Adaptive Radiation of the Hawaiian Silversword Alliance

Chromosomal Diversity

Last Modification: July 31, 2000

Fourteen pairs of chromosomes at meiotic metaphase I in Wilkesia gymnoxiphium. All species of Wilkesia and Argyroxiphium and 12 species of Dubautia share this chromosome number. However, a study of chromosome pairing in hybrids among various 14-paired species indicates that their genomes are not all structurally uniform.
Thirteen pairs of chromosomes at meiotic metaphase I in Dubautia linearis subsp. opposita. Nine species of Dubautia (D. arborea, D. ciliolata, D. herbstobatae, D. linearis, D. menziesii, D. platyphylla, D. reticulata, D. sherffiana, and D. waianapanapaensis) share this chromosome number. Furthermore, a study of chromosome pairing in hybrids among these 13-paired species suggests that their genomes are structurally uniform.
Crosses among the species with 13 pairs of chromosomes yield hybrids with 13 pairs of chromosomes and mean pollen stainability of 95% or higher. Moreover, any 13-paired Dubautia species crossed with Dubautia scabra yields a hybrid with 12 pairs and a linear chain of 3 chromosomes with mean pollen stainability ranging from 73-91%.
Meiosis metaphase I in Dubautia ciliolata (n=13) x D. scabra (n=14). Note 12 pairs and a "v"-shaped chain of three chromosomes (arrow). This configuration is typical of all hybrid combinations involving D. scabra with any 13-paired Dubautia species. These data and other information support the conclusion that the 13-paired species have a single common origin from a 14-paired genome comparable to that of D. scabra. This origin apparently involved translocation of all of the essential genetic material from two nonhomologous chromosomes of the 14-paired genome onto a single centromere in the 13-paired genome. The centromere with no remaining essential genetic material was subsequently lost, resulting in the dysploid reduction from n=14 to n=13 chromosomes.
Crosses involving different 14-paired species with each other or with 13-paired species yield hybrids with potentially quite different meiotic configurations. The hybrids that have been available for study indicate that there are at least 8 different genomic arrangements among the 14-paired species. Meiosis in hybrids involving 14-paired species ranges from 14 normal bivalents in hybrids involving undifferentiated species to 9 bivalents + a chain of 6 (or 2 chains of 4) + a chain of 3 chromosomes in hybrids involving species differentiated by two translocations and a dysploid reduction. Pollen stainability ranges from essentially 100% in hybrids involving parents that share the same structural genome to 11% in hybrids involving parents whose genomes are strongly structurally differentiated.
Meiosis metaphase I in Argyroxiphium sandwicense subsp. macrocephalum (n=14) x Dubautia menziesii (n=13), note 9 pairs and two "v"-shaped chains of 3 (left and right arrows) and one "straight" chain of three (center arrow). Even though the parental genomes are differentiated by at least two reciprocal chromosome translocations and dysploid reduction, and the pollen stainability of this hybrid is only about 11%, backcrossing occurs in the field and gene flow between the parental species apparently is realized (see hybridization).

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