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

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. On 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.