Gametophyte ecology

DONALD R. FARRAR, CYNTHIA DASSLER, JAMES E. WATKINS, JR., AND CHANDA SKELTON

9.1 Introduction

Seed plant ecologists would find incredulous a proposal to study the ecology of a species that did not include critical examination of all aspects of recruitment in the field, relying instead on laboratory studies of seed germination, seedling growth and mortality, etc. Yet students of fern and lycophyte ecology are limited to data on gametophyte growth and reproduction collected almost exclusively from laboratory studies. They are expected to assume that these studies accurately reflect growth and reproduction in nature. Field investigations of gametophyte biology are minimal on such critical topics as: the role of morphological and physiological diversity among gametophyte taxa in habitat selection; the time frame and method of gametophyte development, maturation, sexual differentiation, and sporophyte production; the breeding systems and habitats effectively contributing new recruits to sporophyte populations; or the number and frequency of recruits. In addition to producing a decidedly unbalanced view of fern and lycophyte ecology, the absence of ecological data on the gametophytic phase of fern biology has left science with important misconceptions regarding this critical phase of the life cycle. The gametophyte not only provides the opportunity for sexual reproduction (and thus controls genetic diversity) but also determines (along with vegetative reproduction) recruitment, species habitat selection, species migration, and, ultimately, fern and lycophyte evolution.

Reasons for the dearth of field studies on gametophyte ecology are both perceived and real. It is much more difficult to find and to identify gametophyte

Biology and Evolution of Ferns and Lycophytes, ed. Tom A. Ranker and Christopher H. Haufler. Published by Cambridge University Press. © Cambridge University Press 2008.

plants than to do the same with sporophytes. Following the fate of gametophyte populations and newly produced sporophytes requires continuous access to study sites and dedicated observation for long periods. Although difficult, such studies are possible and the rewards, in the context of current knowledge, are immense. In this chapter we will examine the problems posed by field studies and will

• discuss why field studies are important to understanding fern biology,

• describe methods that have been successfully employed, and

• present recent advances in our understanding of the gametophyte generation that promise deeper insight into the role of that generation in determining the ecology and evolution of fern and lycophyte species.

Ferns and lycophytes are vascular land plants composed of two alternating and morphologically very different growth forms, each of which is free-living, i.e., the majority of the life span of each is nutritionally and physically independent of the other. We refer to these two forms as "sporophyte," the relatively large and familiar spore-producing plant and "gametophyte," the generally obscure and relatively featureless gamete-producing plant (see Chapter 2). Even apogamous species, which avoid complications of sexual reproduction, have a gametophytic phase, and most ecological considerations apply to their prothalli.

With few exceptions, the sporophyte phase of ferns and lycophytes is a perennial plant of potentially indeterminate growth. As long as the local environment is suitable, an individual plant or clone can exist indefinitely. It is likely that terrestrial and epipetric ferns and lycophytes are the oldest members of their community, having persisted vegetatively in their local area for centuries or perhaps millennia.

Gametophytes are usually not considered to be perennial, and for many, especially terrestrial species with photosynthetic gametophytes, this may be the rule. However, a large and underappreciated number of epiphytic and epipet-ric (and some terrestrial) species do have gametophytes that persist indefinitely through vegetative growth and proliferation. Furthermore, a large number of these species are capable of local dispersion through production of vegetative propagules (gemmae). Thus both gametophyte and sporophyte phases of ferns and lycophytes can persist indefinitely in a local supportive habitat. Regardless, the initial introduction of a species into a local area is through successful spore dispersal, spore germination, gametophyte development, gametangia formation, fertilization, and production of a viable sporophyte. From this it follows that studies of species distributions, species migration, habitat selection, etc. must take into account the particular growth form and the ecology of the gametophyte generation.

Important historical studies on the gametophyte generation of ferns and lycophytes began with the elaboration of the fern life cycle by Hofmeister (1862). Made aware of the vital role of the gametophyte, natural historians of the late nineteenth and early twentieth centuries sought and illustrated gametophytes of a number of fern and lycophyte species (e.g., Campbell, 1905; Goebel, 1905; Bower, 1923). It was not until the mid twentieth century, however, after detailed description of a wide diversity of taxa by Stokey (1951), Atkinson and Stokey (1964), Nayar and Kaur (1971), Atkinson (1973), and others that important patterns in development and mature morphologies were detected within the diverse array of gametophyte forms. Similarly, Nayar and Kaur (1969) detected four fundamentally different pathways of early development in fern gametophytes that segregate along genus and family lines. In addition to describing characters useful in phylogenetic classification, these studies also formed the basis for field identification of gametophytes, especially to the level of genus, and laid the groundwork for future gametophyte-based ecological studies.

Development of culture methods (e.g., Andersson, 1923) and recognition of systematic differences among taxa also led to more elaborate studies on the developmental response of gametophytes to environmental stimuli. Effects of light quantity, quality, and direction, as well as reponses to chemical signals, revealed a highly complex physiology within these morphologically simple plants (Miller, 1968; Raghavan, 1989). Of special importance was the elaboration of an antheridiogen system by which older or developmentally more advanced plants within a population secrete a gibberellin-like chemical into the substrate that promotes antheridium formation in younger or developmentally less advanced plants (Dopp, 1962; Naf, 1979; Chapter 5). This "antheridiogen" also promoted dark germination of spores of species otherwise requiring light for germination (reviewed in Voeller and Weinberg, 1969). Thus sexual differentiation in gametophyte populations could involve not only gametophytes developed from recently deposited spores, but also gametophytes developed from spores that had arrived earlier and were dormant in a shallow but dark spore bank.

Detailed field studies of natural populations of fern gametophytes began in the 1970s (e.g., Lloyd, 1974; Farrar and Gooch, 1975; Cousens, 1981, 1988; Peck et al., 1990). These studies greatly elevated holistic ecological studies by (1) analyzing gametophyte establishment and persistence/mortality through time, (2) considering the nature of ''safe sites" that can support new sporophyte recruitment, and (3) recognizing the interplay of genetic variability, breeding systems, and gametophyte density in governing the migratory ability of species. In this regard the ''isolate potential," the potential for viable sporophyte production by individual gametophytes developing in isolation following long-distance dispersal,

Table 9.1 Classification of leptosporangiate fern gametophytes into five types in three ecologically functional groups based on form, type of meristem, type of proliferation and

longevity

Type

Shape

Meristem

Description

Functional group

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  • Keira
    What is the role of the gametophyte in ferns?
    2 years ago

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