Distribution of cytotypes and seasonal variation in the Odontites vernus group in central Europe.

Based on differences in chromosome number two cytotypes were reported in the Odontites vernus group in central Europe by earlier studies. These cytotypes were also considered to correspond to two seasonal ecotypes that differ in phenology, morphology and ecology. In this study, we conducted a broad screening of central European populations of the O. vernus group using flow cytometry and morphological analysis of characters underpinning the seasonal variation (number of internodes).We confirmed the existence of a widespread diploid (2n = 2x = 18) with a high but variable number of internodes and an early-flowering tetraploid (2n = 4x = 40) with a low number of internodes occurring on fallows or as an agricultural weed. In contrast to previous studies, we discovered an additional type, which is a distinctly late-flowering tetraploid (2n = 4x = 40) that has the highest number of internodes of all the three types. These plants were mostly recorded in disturbed places in thermophilous steppic grassland and sporadically also in meadows in moderately warm regions. Thus, the close association between seasonal and cytotypic variation was rejected in favour of a concept of a seasonally undifferentiated diploid type, which is an ecological generalist, and two seasonally and ecologically distinct tetraploid types. The reproductive isolation may be based mainly on incompatibility between the ploidy levels (diploid vs. tetraploid plants) and phenological differentiation in the time of reproduction (early vs. late tetraploids).