Argyroxiphium sandwicense subsp. macrocephalum x Dubautia menziesii
Among the more spectacular hybrids that occur naturally is the one between Argyroxiphium sandwicense subsp. macrocephalum with 14 pairs of chromosomes and Dubautia menziesii with 13 pairs of chromosomes. In addition to the difference in chromosome number, the genomes of these species are further differentiated by at least two reciprocal chromosome translocations. This results in a common meiotic configuration of 9 pairs and 3 chains of three chromosomes, and a concomitant reduction in fertility to approximately 9%, based on pollen stainability in F1 hybrids. Despite the tremendous morphological differences of the parents and the reduced fertility of the F1, backcross progeny are produced in nature. Some of these (e.g. 1-2 above) have been inadvertently cultivated in Haleakala National park and several others have been grown experimentally at the University of Hawaii-Manoa. One of the experimentally grown plants that represented a backcross to Dubautia menziesii flowered (3-4 above). Note that many heads were removed from this plant (3) for chromosome analysis. It had a simplified chromosome pairing configuration of 12 pairs and a chain of 3 chromosomes, was about 80% fertile, and was used to generate a vigorous second backcross progeny of several individuals with Dubautia menziesii as the recurrent parent. These individuals were remarkably uniform morphologically (5) and one that flowered (6-7) exhibited 13 pairs of chromosomes and 99% pollen stainability. Some plants seen in the field closely approximate the cultivated backcross progeny of this intergeneric hybrid combination and probably originated in the same manner. Specifically, the type material of D. dolosa appears to represent such an unstabilized hybrid product and is no longer equated with D. waianapanapaensis which is geographically distinct and clearly a reproductively stabilized taxon. However, the similarity of these plants suggests a possible hybrid origin for D. waianapanapaensis. Indeed, the ease of recombination between such strikingly differentiated plants as Argyroxiphium sandwicense subsp. macrocephalum and Dubautia menziesii underscores the potential of hybridization in the evolution of this and other plants, especially in Hawaii, where hybridization appears to be a way of life. For more details see G. D. Carr, 1995, Amer. J. Bot. 82:1574-1581.
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