Induced apogamy in Tmesipteris (Psilotaceae)

Gametophytes of Tmesipteris lanceolata Dang., which are mycorrhizal in nature, were grown in axenic culture. If cultured in the light on a nutrient medium containing minerals and 0.5% glucose, they did not become photosynthetic; however, about 15% of them produced apogamous sporophytes with stems and microphylls. The gametophyte–sporophyte junction had a direct connection between the gametophyte and sporophyte tissues and lacked a foot, which is typical for apogamy. Gametangia were limited to the gametophyte portions of these gametophyte–sporophyte growths, and the vascular tissue was present only in the sporophyte regions. The apogamous aerial stems had the normal anatomy for a sporophyte, with vascular tissue, epidermal cells, stomata, and chlorenchyma. The origin of the apogamous sporophytes was different from the origin in fern gametophytes. The Tmesipteris sporophytes arose terminally from the gametophyte apices. It appears that the apical meristem of the gametophyte is converted to a shoot apical meristem to form the apogamous aerial shoot.Key words: Tmesipteris, Psilotaceae, apogamy, sporophyte, gametophyte.

Meiosis and sporogenesis in a haploid plant of Pellaea glabella var. occidentalis

In spore mother cells of a sporophyte of Pellaea glabella var. occidentalis (E. Nelson) Butters resulting from induced apogamy, the chromosomes did not pair during prophase I and separated randomly, usually into two groups. Meiosis II generally produced tetrads with nuclei of unequal sizes. Spore walls began to form around these nuclei, but most of the spores eventually aborted. One apparently normal spore, which later germinated, is assumed to have resulted from the formation of a restitution nucleus at the end of meiosis I.The total lack of pairing among the 29 chromosomes of this plant is evidence that this number represents the basic haploid complement of the species.

The effect of mineral nutrition on apogamy in Pteridium

The effect of mineral nutrition on apogamy was studied because earlier investigators proposed that low mineral levels induced apogamy. The weight of the gametophytic tissue and the number of apogamous plants per culture and per gram of gametophytic tissue were determined in relation to variations in the levels of mineral elements. The apogamous response was promoted only by high concentrations of phosphorus, and an increase in the gametophytic weight was produced by high levels of potassium. The omission of any of the elements from the nutrient medium inhibited apogamy and, except calcium, reduced the gametophytic weight.

INDUCED APOGAMY IN DIPLOID GAMETOPHYTES OF PTERIDIUM

Diploid gametophytes were produced via self-fertilization and apospory from a strain of haploid gametophytes which underwent apogamy in sterile culture. These diploid gametophytes of the fern, Pteridium, demonstrated quantitative differences from the haploid prothalli. The diameters of the rhizoids and antheridia were larger on the diploid prothalli. The diploid gametophytes grew slightly faster than the haploids. Under conditions which induced apogamy in the haploid prothalli, the diploids produced more apogamous sporophytes on a culture or on a weight basis than did the haploid prothalli.

FURTHER STUDIES ON INDUCED APOGAMY IN FERNS

Apogamous sporophyte formation has been induced in 14 strains of seven species of normal fern gametophytes using a technique similar to the one reported earlier (Whittier and Steeves, Can. J. Bot. 38, 925–930 (1960)). These gametophytes were supplied with suitable concentrations of sugar in the sterile culture medium for certain lengths of time which resulted in the formation of apogamous sporophytes. This investigation demonstrated that sucrose is more effective in inducing apogamy probably because it produces about half the osmotic potential as an equivalent percentage of glucose. Therefore more sugar can be made available to gametophytes before the osmotic potential deleteriously affects the prothallial growth and apogamy. This confirms the work carried out previously using sugar in the medium and it demonstrates that this method has general applicability for inducing apogamy in ferns. The sugar in the medium is felt to modify the carbohydrate metabolism enabling the prothalli to undergo a thickened growth and develop apogamous sporophytes.