Mc. Boisselierdubayle et H. Bischler, ENZYME POLYMORPHISM IN PREISSIA-QUADRATA (HEPATICAE, MARCHANTIACEAE), Plant systematics and evolution, 205(1-2), 1997, pp. 73-84
Preissia quadrata has a high habitat specificity and a reproductive sy
stem including frequent sexual reproduction and absence of specialized
asexual propagules. Fertilization is aquatic. Colonies collected in E
urope, Asia, eastern and western Canada show the species to be polymor
phic at the eleven isozyme loci studied. Most often, thalli of the sam
e colony appeared genetically identical. Partitioning of genetic varia
tion was among rather than within colonies, suggesting a clonal struct
ure of the colonies and a species structure consisting of a series of
small populations reproductively isolated from each other. Little gene
tic exchange between colonies growing nearby was present, but the spec
ies 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 h
olding liverwort species together, both morphologically and geneticall
y, remains an open question. The genetic structure of the species migh
t be due to its past history. It might have had a continuous distribut
ion in the northern part of the Northern Hemisphere. If so, little sub
sequent genetic diversification, perhaps linked to its haploid-dominan
t life cycle, must be supposed.