P1. Syst. Evol. 177: 139-148 (1991) Plant Systematics and Evolution © Springer-Verlag 1991 Printed in Austria Allozymic and morphometric variation in Lemna minor (Lemnaceae) LIETTE VASSEUR, LONNIE W. AARSSEN, and DANIEL D. LEFEBVRE Received November 14, 1990; in revised version April 16, 1991 Key words: Angiosperms, Lemnaceae, Lemna minor.- Allozymic variation, clonal propa- gation, morphometry, cosmopolitan distribution. Abstract: Allozymic and morphometric variation was studied in 28 clones of Lemna minor. This variation was compared with the corresponding variation in four clones of Lemna gibba and four clones of Spirodela polyrrhiza. A high level of allozymic variation was observed among the clones, despite having been grown under uniform laboratory conditions for several years and despite its quasi-exclusive clonal means of propagation. Based on degree of allozymic similarity, Spirodela polyrrhiza was distinguished from the two Lemna species but the latter species were genetically indistinguishable. Allozymic similarity among clones of Lemna minor was not related to morphometric similarity, nor was it related to the degree of geographic separation or climatic similarity of their sites of origin. The results suggest that allozymic variation among these clones of Lemna minor may be largely neutral and not a consequence of differential selection. Recent studies on populations of exclusively or predominantly clonal plant species have reported that they are commonly composed of several genetically different clones (ELLSTRAND • LEVIN 1982, WEIDER & al. 1987, Tuc~c & al. 1988). Most of these studies, however, have focused on a relatively small geographical area. Few studies have investigated the genetic structure among clones that are repre- sentative of the overall distribution of a species. In the present study, this scale of genetic variation was examined in the widely distributed species Lemna minor L. (Lemnaceae). Lemna minor, the common duckweed, is a small floating aquatic plant with an extremely simple structure consisting of 1 - 12 leaves or fronds of 0.8 cm maximum length and grouped in a rosette. L. minor multiplies rapidly by clonal propagation. Its life history is very simple since it rarely flowers (HILLMAN 1961, LANDOLT 1986). L. minor grows on still or slightly flowing waters and can be found under a wide range of environmental conditions. The geographical distribution of L. minor is virtually cosmopolitan, ranging from 66 ° north in Scandinavia to 40 ° south in New Zealand (HILLMAN 1961, LANDOLT 1986). Members of the Lemnaceae are successful colonizers of aquatic habitats. They possess the "colonizing" features of clonality, self-fertilization, polyploidy, and