Towards an understanding of sex determination in Equisetum: an analysis of regeneration in gametophytes of the subgenus Equisetum

Abstract

Equisetum gametophytes possess extensive powers of regeneration without showing any signs of diminished vigour or abnormal development. Isolated lamellae, pieces of antheridial and archegonial branches were used to analyse gametophyte morphogenesis and sex determination. Intact individuals and subcultured sexual branches never revert to a vegetative state. Transplantation of apical pieces of archegonial branches on to fresh medium delays the onset of antheridium formation indicating phenotypic control. O n the other hand, male branches invariably continue producing antheridia. Experimentally induced adventitious branches on lamellae are interpreted as the result of physiological isolation from inhibitors. In intact individuals these adventitious branches are always antheridial: after subculturing, archegonial branches are also produced, demonstrating that the gametophytes of Equisetum are all potentially bisexual. Isolated lamellae regenerate secondary gametophytes, the sexual ratios between which vary between species, but lamellae from archegonial tissues invariably produce more female individuals, probably reflecting the different sizes of the respective lamellae. The view that mixtures of intact male and bisexual Equisetum gametophytes result from inherent physiological differences between the spores is not supported.

Reproductive biology and population dynamics of wild gametophytes of Equisetum

“…Data on the sexuality of cultured gametophytes are taken from Duckett (1968Duckett ( , 1970aDuckett ( , 1977 where details ofmedia and conditions are also given.…”

“…With the knowledge that the sex change of initially female Equisetum gametophytes to antheridium formation is mediated by accumulation of metabolites in culture (Duckett, 1977) a factor clearly of much lesser importance in nature, the rarity o f wild bisexual individuals is readily explained. The latter situation is almost certainly one of'the principle reasons why many early workers described the gametophytes as strictly dioecious.…”

“…As far as we are aware, the precise conditions for sporeling development in the wild have not been investigated for a single pteridophyte, although the phenomenon provides a powerful tool for morphogenetic research (see Miller, 1968, for review). Culture experiments may shed considerable light on the mechanism of sex determination (Duckett, 1970a(Duckett, , 1972(Duckett, , 1977Holbrook, Walker & Lloyd, 1973;Klekowski, 1969a,b, 197Oa,b,c, 197 1, 1972a,b, 1973Lloyd, 1974a,b) and nature of any self-incompatibility mechanisms. Without recourse to field observations, the sexual behaviour of gametophytes must remain theoretical, in relation to sporophyte production and the assessment of the significance of criteria for sexual reproduction, out-breeding and attendant possibilities for hybridization.…”

“…Against a background of predictions about the natural reproductive biology of Equzsetum based on sexual behaviour and sporophyte production in axenic cultures (Duckett, 1970a(Duckett, , 1972(Duckett, , 1977 our sampling of wild gametophyte populations was specifically designed to investigate the following points.…”

SEX RATIO VARIATION IN THE LESSONIA NIGRESCENS COMPLEX (LAMINARIALES, PHAEOPHYCEAE): EFFECT OF LATITUDE, TEMPERATURE, AND MARGINALITY1

“…A case of complete ESD (i.e., no evidence of GSD) was reported in Equisetum sp., a horsetail with haplo‐diploid life cycle. Indeed, it was shown that haploid gametophytes were potentially bisexual (Duckett 1977), and that sex expression was, at least partly, determined by environmental factors (Guillon and Fievet 2003). Other mechanisms involved in sex determination evolved in the fern Ceratopteris richardii , where male:hermaphrodite ratios are determined epigenetically by an antheridiogen pheromone; thus, male frequencies increased with the density of the population (Tanurdzic and Banks 2004).…”