Cryptic invasion within the Holosteum umbellatum complex (Caryophyllaceae) in central Europe

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Abstract Botanical inventories across Europe increasingly reveal rapid and often underestimated change, driven by ongoing introductions and improved detection of overlooked taxa. Cryptic invasions—where non-native or unrecognized lineages remain hidden due to morphological similarity or taxonomic complexity—contribute substantially to this underestimation, particularly among small, short-lived ruderal plants. We combined flow cytometry, chromosome counting, morphometric analyses, and nuclear ITS sequence data to investigate diversity within the genus Holosteum in central Europe. Sampling in Czechia, Hungary, and Slovakia revealed the coexistence of two distinct cytotypes: diploids (2n = 20) corresponding to H. umbellatum s.s., and an as yet unknown tetraploid cytotype (2n = 40) not assignable to any described taxon. Morphometric analyses showed clear differentiation, with leaf shape, pedicel indumentum, and stamen number providing the most reliable diagnostic characters. ITS data indicate the tetraploid is an allotetraploid sharing one ribotype with diploid H. umbellatum and another of unknown origin. Based on combined morphological and molecular evidence, we describe the tetraploids as representing a new species, Holosteum cerastioides sp. nov. It is currently known from salt-affected roadside habitats and is apparently undergoing rapid expansion. Our results highlight the importance of integrative taxonomy for detecting cryptic diversity and overlooked components of changing European floras.

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