Abstract
Preissia quadrata has a high habitat specificity and a reproductive system including frequent sexual reproduction and absence of specialized asexual propagules. Fertilization is aquatic. Colonies collected in Europe, Asia, eastern and western Canada show the species to be polymorphic at the eleven isozyme loci studied. Most often, thalli of the same colony appeared genetically identical. Partitioning of genetic variation was among rather than within colonies, suggesting a clonal structure of the colonies and a species structure consisting of a series of small populations reproductively isolated from each other. Little genetic exchange between colonies growing nearby was present, but the species does not appear to consist of regionally circumscribed genetically cohesive entities. Some European and Canadian colonies had identical electrophoretic patterns. The problem of understanding the phenomena holding liverwort species together, both morphologically and genetically, remains an open question. The genetic structure of the species might be due to its past history. It might have had a continuous distribution in the northern part of the Northern Hemisphere. If so, little subsequent genetic diversification, perhaps linked to its haploid-dominant life cycle, must be supposed.
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