Morphological, flow cytometric and cytogenetic evidence of a hybrid swarm between Elymus hispidus and E. repens in the Bílé Karpaty Mts, Czech Republic

Grasslands in the Bílé Karpaty Mts (Czech Republic, central Europe) are known for their extraordinary species richness and characteristic composition. At one of the local sites, the Čertoryje nature reserve, the structure of a peculiar population of Elymus plants (Triticeae) was investigated. The site is cohabited by the closely related species E. hispidus and E. repens, but a substantial percentage of the plants based on their morphology are hybrids. A combination of flow cytometric analyses (DNA content – genome size, GC content), morphometrics (36 characters) and molecular cytogenetics (genomic in situ hybridization – GISH) was used to shed light on the hybridization processes occurring within this population. One hundred and twenty three plants were collected along 14 transects. A preliminary determination based on two discriminative morphological characters suggested that more than 30% of all individuals were hybrids. The hexaploid cytotype prevailed in the dataset (85%), but 15% of the plants were higher polyploids, namely heptaploids (7x) and octoploids (8x). Morphometric and flowcytometric data indicated an introgressive pattern of hybridization biased towards E. hispidus. GISH was used to reveal the genomic composition of plants selected to cover the spectrum of genome size values. Fifteen of the 20 plants, including higher polyploids, had a hybrid genome composition with signs of successive hybridization (later than the F1 generation). There were inconsistencies between morphology, genome size and cytogenetic data. In particular, some morphologically identified hybrids had a DNA content and a genomic composition corresponding to the parental species E. hispidus. A shift in morphology caused the characters that in a previous extensive study were used to differentiate between E. repens and E. hispidus, to completely fail with this population. The high percentage of hybrids, unparalleled in central Europe, coupled with the occurrence of high-ploidy cytotypes and successive hybrid generations, suggest that the population is best characterized as a hybrid swarm in which interspecific hybridization is the major driving force generating progeny, which thrives at this locality and participates in successive introgressive hybridization.