spore larger viable spore morph antheridia and archegonia borne on separate conspecific gametophytes antheridia and archegonia borne on the same conspecific gametophyte male and female reproductive organs borne on separate conspecific sporophytes male and female reproductive organs borne on the same conspecific sporophyte obligately male viable spore morph obligately or facultatively female viable spore morph spore dimorphs apportioned among different sporangia mature gametophyte largely enclosed by spore \\all endomicrospory : mature male gametophyte largely enclosed by spore wall endomegaspory : mature female gametophyte largely enclosed by spore wall monomegaspory : single viable megaspore per megasporangium endomegasporangy : megaspore(s) routinely dispersed within megasporangium integumentation :…”