Adaptive Radiation in the Hawaiian Silversword Alliance (Compositae‐madiinae). Ii. Cytogenetics of Artificial and Natural Hybrids

Abstract

The Hawaiian silversword alliance of Argyroxiphium, Dubautia, and Wilkesia, in spite of exhibiting spectacular morphological, ecological, physiological, and chromosomal diversity, is remarkably cohesive, genetically. This is attested to by the ease of production of artificial hybrids and by the high frequency of spontaneous hybridization among such life forms as mat-forming subshrub, monocarpic rosette shrub, polycarpic shrub, cushion plant, tree, and vine. Even the least fertile of these hybrids is capable of producing backcross progeny. Moreover, first generation interspecific and intergeneric hybrids have been successfully used to produce trispecific hybrids in a number of instances. In general, the widest hybrid combinations have been as readily produced as crosses within a species. At present eight genomes or chromosome races distinguished by reciprocal translocations are recognized on the basis of meiotic analysis of artificial and spontaneous hybrids. Seven of these races are found among those species with 14 pairs of chromosomes. The eighth genome very likely characterizes all nine species of this alliance that have 13 pairs of chromosomes. The cytogenetic data indicate that redundancy of translocations involving the same chromosomes has been a recurrent theme in the chromosomal differentiation of these taxa. There appears to be little, if any, correlation between chromosomal evolution and adaptive radiation as assessed by gross habital differentiation in this group. However, within Dubautia, a novel ecophysiological trait associated with colonization of xeric habitats is restricted to species with n = 13. In contrast to the bulk of the Hawaiian flora, which is characterized by self-compatibility and chromosomal stability, it is suggested that the occurrence of self-incompatibility in the Hawaiian Madiinae may have favored selection of supergenes via chromosomal repatterning, and this may account for the diversity of chromosome structure seen in this group.

Hybrids and Phylogenetic Systematics I. Patterns of Character Expression in Hybrids and Their Implications for Cladistic Analysis

“…The literature is replete with examples of hybrids that are intermediate in the same sense as the Aphelandra hybrids (for recent examples see Balick et aI., 1987;Carr and Kyhos, 1986;Hauber, 1986;Rieseberg and Warner, 1987;Sanders, 1981;Tortosa, 1988;Wanntorp, 1983;Woodruff and Gould, 1987). This kind of intermediacy was pointed to by Wagner (1968Wagner ( ,1983 in suggesting that hybrids will often possess partially derived or "hemiapomorphous" character states.…”

Extinction and recolonization of local populations on a growing shield volcano.

“…In particular, Hawaiian volcanoes have many examples of such evolutionary diversity (15)(16)(17). As volcanism continually requires the founding of new local populations, genetic shifts and/or other episodic evolutionary change would be expected to accelerate during the growth phase of each successive Hawaiian volcano.…”

Accelerated regulatory gene evolution in an adaptive radiation

“…Both cytogenetic (14) and allozyme (15) data indicate that the Hawaiian species are tetraploids (n ϭ [13][14], in contrast to the basally diploid condition (n ϭ 6-9) in the most closely related North American species within the ''Madia'' lineage (13,16). Results of our recent phylogenetic analyses using two floral homeotic genes have led us to conclude that the Hawaiian silversword alliance descended from an interspecific hybrid between members of the Anisocarpus scabridus and Carlquistia muirii lineages of North American tarweeds (16).…”