Hybrid Speciation and Introgression Both Underlie the Genetic Structures and Evolutionary Relationships of Three Morphologically Distinct Species of Lilium (Liliaceae) Forming a Hybrid Zone Along an Elevational Gradient

Abstract

We studied hybrid interactions ofLilium meleagrinum,Lilium gongshanense, andLilium saluenenseusing an integrative approach combining population genetics, fieldwork, and phenological research. These three species occur along an elevational gradient, withL. meleagrinumoccurring at lower elevations,L. saluenenseat higher elevations, andL. gongshanensebetween them. The species show strong morphological differentiation despite there being no clear environmental barriers to gene flow among them.Lilium gongshanenseis likely to have a hybrid origin based on our prior work, but its progenitors remain uncertain. We sought to determine whether gene flow occurs among these three parapatric species, and, if so, whetherL. gongshanenseis a hybrid ofL. meleagrinumand/orL. saluenense. We analyzed data from multiple chloroplast genes and spacers, nuclear internal transcribed spacer (ITS), and 18 nuclear Expressed Sequence Tag-Simple Sequence Repeat (EST-SSR) microsatellites for accessions of the three species representing dense population-level sampling. We also inferred phenology by examining species in the field and using herbarium specimens. We found that there are only two types of chloroplast genomes shared among the three species and thatL. gongshanenseforms two distinct groups with closest links to other species ofLiliumbased on ITS. Taken together,L. gongshanenseis unlikely to be a hybrid species resulting from a cross betweenL. meleagrinumandL. saluenense, but gene flow is occurring among the three species. The gene flow is likely to be rare according to evidence from all molecular datasets, and this is corroborated by detection of only one putative hybrid individual in the field and asynchronous phenology. We suspect that the rarity of hybridization events among the species facilitates their continued genetic separation.