Abstract
Abstract: Astrophytum coahuilense and Astrophytum capricorne var. senile have partly sympatric distributions in southern Coahuila and adjacent Zacatecas, Mexico. The apparent absence of natural hybrids in areas of sympatry has been noted by several authors, leading to speculation concerning the mechanisms of reproductive isolation. Experimental crosses between the two species revealed prezygotic isolating mechanisms in some cases and postzygotic mechanisms in others. Several different results were obtained from the cross-pollinations: 1) fruit was not produced and the ovules in the ovary of the floral remnant showed no development; 2) fruit contained incompletely developed, mostly non-germinant seeds; 3) apparently normal seed was produced, but many of the hybrid seedlings manifested morphological anomalies and mortality was high (>97%) due to chlorophyll deficiency. A series of experimental crosses between A. coahuilense and several allopatric taxa of the A. capricorne group revealed only postzygotic iso...
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