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Lacy bracken fern (Pteridium aquilinum var. caudatum)
Scanning electron micrographs of the lower epidermal surface of frond laminae in Pteridium. Fig. 1. LINN 1246.14. Scale bar = 200 µm. Fig. 2. LINN 1246.15. Scale bar = 200 µm. Fig. 3. P. caudatum, Venezuela, NSW 361276. Scale bar = 50 µm. Fig. 4. P. arachnoideum, Venezuela, NSW 361275. Scale bar = 50 µm. Fig. 5. P. arachnoideum, Surinam, NSW 360580. Scale bar = 50 µm. Fig. 6. P. caudatum, Costa Rica, NSW 420297. Scale bar = 25 µm. Fig. 7. LINN 1246.14. Scale bar = 25 µm. Fig. 8. P. arachnoideum, Costa Rica, NSW 420304. Scale bar = 25 µm.
Clarification of the taxonomic status and relationships of Pteridium caudatum (Dennstaedtiaceae) in Central and South America.
by Thomson J., Alonso-Amelot M. E. (2002)
John A. Thomson, National Herbarium of New South Wales, Royal Botanic Gardens, Mrs Macquaries Road, Sydney, NSW 2000, Australia
Miguel E. Alonso-Amelot
in Botanical Journal of the Linnean Society 140: 237–248 – https://doi.org/10.1046/j.1095-8339.2002.00089.x
Contemporary systematic treatments of the Central and South American bracken ferns in the genus Pteridium Gled. ex Scop. recognize morphotype caudatum as either a full species or a variety of P. aquilinum (L.) Kuhn.
Geographically representative sporophytes of morphotype caudatum, including the type in the Linnaean Herbarium, are shown using spore size, guard-cell length and morphology of the cells of the false indusium to be tetraploid (based on 4n = 208).
DNA fingerprinting of field-collected Venezuelan samples supports the generalization that morphotype caudatum is a fertile allotetraploid containing genomic elements otherwise distinctive of the southern hemisphere diploid P. arachnoideum (Kaulf.) Maxon, together with elements characteristic of northern hemisphere diploids including the North American P. aquilinum var. pubescens Underw. and P. aquilinumvar. pseudocaudatum (Clute) A. Heller.
Evidence of genetic isolation from taxa with overlapping distributions, as well as morphological, biochemical and ecological data, validate recognition of P. caudatum (L.) Maxon at species level.
Heterogeneity observed within P. caudatum is consistent with multiple origins through independent hybridization events.
Pteridium caudatum is strikingly analogous to the tropical Asian/Australasian allotetraploid P. semihastatum (N. Wallich ex J. G. Agardh) S. B. Andrews [=P. yarrabense (Domin) N. A. Wakef.].
Knowledge of ploidy level is of considerable potential value in systematic and evolutionary studies, but attains significance only when generalization to an entire taxonomic unit becomes possible. Methods for unequivocal direct assessment of ploidy, including chromosome counts, determination of nuclear DNA content or molecular genome and isozyme analyses, typically require fresh or specially preserved material and are inapplicable to routine dried herbarium specimens. Correlates of ploidy level, especially stomatal guard-cell length and spore size (Barrington, Paris & Ranker, 1986), must therefore be used to generalize results obtained on a few individuals by direct methods to the taxon as a whole. The relationship between guard-cell length and ploidy level has been established for P. arachnoideum and P. caudatum (Thomson, 2000a) by (i) measurement of guard-cell length in specimens of P. arachnoideum and a presumed hybrid from the Galapagos Islands that showed chromosome complements of 2n = 104 and 4n = 208, respectively (Jarrett, Manton & Roy, 1968), and (ii) measurement of nuclear DNA content (Table 1;2c = 16.5 pg for P. arachnoideum and 4c = 30.9 pg for P. caudatum; Tan & Thomson, 1990). SEMs of the guard cells of P. caudatum (NSW 420297, Costa Rica) are shown in Figure 6 for comparison with those of Linnaeus’ specimen 1246.14 (Fig. 7) and P. arachnoideum (NSW 420304, Fig. 8). Guard cells of the type of P. caudatum (LINN 1246.15) could not be visualized with the SEM using the limited tissue available for examination due to the dense overlying indumentum in this specimen (Fig. 2, cf. Fig. 1). Light micrographs of guard cells in LINN 1246.14 and LINN 1246.15 are shown in Figures 13 and 14, respectively, with P. caudatum (NSW 360578) from Peru (Fig. 15) and P. arachnoideum (NSW420304) from Costa Rica (Fig. 16) for comparison. Consistent with a difference in ploidy level between them, the larger size and less crenulate margins of the epidermal cells in P. caudatum (including the Linnaean specimens) compared with those of P. arachnoideum are evident in both the SEM photographs of Figures 6–8and the light micrographs of Figures 13–16.
Mean guard-cell lengths, based on measurements for 30 stomata for each accession, are shown in Table 1. These data greatly extend the preliminary observations reported by Thomson (2000a,b) and suggest that all specimens assigned to P. arachnoideum on morphological grounds are diploid, and are consistent with the conclusion that all accessions of P. caudatum, including LINN 1246.14 and the type (LINN 1246.15) are tetraploid. Within both P. arachnoideum and P. caudatum there is significant between-accession variation in guard cell length. The ranges of the means for the two groups do not overlap. The smallest difference between mean guard-cell lengths for any pair of P. caudatumand P. arachnoideum specimens represented in Table 1 is significant at P < 0.001. In overview, accessions of P. arachnoideum have a mean guard cell length less than 34 µm; those of P. caudatummore than 38 µm. Guard-cell length in the North American diploid brackens (vars latiusculum, pseudocaudatum and pubescens) is significantly greater than in the southern diploid P. arachnoideum(Thomson, 2000a), limiting the value of this character as an independent indicator of ploidy level. Similar differences in the relationship of ploidy to guard-cell length have been reported for other fern taxa (Barrington et al., 1986), and presumably reflect the adaptive significance of stomatal morphology in gas and water economy.