Is the presence of simple or glandular hairs a good trait for distinguishing species in Caryophyllaceae? A case study of Arenaria serpyllifolia sensu lato in southern Poland

The presence of different types of indumentum is regarded as a valuable taxonomic trait for describing and differentiating between species in many different families and genera. In Caryophyllaceae, however, this character is sometimes a subject of discussion and scientific conflicts. For instance, within Arenaria serpyllifolia sensu lato (s. l.), two or three taxa have been distinguished based on the presence or absence of glandular and/or eglandular hairs on the calyx and uppermost leaves, namely A. serpyllifolia , A. viscida , and A. serpyllifolia var. intermedia . The most common, based on material from Poland, is the glandular morphotype of Arenaria serpyllifolia s. l. (315 specimens), which is in contrast to the eglandular and intermediate morphotypes represented by 174 and 24 specimens, respectively; however, the ranges of distribution of these morphotypes fully overlap. Based on our macro- and micromorphological examination of specimens belonging to the group, as well as numerical and molecular studies, we conclude that the occurrence and abundance of eglandular and glandular hairs may vary in particular parts of specimens of Arenaria serpyllifolia s. l., i.e., on their calyces, bracts, and uppermost and middle cauline leaves. The width of the capsule was the only character for which significant differences between the SERP and VIS morphotypes were found; however, considering all studied morphotypes together, the capsule characters overlap considerably. Moreover, morphological variation expressed by different types of indumenta is not paralleled by internal transcribed spacer (ITS) polymorphism patterns. The probable lack of genetic barriers between populations of individuals with glandular or eglandular hairs supports the hypothesis that the type of indumentum is of minor taxonomic importance. However, it is possible that we are dealing with sampling in the hybrid zones of both taxa, and that individuals demonstrating morphological traits typical of both taxa (eglandular and glandular) do not in fact represent pure taxa but only different kinds of hybrids, backcrosses, or introgressive forms.