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A phylogeographic study of the endemic Mexican tulip poppy Hunnemannia fumariifolia (Papaveraceae) was conducted to determine: (1) the historical processes that influenced its geographical pattern of genetic variation; (2) whether isolation by distance was one of the main factors that caused genetic divergence in populations of this species; and (3) whether genetic flow still exists between populations from northern arid zones (Chihuahuan Desert and Sierra Madre Oriental) and those from southern arid zones (Tehuacan-Cuicatlan Valley) - populations that are separated by the Transvolcanic Belt.

Xerophytic vegetation in Mexico.

Chloroplast DNA (cpDNA) sequences of three regions, trnH-psbA, rpl32-trnL(UAG) and ndhF-rpl32, were obtained for 85 individuals from 17 populations sampled in the field, covering the entire range of H. fumariifolia. The evolutionary history of these populations was investigated using a nested clade phylogeographic analysis and also by conducting various population genetic analyses.

In total, 17 haplotypes were detected, 14 of which were found in the Sierra Madre Oriental. Differentiation among populations based on cpDNA variation (G(ST) = 0.787, SE 0.0614) indicated population structure in H. fumariifolia, corroborated by a fixation index (F(ST)) of 0.907. Results from analysis of molecular variance found that most of the total variation (90.71%, P < 0.001) was explained by differences among populations. Three regions were determined based on geological correspondence - the Chihuahuan Desert, Sierra Madre Oriental and Tehuacan-Cuicatlan Valley - and the variation between them was significant (43.39%, P < 0.001). Results of a Mantel test showed a significant correlation between genetic and geographic distances (r = 0.511; P = 0.0001), suggesting a pattern of isolation by distance, which was corroborated by nested clade phylogeographic analysis. Mismatch distribution analysis indicated a sudden demographic expansion.

Our study found that isolation by distance influenced genetic divergence in populations of H. fumariifolia. The finding that allopatric fragmentation influenced genetic divergence in populations in the Sierra Madre Oriental may be a reflection of the complex geology of the area. Our results suggest that the areas located in the north of the Sierra Madre Oriental acted as post-glacial refugia for some populations.