Landoltia (Lemnaceae), a New Genus of Duckweeds

Abstract

Morphological, allozyme, and cpDNA (rbcL) sequence data provide evidence for the distinctness of Spirodela punctata from species in both Lemna and Spirodela (Lemnaceae). We propose the recognition of a new genus, Landoltia, to better reflect current phylogenetic concepts in the Lemnaceae. Landoltia is distinguished by its reduced frond prophyllum, frond nerves (3 to 7), roots (up to 7), root tracheids, external anther locules, and also by well-supported molecular evidence provided by allozymes and cpDNA sequences. The new combination Landoltia punctata is made to accommodate this taxonomic modification. Duckweed classification remains equivocal because phylogenetic relationships are difficult to reconcile in this diminutive, reduced family (Les et al., 1997b). The most comprehensive phylogenetic hypotheses for duckweed taxa were presented in a monograph by Landolt (1986) and form the basis of modern classification for the family Lemnaceae. In the most recent monograph of the duckweed family (Lemnaceae), Landolt (1986) recognized four genera: Lemna, Spirodela, Wolffia, and Wolffiella. Few additional genera have been proposed in past taxonomic treatments of duckweeds. One example is Staurogeton Reichenbach, which was elevated from subgeneric to generic rank by Schur (1866) to accommodate the morphologically distinctive Lemna trisulca L. (Landolt, 1986). Most contemporary classifications continue to assign this taxon to the genus Lemna. Den Hartog and van der Plas (1970) subdivided Woffiella to create the two genera Pseudowolffia and Wolffiopsis. Few taxonomic treatments recognize either of these segregate genera as distinct from Wolffiella. To our knowledge, division of either Spirodela or Wolffia into subsidiary genera has not yet been suggested, although some authors have transferred certain species from these genera into either Lemna or Wolffiella. Landolt's recent generic concept of Spirodela recognizes a paraphyletic taxon, with S. polyrhiza and S. intermedia as sister species, but with S. punctata associating with Lemna (Fig. 1A). Although Landolt's evolutionary trees reflect a keen understanding of duckweeds and a comprehensive evaluation of published taxonomic literature on the group, they were constructed using nonexplicit phylogenetic methods, i.e., not by cladistic analyses. We have re-analyzed results of these earlier studies using cladistic methodologies to test hypothetical relationships proposed by Landolt. In addition, we have generated several molecular data sets to supplement the clearly limited number of phylogenetically informative characters available for these morphologically simple plants. Although this work remains in progress, the results of our preliminary analyses of morphological, biochemical, allozyme, and DNA (rbcL) sequence data (Crawford & Landolt, 1993, 1995; Crawford et al., 1995, 1997; Crawford et al., 1996; Les et al., 1994, 1997a, 1997b) are reasonably concordant with Landolt's classification, but differ in a number of details from his phylogenetic diagrams. In particular, generic subdivisions used in his classification of Lemnaceae are inconsistent with results of these phylogenetic analyses and merit reconsideration. Herein we summarize evidence that warrants the taxonomic segregation of a new duckweed genus. We consider this taxonomic action essential for a classification that reasonably depicts our best, current estimate of phylogenetic relationships in the Lemnaceae. A specific diagram of intergeneric duckweed relationships that summarizes the phylogenetic trees originally appearing in Landolt (1986) was provided to us by E. Landolt. We compared these hypothetical relationships to published allozyme studies (Crawford & Landolt, 1993) and to previous cladistic analyses of morphological, anatomical, and biochemical data (Les et al., 1997b). Intergeneric duckweed relationships were also examined using preliminary results of a phylogenetic analysis of NovoN 9: 530-533. 1999. This content downloaded from 157.55.39.136 on Thu, 19 May 2016 06:11:23 UTC All use subject to http://about.jstor.org/terms Volume 9, Number 4 1999 Les & Crawford New Duckweed Genus 531