Gametophyte Sex Expression in Some Species of Dryopteris

Abstract

The study of sexuality in fern gametophytes has stimulated research in the genetic consequences of gametophyte sex expression (Klekowski, 1971) and in the physiology of sex determination in ferns (Dopp, 1950; Voeller, 1971). A class of antheridia-inducing hormones, antheridogens, has been discovered in several genera of ferns including Dryopteris (Dopp, 1950; Voeller, 1964). Homosporous ferns differ fundamentally from heterosporous plants in their potential for intragametophytic selfing, which eliminates heterozygosity by the union of genetically identical egg and sperm derived mitotically from a single gametophyte (Klekowski & Lloyd, 1968). Homeologous pairing in polyploids is a possible mechanism for expression of heterozygosity following intragametophytic selfing (Klekowski & Hickok, 1974). Intragametophytic selfing and intergametophytic crossing are genetically equivalent to self pollination and cross pollination in heterosporous plants. The occurence of bisexual, male, and female gametophytes in multispore cultures of Dryopteris ludoviciana suggested that gametophytes of this genus are not regularly bisexual (Cousens & Horner, 1970). This paper extends the observation of gametophyte sex expression to seven additional species of Dryopteris. Various patterns of sex expression may be adaptive in allowing different balances of intraand intergametophytic mating in different environments (Klekowski, 1972). Dryopteris is a large genus of ferns notable for frequent hybridization between species (Wagner, 1970). Three tetraploid species studied, D. campyloptera, D. spinulosa, and D. celsa, probably arose by chromosome doubling of sterile hybrids; their ancestral taxa are either known or hypothesized (Wagner, 1970; Walker, 1961, 1962). Dryopteris clintoniana is a hexaploid hybrid of D. goldiana and D. cristata (Wagner, 1970). Dryopteris intermedia, D. goldiana, D. marginalis, and D. ludoviciana are all sexual diploids. Cultures of Dryopteris gametophytes were initiated by inserting a sterile wooden applicator into 25 ml spore vials and transferring 400-600 adherent spores to a 5 ml beaker which was then filled with sterilized water to which a trace of detergent had been added. Each species studied was represented by spore samples from two or more sporophytes. After the spores were dispersed in the liquid, the suspension was divided among three sterile 60 mm petri dishes filled with sterile clay-loam soil. Only cultures which resulted in a gametophyte population density of 1-3 gametophytes per cm2 were used because previous work has shown that culture density modifies sex expression (Cousens & Horner, 1970; Rashid,