INTRACLONAL VARIATION IN MORPHOLOGY, GROWTH RATE, AND COPPER TOLERANCE IN THE MOSS, FUNARIA HYGROMETRICA.

Abstract

Homosporous plants such as bryophytes, ferns, and fern allies, form one kind of sporangium in which meiosis occurs, and the resulting spores produce multicellular, haploid gametophytes. Gametes formed by a single gametophyte are produced mitotically and are therefore genetically uniform (barring mutation). Homosporous plants can undergo outbreeding (crossing between unrelated gametophytes) or various degrees of inbreeding (crossing between gametophytes derived from related sporophytes). In addition, homosporous plants can undergo true self-fertilization: union of egg and sperm from a single gametophyte. Inbreeding, including intragametophytic self-fertilization (the union of egg and sperm from a single, bisexual gametophyte; Klekowski, 1979) reduces the amount of genetic variability among haploid siblings produced by meiosis in the diploid sporophytes. Selfing, in fact, results in completely homozygous sporophytes, and meiosis in such sporophytes is expected to give rise to genetically identical spores. The diploid and haploid life-history stages of bryophytes (mosses, liverworts, and hornworts) are welldeveloped, and the stages coexist within populations. Diploid sporophytes bear a single terminal sporangium in which meiosis occurs, producing families of haploid spore progeny. The total amount of genetic variation existing among spore progeny from a can be partitioned into components attributable to differences among families of haploids derived from different sporophytes and to differences among siblings derived from the same sporophyte. Such families of haploids, related as meiotic derivatives of individual sporophytes, can be referred to as haploid sib families (Shaw et al., 1989). Genetically, haploid sib families are analogous to the of gametes produced by a diploid organism. In the present study, a synthetic of sporophytes was produced by intragametophytic selfing in a single gametophytic clone as a basis for comparison with natural populations. Growth rate and copper tolerance were measured in 84 gametophytes representing 21 haploid sib families, all derived by selfing in one gametophyte. The obvious prediction is that no genetic variation exists, either within or between haploid sib families. The species utilized in this study, Funaria hygrometrica, exhibits a typical bryophytic life cycle, which can be summarized as follows. Haploid spores germinate to form a filamentous gametophytic stage, the protonema. The protonema from a single spore is shortlived but gives rise to hundreds or thousands of leafy stems. The stems of F. hygrometrica are autoecious, forming clusters of antheridia (bearing sperm) in inflorescences terminating the main stem and clusters of archegonia (bearing eggs) terminating side branches formed a few millimeters below the antheridia. Fertilization results in a diploid sporophyte, consisting of an unbranched stalk, or seta, terminating in a single sporangium, or capsule, in which meiosis occurs. Each capsule of F. hygrometrica contains several hundred thousand spores (Kreulen, 1972). Since a single spore produces a mat of protonema and thousands of stems, each bearing archegonia and antheridia, numerous genetically identical sporophytes can be formed. The clonal population of sporophytes and gametophytes used in the present work was produced in this manner, as summarized in Figure 1. In nature, the haploid gametophytes are perennial and free-living, whereas the sporophytes are produced once annually and remain attached to the gametophytes throughout their development.