Morphological and anatomical evidence supports differentiation of new interspecific hybrids from native Spartina maritima and invasive S. densiflora (Poaceae, subfamily Chloridoideae)

Interspecific hybridization is a major mechanism for generating evolutionary novelty in plants. Halophytic Spartina species are often abundant in coastal salt marshes around the world, and they frequently hybridize. Despite a large body of knowledge on the ecology and evolution of Spartina species, taxonomic resources for distinction of complex taxa in the genus are scarce. The general aim of this study was to characterize parental species S. maritima (native) and S. densiflora (invasive) and their reciprocal hybrids formed in the Gulf of Cadiz (Southwest Iberian Peninsula). Our approach was to quantify and compare morphological and anatomical characters of tufts, culms, leaves and inflorescences and the limits of their variation in different ecological settings. Our results document discriminating trait variables from populations of both hybrids that can be used to distinguish them from their parental species and each other. From these findings and the verification of type specimens, we describe two new nothotaxa, Spartina × onubensis subsp. onubensis and Spartina × onubensis subsp. tartessiana , and develop taxonomic keys for identification of the four studied taxa. Floral traits such as the length of the terminal spike, the spikelet and its glumes and lemma allow us to differentiate S. densiflora (with lower values) from the other three taxa. The presence of exerted stamens, shorter culms and shorter leaf blades separated S. maritima from both hybrids. Each hybrid can also be distinguished since S. × onubensis subsp. onubensis develops more spikes per inflorescence and consistently has longer leaves than S. × onubensis subsp. tartessiana .