Morphological comparison of laboratory and field grown gametophytes

By comparing field and laboratory grown gametophytes of the same species, we can consider which morphological characters remain constant across growth conditions. Such characters are presumably genetically determined, they form the basis for field identification of gametophyte taxa, and may be used for phylogenetic analyses. If culture conditions are altered, it may be possible to determine the causes of differences we observe in field grown gametophytes, especially with regard to sexual development and sex ratios. Results of these studies are relevant to determination of breeding systems.

The work of Stokey (1951), Atkinson (1973), Nayar and Kaur (1971), and others identified developmental and morphological characters that remained constant within genera and higher taxa. These characters, along with juvenile sporo-phytes, enabled later workers (e.g., Peck et al., 1990) to identify the species that produce natural populations of fern gametophytes. However, no one has determined the constancy/non-constancy of morphological and developmental characteristics between field and laboratory grown gametophytes of any species.

In our studies (Skelton, 2007), in addition to taxon specific gametophyte characters, identification of most fern gametophyte populations in the field was aided because young sporophytes at various ages were found associated with gametophytes and could be traced through development to stages old enough to exhibit mature frond characters. Comparisons between laboratory and field gametophytes of 15 species from a range of eusporangiate and leptosporangiate fern families showed far more similarities than differences (Table 9.2). Basic growth form was conserved, as well as structure and position of rhizoids, hairs, and gametangia. Field gametophytes of epiphytic species tended to be more elongated and highly branched, possibly due to bryophyte competition that was present in field populations but absent from laboratory cultures. Field plants may also have been older than the cultured plants. The most frequent difference was the common production by laboratory gametophytes of some archegonia on the dorsal side of the thallus, instead of exclusively on the ventral side as in field samples.

Other differences included density and intensity of color of rhizoids and hairs in some taxa, but other than the degree of rhizoid branching these were quantitative differences and not consistently characteristic of field plants. Alteration of densities of hairs and rhizoids could have resulted from inadvertent removal in the process of cleaning field gametophytes. Hairs could also have been removed in the field by invertebrate activity or other disturbances.

In summary, the basic morphological structure of fern gametophytes appears to be highly conservative across growth environments. There is little suggestion of high plasticity in development or in mature morphology that would preclude use of morphological characteristics for gametophyte identification in the field or in use of these characters in phylogenetic classification, especially at the level of genus.

Table 9.2 Comparison of morphological characters in laboratory and field grown gametophytes of the same species

Blank entries indicate characters that were consistent between laboratory and field samples, others indicate differences. A. Some cultured gametophytes produced elongate projections from the apical meristem (percent observed —33%). B. Field gametophytes were more elongate, strap-like, and branched (percent observed ~82%). C. Some field gametophytes were more elongate (percent observed ~57%). D. Field gametophytes were more strap-like (percent observed ~66%). E. Rhizoids were darker in laboratory gametophytes (percent observed ~85%). F. Rhizoids were darker in field gametophytes (percent observed ~75%). G. Some field gametophytes had branched rhizoids (percent observed ~66%). H. Hairs were darker in field gametophytes (percent observed ~80%). I. Fewer hairs in field gametophytes (percent observed ~80%). J. Fewer hairs in cultured gametophytes (percent observed ~90%). K. Antheridia on the "wings" of field gametophytes (percent observed ~16%); at base of thallus among rhizoids in the cultured gametophytes. L. Archegonia on dorsal surface of cultured gametophytes (percent observed ranged from —25% to 50%).

Species

Field Laboratory gametophytes gametophytes Rhizoid Presence/ Hair Abundance Abundance total I number total / number Gametophyte structure Rhizoid absence structure of marginal of suficial Gametangial Antheridia Archegonia of populations of populations outline and color position of hairs and color hairs hairs structure position position

Angiopteris lygodiifolia 23/1 4/1

Dicranopteris lineara 87/1 38/1

Adiantum latifolium 101/1 82/2

Adiantum petiolatum 82/2 74/1

Cyathea podophylla 15/1 39/2

Histiopteris incisa 166/2 59/4

Dictyocline griffithii var. 106/3 102/5 wilfordii

Diplazium cristatum 35/1 22/1

Dryopteris subexaltata 81/1 27/1

Elaphoglossum peltatum 38/1 11/2

Tectaria subebeana 96/1 27/1

Asplenium antiquum 88/1 53/2

Blechnum orientale 88/1 76/2

Blechnum gracile 41/1 63/2

Campyloneuron 92/3 42/1 brevifolia

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