DOI: 10.5061/dryad.2547d7wtd
title: Tension zone trapped by exogenous cline: analysis of a narrow hybrid zone between two parapatric Oxytropis species (Fabaceae)

description: Hybrid zones have been widely highlighted for their interest in
 understanding evolutionary processes. It is generally accepted that hybrid
 zones can be maintained in a balance between dispersal and selection.
 However, the selective forces can either be endogenous (i.e., genetic
 incompatibilities between parental taxa) or exogenous (i.e., parental taxa
 are adapted to different environments). In this study, we evaluated these
 alternatives and determined the maintenance of a narrow hybrid zone
 between parapatric distributed Oxytropis diversifolia and O. leptophylla
 in Nei Mongol, China. For 507 individuals sampled from two populations in
 the hybrid zone, 12 O. diversifolia populations and five O. leptophylla
 populations, we measured leaf-morphological characteristics, quantified
 genetic structure using 11 nuclear microsatellite loci and five
 chloroplast DNA intergenic regions, collected micro- and macrohabitat
 data, and conducted geographical cline analysis. We found that the two
 species differed in leaf morphology, and putative hybrids showed either
 intermediacy or a bias to O. diversifolia. Parental taxa formed two
 genetically distinct clusters, while populations in the hybrid zone
 consisted of both parental forms and various admixed individuals,
 exhibiting a bimodal pattern. The hybrid zone was coupled to ecological
 transitions of both microhabitat (i.e., the slope) and macroclimatic
 conditions. However, the genetic clines were significantly narrower than
 the environmental cline. Our results indicate that endogenous selection
 can be primarily responsible for maintaining the hybrid zone, while local
 adaptation accounts for the position of the zone. We further suggest the
 probable outcome of hybridization could be introgression.
description: 1. Morphology:  Firstly,
 six leaf-morphological traits were measured by digital caliper for all 507
 individuals of 19 populations: early leaf length, early leaf width, mature
 leaf length, mature leaf width, petiole length of early leaf, petiole
 length of mature leaf. Secondly, we quantified the trichome density on
 leaf upper surface with an Olympus SZ61 stereo microscope (Olympus, Tokyo,
 Japan), and the number of trichomes on a 1mm×1mm surface was
 counted. In total we
 quantified 211 individuals of all the 19
 populations. 2.
 Microsatellite genotyping: 11 nuclear
 microsatellite loci (N745892, N145635, N2724893, N2717495, N178451, N161850, N49251, N350553, N935993, N2528349, and N2697375) were scored across all 507 individuals
 according to the methods described by Wang et al.
 (2018). Wang,
 H., Yang, H., Liu, P. L., Su, C., Xiao. L., & Chang, Z. Y. (2018).
 Isolation and characterization of microsatellite loci from
 <em>Oxytropis diversifolia</em> (Fabaceae).
 <em>Applications in Plant Sciences</em>,
 <em>6</em>, e1168. https://doi.org/10.1002/aps3.1168 3. Microhabitat For each
 individual sampled, a 40 cm × 40 cm square plot was surveyed, slope (by
 geologic compass), the percent of total vegetation cover, the percent of
 rocky ground (rock diameter &gt; 0.2 cm), and the percent of bare
 ground (sand and soil) were measured. The latter three were
 scored on pictures by eye, and always done by the same experimenter (Hui
 WANG). In total, microhabitat data for 477 individuals in 18 populations
 were collected. 4.
 Macroclimate In
 addition to our field sampling localities (19 records), occurrence records
 of the two parental species were collected from the Chinese Virtual
 Herbarium (CVH, http://www.cvh.ac.cn), the National Specimen Information
 Infrastructure (NSII, http://www.nsii.org.cn), and the Global Biodiversity
 Information Facility (GBIF, http://www.gbif.org). After removing wrong
 identifications, duplicate records and observations, and records without
 georeferences, in total we got 35 additional records for <em>O.
 leptophylla</em>. Nineteen contemporary bioclimatic variables of the
 time period 1970 – 2000 at 30-second resolution were downloaded from
 WorldClim (http://www.worldclim.org, Fick & Hijmans, 2017). To
 conduct PCA, the entire environmental space was built on variable values
 associated with 10,000 random points sampled from the whole study
 area. Fick, S. E., & Hijmans, R. J. (2017).
 WorldClim 2: new 1‐km spatial resolution climate surfaces for global land
 areas. International journal of climatology, 37, 4302–4315.
 https://doi.org/10.1002/joc.5086
description: 1. Morphology and 3. Microhabitat: integrated in one csv file,
 and can be opened in R. 2. Microsatellite genotyping:
 txt file can be opened in GENEPOP 4.7. 4. Macroclimate:
 rds file can be opened in R.