Jurg Steiger in "Pinguicula (Lentibulariaceae) : The cool climate species of the northern hemisphere - Morphology, Biology, Cultivation" a text from the second conference of the International Carnivorous Plants Society, Bonn (Germany), May 30 - June 1st, 1998, explain this theory :


" During the process of changing environmental conditions three strategies are observed in the evolution of plants : (1) migration, (2) adaptation; (3) extinction. During many millions of years most parts of the earth had subtropical climate. But approximately 1 million years ago a series of ice ages began to force hundreds of plants species to migrate and/or to adapt which resulted in completely new patterns of distribution areas and genetic information. Geographical, morphological and caryological data of Pinguicula may provide some evidence concerning the adaptation, migration and differentiation strategies within the genus. 


In the thirties Hagerup, Tischler and other authors came up with a hypothesis ('Hagerup-Tischler Rule') saying that periods of hard environmental conditions - particularly unstable, unpredictable conditions - will increase the chromosome number of plants (e.g. ages/interglacial periods, heat periods, extreme temperature amplitude, intensive ultraviolet radiation). This would mean that plants with higher chromosome numbers are likely to have been able to survive one or several ice ages, while lower chromosome numbers may be an evidence for relatively stable environmental conditions (e.g. Hagerup 1932).


Stable environment

Unstable environment

LOW chromosome numbers

(Haploid gametes)

Sufficient recombinaison possibilities


Insufficient recombination possibilities

 -> long-term survival genetically not guaranteed


HIGH chromosome numbers

(e.g. diploid gametes)

No necessity for higher recombination possibilities

Necessity of higher recombination possibilities


Simply stated this may be explained as follows : Hard and unpredictable environmental conditions are likely to cause an increased rate of mutations. An unfavourable mutation - particularly if it is dominant - in one of the gametes may cause a selection disadvantage to the offspring. Usually the pollen grains and the female gametes are haploid, resulting in a diploid somatic cell when joining. But it is known from other plants groups that in average 1 per thousand plant specimens is tetraploid due to defective processes during meiosis of their parent gametes or during their generative fusion (Briggs & Walters 1973). If a mutation happens in a diploid gamete it is very unlikely that just the very same mutation will affect also the corresponding gene in the second set of - homologous - chromosomes. If two diploid gametes now join to a tetraploid cell the mutation will be present only in one of four homologous chromosomes : Thus the three healthy chromosomes may protect the plant from unfavourable mutations, but the multiple representation of the genetic code for the same feature may result in an increased ecological tolerance due to some heterogeneity of the homologous genes. "


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