Species delimitation in the Stenocereus griseus (Cactaceae) species complex reveals a new species, S. huastecorum

Abstract

The Stenocereus griseus species complex (SGSC) has long been considered taxonomically challenging because the number of taxa belonging to the complex and their geographical boundaries remain poorly understood. Bayesian clustering and genetic distance-based methods were used based on nine microsatellite loci in 377 individuals of three main putative species of the complex. The resulting genetic clusters were assessed for ecological niche divergence and areolar morphology, particularly spination patterns. We based our species boundaries on concordance between genetic, ecological, and morphological data, and were able to resolve four species, three of them corresponding to S. pruinosus from central Mexico, S. laevigatus from southern Mexico, and S. griseus from northern South America. A fourth species, previously considered to be S. griseus and commonly misidentified as S. pruinosus in northern Mexico showed significant genetic, ecological, and morphological differentiation suggesting that it should be considered a new species, S. huastecorum, which we describe here. We show that population genetic analyses, ecological niche modeling, and morphological studies are complementary approaches for delimiting species in taxonomically challenging plant groups such as the SGSC.

Fig 1. Distribution of the Stenocereus griseus species complex (SGSC) taxa, as considered by Arreola-Nava [15].

Fig 2. Kriging interpolation of individual Q-matrix by each clustering method.

The first three maps columns (left to right) depict the interpolated assignment probability for each method (labeled in the heading) and K is the number of groups detected by each. Groups containing the same populations across methods (EGGs) are placed alongside and keep the same color hue, whereas the color gradient saturation represents higher probability: red = S. griseus-Mexico (here designated S. huastecorum); green = S. pruinosus and its subgroups (green shades in Geneland maps); blue = S. laevigatus and its subgroups (blue shades in Geneland maps), and dark gray = S. griseus. White bullets represent populations. EGGs populations are represented at the rightmost column by bullets which follow the color code before described.

Fig 3. Genetic clustering summary.

Color gradients represent BC belonging probability starting from the 75% threshold value, purple bars with green types bootstrap values are the three genetic barriers detected by Barrier, at the left the UPGMA dendrogram following the color code used in the BC.

Fig 4. Structural characters measured.

CLSL: central-left spine length, CRS: central-right spine, LCS: lower central spine, UCS: upper central spine, RCSL: radial c-homologous spine length, lower-case letters: radial spines (pairs represented by the same letter).

Fig 5. Niche suitability pairwise comparisons based on t-tests.

Different letters represent levels of significant differences.

Fig 6. Areolar morphometrics t-tests.

(A-D) areolar features lengths comparisons, (E-F) spines counts comparisons. Levels not connected by the same letter are significantly different.

Fig 7. Vegetative features of S. huastecorum.

(A) general aspect of S. huastecorum (H. Alvarado-Sizzo 350), (B) rib details (H. Alvarado-Sizzo 352), (C) typical young (upper) and mature (lower) areolas (H. Alvarado-Sizzo 245), (D) apex of a young branch. Credits: (A) I. Torres-García, (B-D) H. Alvarado-Sizzo.

Fig 8. A newly opened S. huastecorum flower.

Lateral (A) and top (B) view, (C) longitudinal cut of the floral tube. (H. Alvarado-Sizzo 350). Credits: H. Alvarado-Sizzo. Scale bars = 1 cm.

Fig 9. Ripe fruit of S. huastecorum.

Falling areolas (A, upper) and longitudinal cut (A, lower), aspect of a reproductive branch with immature fruits (B), and focus stacking micrograph (4X) of a seed hilum-micropylar region (C) and lateral view(D). Credits: (A, B) H. Alvarado-Sizzo (C) A. González-Murillo & H. Alvarado-Sizzo. Scale bars (A) = 1 cm; (C, D) = 1 mm.