Ecological adaptations of gametophyte types

Fast-growing, annual, cordiform gametophytes appear well adapted to short-lived habitats produced by disturbance on the forest floor by erosion, tip-up mounds, trail and roadsides, etc. For a period of time, gametophytes in these sites would be largely free of competition and free from shading effects of dense forest litter. Annual cordiform gametophytes tend to grow in large populations sufficiently dense for intergametophytic interactions, including responses induced by antheridiogens (e.g., Tryon and Vitale, 1977; Hamilton and Lloyd, 1991). Successful populations would need to produce sporophytes before litter deposition or competing plant growth renders the site unsuitable for their continued growth.

More slowly growing, perennial, strap and ribbon gametophytes are ill equipped for reproduction in the fast lane of temporary habitats. Through their longevity and branching habit, they appear better suited to more or less mature habitats already occupied by competing vegetation, especially in bryophyte mats. Gametophytes of these types can grow intertwined among bryophyte stems, which may allow them to survive until favorable conditions for sporophyte production arise and, through elongation and branching, explore their habitat for more supportive microsites.

A putatively important capability of perennial strap and ribbon gametophytes is to provide increased opportunity for sporophyte production via outcrossing. For species carrying high genetic variability that includes recessive deleterious alleles, outcrossing may be necessary for production of viable sporophytes (see Chapter 4 and references therein). To this end, long-lived gametophytes and gametophyte clones may provide increased opportunity in both space and time for breeding between different genotypes arising from spores that do not arrive in the spatial and temporal proximity that is required by annual gameto-phytes. This hypothesis becomes especially relevant in considering long-distance migration.

Perennial ribbon gametophytes grow and branch more rapidly than do strap-shaped gametophytes (Farrar, personal observations). Ribbon gametophytes begin branching almost immediately upon germination, and, in their narrow, one-cell-thick thalli, seem to invest fewer resources per linear unit than do strap-shaped gametophytes. This reduction of morphological complexity reaches the extreme in the permanently filamentous gametophytes of Hymenophyllaceae and Schizaeaceae.

Gemma production by both strap-shaped and ribbon-like gametophytes would seem to expand their capacity for habitat exploration. Propagules derived from a single founding spore can be dispersed to supportive microsites throughout the local area, including different branches of different trees, a feat not achievable by perennial but non-gemmiferous gametophytes. Continuous production and dispersal of gemmae may make possible metapopulation dynamics in which newly supportive habitats can be colonized while older sites become unsuitable, thus accommodating individual population extinction.

The indefinite longevity of all perennial gametophytes, with or without gemmae, may be of special significance in colonization of new habitats that are kilometers, or thousands of kilometers, distant from the spore source (e.g., Rumsey et al., 1998). For species requiring mating between different genotypes, absence of a second genotype in close proximity following long-distance dispersal may severely restrict migratory ability. Indefinite persistence may allow gametophyte clones to avoid this impediment by acting in a manner similar to seed and spore banks. Banks of perennial gametophytes would permit interaction among genotypes (spores) arriving at different times, thus greatly enhancing colonization by outbreeding species of sites infrequently receiving spores. It is important in this regard to recognize the dual role of gemmae as both vegetative propagules and dispersible male gametophytes that may promote breeding between spatially separated colonies.

Gametophyte banks may also permit species' survival in areas that are unable to support sporophyte growth during unfavorable climatic periods. This is possible because of the gametophyte's ability to grow in smaller and more moderated microsites, and because of their possible greater tolerance of desiccation and/or freezing than their sporophyte counterparts (see Section 9.3). Extreme examples are the independent (non-sporophyte producing) gametophytes of several tropical species in temperate regions of the eastern USA (Farrar, 1998).

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