A morphological study on the question of the hybrid origin of Astrophytum coahuilense (Cactaceae)

Two alternative hypotheses may explain the origin of Astrophytum coahuilense. One hypothesis is that A. coahuilense is an ancient diploid hybrid derived from interbreeding between A. capricorne and A. myriostigma. An alternative is that A. coahuilense arose via allopatric speciation and its similarity to A. myriostigma in vegetative characters is due to convergent evolution. The two hypotheses are tested, applying univariate analysis and Non-metric Multidimensional Scaling analysis to morphological datasets. Univariate analysis, which is based on 13 vegetative and 10 reproductive characters, shows that the morphology of A. coahuilense is complex, consisting of a combination of vegetative and reproductive characters shared with both A. capricorne and A. myriostigma as well as several intermediate traits. Furthermore, the morphology of A. coahuilense lacks unique, fixed characteristics. Non-metric Multidimensional Scaling analysis of 15 characters (three vegetative and 12 reproductive traits) produced a two-dimensional scatterplot in which the cluster representing A. coahuilense occupies a position intermediate between the clusters of the two parental species along Dimension 1. Thus, both analyses support the hypothesis that Astrophytum coahuilense originated through hybridization between A. capricorne and A. myriostigma. Biological and ecological factors that may have facilitated homoploid hybrid speciation are discussed.