Hybridisation, introgression events and cryptic speciation in Stipa (Poaceae): A case study of the Stipa heptapotamica hybrid-complex

Hybridisation, considered one of the major drivers of speciation in plants, commonly occurs among many genera of grasses, including the genus Stipa L. one of the largest in the family Poaceae in the Old World. In the course of research on Stipa, we studied S. richteriana, S. lessingiana, and their putative hybrid S. heptapotamica, all of which grow in south-eastern Kazakhstan. Stipa heptapotamica is treated as a local endemic, known only from the eastern part of the country. We used an integrative approach, combining morphological and molecular data, to validate the status of the hybrid taxon and present for the first time clear data documenting natural hybridisation in Stipa. The macro- and micromorphological analyses presented here show S. heptapotamica to be intermediate to its parental species in the characters studied; pollen analysis demonstrates low viability of pollen grains (below 50% compared to 94% and 87% in S. lessingiana and S. richteriana, respectively); molecular analyses based on inter simple sequence repeat markers (ISSR) and next-generation sequencing (NGS) data prove that S. heptapotamica originated strictly from hybridisation events involving S. richteriana and S. lessingiana. ISSR markers reveal possible introgression events between S. lessingiana and S. heptapotamica, whereas NGS data analyses confirm the maternal inheritance of a plastome from S. lessingiana in S. heptapotamica and support the finding of cryptic lineages within S. richteriana.