Abstract
Species of duckweeds (Letnnaceae) that were resolved as sister taxa in a phylogeny based on combined molecular and non-molecular data were compared for morphological, physiological, and ecological attributes to infer factors important in the initial divergence leading to speciation. The ability to survive extreme conditions such as desiccation and cold temperatures is the most common difference identified between species. Two morphological characters facilitating survival in extreme environments are production of special resting buds called turions and increased seed production. The prevalent geographic pattern for species pairs consists of one restricted species occurring on the periphery of a more widespread taxon; this pattern indicates that divergence of peripheral isolates is a common geographical mode of speciation in duckweeds. Other species differ in optimal environmental conditions for growth, and these species are largely sympatric. In only one instance does it appear that divergence and speciation occurred following long-distance dispersal. Sympatric species pairs have the lowest molecular divergence; widespread primarily allopatric, and distantly allopatric species have the highest molecular divergence. Despite infrequent sexual reproduction, some degree of detectable variation (molecular, physiological, etc.) occurs within populations and among populations of the same species. Molecular evidence indicates that variation within duckweeds extends from the population and intraspecific levels to very different levels of divergence among recognized species. Contrary to the appearance of morphological and ecological uniformity implied by their reduced morphology and restricted occurrence in fresh water habitats, duckweeds are not a group in evolutionary stasis. Rather, these smallest of all flowering plants are dynamic evolutionarily, and comprise a model system for studying plant evolution and speciation.
Highlights
The family Lemnaceae comprises highly reduced aquatic monocots in which there has been extreme reduction in both the size and presence of organs (Landolt 1986)
Plastid sequences and allozymes have been used to calculate divergence times, something that is essentially impossible to estimate with morphology alone in most flowering plants, let alone in a group so reduced as duckweeds
There are two sister species pairs in which one species can survive a cold season and the other cannot, and this may have been the key factor in their initial divergence leading to speciation (Table 1; Fig. 1)
Summary
The family Lemnaceae (duckweeds) comprises highly reduced aquatic monocots in which there has been extreme reduction in both the size and presence of organs (Landolt 1986). There are two sister species pairs in which one species can survive a cold season (overwinter) and the other cannot, and this may have been the key factor in their initial divergence leading to speciation (Table 1; Fig. 1).
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