Krieger, Jonathan D. .
Dimorphy in Pleopeltis (Polypodiaceae): detection of subtle patterns of variation in fern leaves using geometric morphometrics.
Fertile / sterile leaf dimorphy is widespread throughout the ferns, found in approximately 20 percent of species. The suite of anatomical and morphological changes presumably has adaptive value. However, the simple linear measures and imprecise qualitative measures currently used to quantify fern leaf shape have limited power to answer evolutionary questions or assess the taxonomic value of dimorphy. This study used geometric morphometrics to examine dimorphy in seventeen simple-leaf taxa from Pleopeltis s.l. (Polypodiaceae), including taxa described as dimorphic, subdimorphic, and monomorphic.
Leaf outline and landmark data were collected and mathematically straightened, and linear measurements were extracted for comparison. Extended eigenshape analysis was performed and the dimorphy metric was computed as the distance between fertile and sterile mean shapes in the resulting morphospace, using only the eleven axes corresponding to symmetric variation.
Different linear measurements yielded inconsistent results, with length-to-width ratio the most comparable to the overall dimorphy metric. The magnitude of the dimorphy metric was consistent with subjective assessment of the degree of difference between mean fertile and sterile shapes for each species and generally consistent with quantitative descriptions (e.g., “subdimorphic”). This study demonstrated that dimorphy is far more prevalent than suggested in the literature. Of the taxa examined, only Pl. fuscopunctata and Pl. stolzei exhibited monomorphy. Three taxa previously described as monomorphic exhibited significant dimorphy: Pl. conzattii, Pl. crassinervata, and Pl. polylepis var. erythrolepis. Circular to linear shape variation was the primary component of dimorphic distance: 72-94% for all species with significant dimorphy except Neurodium lanceolatum.
Geometric morphometrics allow the statistical testing of shape differences that are apparent to the investigator, but that have defied detection by simple linear methods. The precise quantification of dimorphy presented here makes it possible to test Wagner and Wagner’s assertion that all fern species have some degree of dimorphy.
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1 - the Natural History Museum, Palaeontology, Cromwell Road, London, SW7 5BD, United Kingdom
Presentation Type: Oral Paper:Papers for Sections
Location: Lake Michigan/Hilton
Date: Monday, July 9th, 2007
Time: 10:15 AM