Interspecific hybridization between rare and common plant congeners inferred from genome size data: assessing the threat to the Czech serpentine endemic Cerastium alsinifolium.

Cerastium alsinifolium Tausch (Caryophyllaceae) is an endemic species restricted to serpentine sites in the Slavkovský les Mts (western Bohemia) in the Czech Republic. Interspecific hybridization with sympatric C. arvense L. has long been suspected due to the substantial and continuous morphological variation observed in the field but it has never been reliably confirmed. Although both parental species share the same number of somatic chromosomes they differ considerably in the size of their monoploid nuclear genomes (~1.5-fold), which makes it easy to identify the species. Flow cytometric investigation of more than 2200 Cerastium samples revealed five distinct genome size categories, corresponding to the two parental species and three types of interspecific hybrids (originating via both reduced and unreduced gametes). F1 interspecific hybrids were very common (nearly 40% of the samples analysed from the Slavkovský les Mts), which indicates the barriers to breeding between the parental species are weak. However, no backcrosses were indicated by the genome size data. In contrast to a widely held view that C. alsinifolium mostly occurs on open serpentine outcrops, this habitat was dominated by interspecific hybrids. The endemic species occurred mainly in moist and (semi-)shaded sites, including springs in spruce forest clearings, seeps and wet margins of forest roads. Multivariate morphometrics revealed that the shape and size of cauline leaves, development of sterile axillary shoots, bract characteristics, and lengths of calyx, petals and anthers are diagnostic for the groups investigated. While the determination of C. arvense usually poses few problems, distinguishing C. alsinifolium from interspecific hybrids on the basis of morphological characters is much more challenging; reduced pollen fertility of hybrids provides the most important clue. Our results indicate that effective conservation of this important component of the Czech flora will require more emphasis on the conservation of forest sites that host core populations of C. alsinifolium.