Recent taxonomic treatments of the genus Harpagonella have included only one lower taxon, H. palmeri A. Gray. However, a larger-fruited variety of H. palmeri from Arizona and Sonora was described by I.M. Johnston in 1924. He continued to recognize this taxon – H. palmeri var. arizonica – in his treatment of the genus in Kearney and Peebles’s Arizona Flora in 1960. Here, we provide two lines of molecular evidence and quantitative morphological evidence from calyx characters showing that plants of Harpagonella from Arizona, Sonora, and central Baja California, corresponding to Johnston’s var. arizonica, are distinct from H. palmeri of southern California and Baja California. We make the new combination Harpagonella arizonica (I.M. Johnston) Guilliams & B.G. Baldwin, comb. nov. for the plants from Arizona, Sonora, and central Baja California.

Amsinckiinae , Boraginaceae , Harpagonella , Pectocarya

Harpagonella A. Gray is a genus of Boraginaceae, subtribe Amsinckiinae (see Chacón et al. 2016 and Luebert et al. 2016) that occurs disjunctly in western North America, with populations in southern California, USA, and adjacent Baja California, México and other populations in southern Arizona, USA, and adjacent northwestern Sonora, México (Figure 1). The only species recognized in the genus, H. palmeri A. Gray, was described in 1876 from an 1875 collection by Edward Palmer on Guadalupe Island, Baja California. In 1924, Ivan M. Johnston recognized two varieties in H. palmeri, var. arizonica and var. palmeri. The former taxon, then known from Arizona and adjacent Sonora, was said to differ from var. palmeri, of California and Baja California, in having longer “cornute processes on the fruiting calyx” and larger nutlets (Johnston 1924). Furthermore, the plants of California and Baja California are often found on clayey soils, while those of Arizona and Sonora often occur in sandy or gravelly soils. In his treatment of the Boraginaceae for the Arizona Flora (Kearney and Peebles 1960), Johnston retained the taxon as a variety, but most other treatments of the genus recognize H. palmeri without varieties (e.g., Munz 1973, Veno 1979, Kelley and Messick 2014).

Figure 1. 

Map of western North America showing Harpagonella collections in major herbaria based on available specimen data from GBIF and Bajaflora. Type collection localities are indicated with black star for H. palmeri and a red star for H. arizonica.

Harpagonella has been regarded as the most morphologically distinctive member of the Amsinckiinae, largely because of ornamentation of the calyx in fruit that is unique to the genus (Johnston 1924, Veno 1979). The genus was placed in its own tribe, Harpagonelleae, for this reason (Gürke 1897). In Harpagonella, the calyx is pentamerous, with the two sepals away from the inflorescence axis connate for >80% of their length and the three other sepals free while in flower. The two fused sepals are strongly accrescent, becoming conduplicate, indurate, and often more or less enveloping one nutlet or sometimes both nutlets at fruit maturity (Figure 2). As the fruit matures, five to ten subterete appendages with distal retrorse barbs develop on the pair of fused sepals, giving the fruit the appearance and function of a grappling hook, which is the common name for the genus. The pedicel is also accrescent. It recurves or rarely coils as the fruit matures, placing the lobes of the fused sepals against the inflorescence axis. As Gray (1876) noted, these modifications effectively result in the transfer of dispersal function from the nutlet, as is typical in many Amsinckiinae, to the calyx. The gynoecium in Harpagonella is also distinctive. It has been reduced from the typical condition in the Amsinckiinae of four ovules and a fruit of four nutlets to two developing ovules and two nutlets, with the other two ovules early abortive. Unlike the nutlets of many close relatives, e.g. Pectocarya, the two nutlets of Harpagonella are largely without ornamentation, bearing only short hairs.

Figure 2. 

Fruits of Harpagonella in lateral view, from A) southern Arizona (Tedford 1043, ARIZ403065) and B) southern California (Bramlet 2301, ARIZ345225). Although morphologically similar, note overall difference in size. Scale bars are each approximately 1 mm. Labels: (AAS) sepals away from inflorescence axis in flower; (IA) inflorescence axis; (N) nutlet; (P) pedicel; (SA) sepal appendages; (TAS) sepals toward inflorescence axis in flower.

We included Harpagonella in broad phylogenetic and taxonomic studies of some members of the Boraginaceae subtribe Amsinckiinae (Guilliams 2015). During the phylogenetic study, we included several samples of H. palmeri from throughout its range with the goal of evaluating phylogenetic structure of the included samples, with attention to historical taxonomy. We also examined herbarium sheets representing both previously recognized varieties of H. palmeri, taking measurements of the calyx appendages and overall size of the fruit. Although a full phylogenetic study will be published later, we present the results of this study here in reduced form so that the resulting new combination can be available for use in the treatment of Harpagonella for the Flora of North America, North of México.

The separate phylogenetic analyses of nrDNA and cpDNA presented here each recover two clades within H. palmeri corresponding to the two named varieties. Statistical support for these groupings was very high, with posterior probabilities above 0.96 and maximum likelihood bootstrap values of 100 in all cases. The Harpagonella-outgroup split as well as clades of samples by variety were each supported by numerous nucleotide substitutions and indels. We take this as strong evidence for two evolutionary lineages in the genus.

Morphologically, these two lineages differ in all measured aspects of fruit size. Plants primarily from Arizona and Sonora are significantly larger in maximum fruit length, maximum fruit width, and appendage length. Box and whisker plots for these features show that the ranges of measurements of these characters between the two lineages are mostly non-overlapping. Although unmeasured here, nutlet size in Harpagonella was suggested by Johnston (1924) to be larger in plants from Arizona and Sonora than in plants from California and Baja California. These differences are quantitative, not qualitative, and absent a formal statistical analysis of morphology, Veno (1979) advocated for recognizing no infraspecific taxa in H. palmeri, stating that “this feature is variable and somewhat clinal, and does not provide a significant or reliable basis for taxonomic delimitation.” The data presented here suggest instead that these quantitative characters appear to be sufficient for reliable delimitation of two taxa corresponding to the evolutionary lineages recovered in the phylogenetic analysis.

Herbarium study of 366 specimens representing 291 gatherings of Harpagonella has permitted the evaluation of the geographic range of these morphologically distinct evolutionary lineages, which is especially critical for specimens collected on the Baja California Peninsula, where both named varieties have been reported. Specimens of plants with larger fruits corresponding to Johnston’s H. palmeri var. arizonica are almost entirely from Arizona and Sonora, with two collections attributable to this taxon made from desert regions of Baja California at mid-peninsula (Moran 12682, 28.29007, -113.12146; Moran 12845, 28.28333, -113.65). We have observed and confirmed the taxonomic identity of a specimen of the former (DS598325) but not the latter. Specimens of plants with smaller fruits corresponding to Johnston’s concept for H. palmeri var. palmeri are known primarily from southwestern California and the adjacent western coastal areas of the Baja California Peninsula, with collections ranging as far to the south as the Vizcaino Peninsula on the Pacific Coast in Baja California Sur.

The biogeographic pattern displayed by Harpagonella – a disjunction between the California Floristic Province sensu Howell (1957) and central, southern Arizona and adjacent Sonora – is somewhat common yet underexplored. Raven and Axelrod (1978) describe this pattern briefly in their important paper on the origin of the California flora, and provide a table of 35 genera, species, or species pairs that have this pattern. To their list of taxa, we add Harpagonella based on evidence presented here.

Based on complete and well-supported reciprocal monophyly in two unlinked genomic partitions, statistically significant morphological differences, and essentially non-overlapping geographic ranges, the two lineages of Harpagonella resolved here merit recognition at the species level under the criteria of phylogenetic species concepts (see Mishler and Theriot 2000) as well as longstanding taxonomic practice. To recognize a taxon at species rank for the large-fruited plants found primarily in the deserts of Arizona and Sonora, the following new combination is needed.

We thank the curators at ARIZ, ASU, CAS, JEPS, RSA, SBBG, and UC for allowing use of herbarium specimens for general study, as well as use of a subset of the specimens for the morphological and molecular analyses. We thank the curators of GH and MO for making high quality digital images of type specimens available. We thank Susan Fawcett for preparing illustrations. We also thank the following sources of funding for this work (listed alphabetically): American Society of Plant Taxonomists, California Native Plant Society, JiJi Foundation, the Lawrence R. Heckard Endowment Fund of the Jepson Herbarium, National Science Foundation (DEB-1210833), Northern California Botanists, and the University of California Institute for Mexico and the United States (UC MEXUS). Finally, we thank the reviewers of the manuscript, whose suggestions improved the paper.