Induction of Apogamy in Pellaea glabella var. occidentalis

Abstract

Apogamy-the formation of a sporophyte from the vegetative cells of a gametophyte-can be divided into two types, obligate and induced. Obligate apogamy is the normal means of reproduction in certain fern species; in other, normally sexually reproducing species apogamy can be induced in response to specific cultural conditions. One condition necessary for the induction of apogamy is the prevention of fertilization. This can be achieved by witholding free water from the gametophytes (Lang, 1898; Manton, 1950; Steil, 1951). Whittier and Steeves (1960) found that sugar in the growth medium promoted the induction of apogamy in certain species of ferns; in a later study (1962) they noted that the optimum sugar concentration varied from one species to another. Other conditions which also affect the induction of apogamy are the presence of various growth substances (Whittier, 1966) and of ethylene (Elmore & Whittier, 1973) in the cultures, and the intensity and quality of the light to which the gametophytes are exposed (Whittier, 1964; Whittier & Pratt, 1971). Apogamy provides a system for studying the development of sporophytic from gametophytic tissue with no change in chromosome number. In addition, when sporophytes resulting from induced apogamy can be raised to maturity, the pairing patterns of their chromosomes at meiosis may indicate the amount of homology among the chromosomes, and thus may provide some evidence as to the evolution of the species (Manton, 1950; Manton & Walker, 1954; Bouharmont, 1972; Vida, 1972). Obligate apogamy occurs in several species of Pellaea (Tryon, 1957), but apogamy has not been experimentally induced in this genus. The object of the present study was to induce apogamy in a sexual variety of Pellaea in order to determine the conditions suitable for induction of apogamy in this taxon and also to obtain apogamous sporophytes to raise to maturity for chromosomal analysis.