Gerald D. Carr

Adaptive radiation and genetic differentiation in the Hawaiian silversword alliance (Compositae: Madiinae)

The Hawaiian silversword alliance consists of the three genera Dubautia, Argyroxiphium, and Wilkesia, and is a classic example of adaptive radiation in an insular setting. Genetic variation and interspecific genetic differentiation based on ten enzyme loci are described for Dubautia and Wilkesia. Genetic identities among species span the range of values expected from interpopulation comparisons within a single species (I = 0.90–1.00) to those typical of interspecific comparisons (I→=0.67) . Genetic‐identity values correspond to biogeographic distribution and morphological distinctiveness, supporting a correlation of increasing genetic distance associated with the time of separation among lineages. It may be inferred that the high genetic identities observed within the Hawaiian Madiinae and other island plant groups are due to limited time spans available for taxa to accumulate new genetic variation through mutation. It appears that species may remain genetically similar (I > 0.90) even after time spans on the order of magnitude of 1,000,000 years.

SELF-INCOMPATIBILITY IN THE HAWAIIAN MADIINAE (COMPOSITAE): AN EXCEPTION TO BAKER'S RULE

responses of early and late successional tree seedlings on three resource gradients. Mem. Torrey Bot. Club 109:451-456. PUTWAIN, P. P., AND J. L. HARPER. 1972. Studies in the dynamics of plant populations. V. Mechanisms governing the sex ratio in Rumex acetosa and R. acetosella. J. Ecol. 60:113-129. SAKAI, A. K. 1978. Ecological and evolutionary aspects of sex expression in silver maple Acer saccharinum. Ph.D Diss. Univ. Michigan, Ann Arbor.

ADAPTIVE RADIATION IN THE HAWAIIAN SILVERSWORD ALLIANCE (COMPOSITAE-MADIINAE). I. CYTOGENETICS OF SPONTANEOUS HYBRIDS

The silversword alliance of the Hawaiian Islands comprises one of the most spectacular arrays of life forms known in a relatively small, clearly natural plant group. Perhaps the best known member of this alliance is the Haleakala silversword (Argyroxiphium macrocephalum Gray). However, A. macrocephalum is only one of six species in a genus which includes plants called greenswords as well as those called silverswords (cf. St. John, 1973). They ordinarily form large basal rosettes of attractive silver or green leaves for a number of years before they finally produce an elongated inflorescence. Argyroxiphium macrocephalum normally does not branch and dies after producing its massive inflorescence (cf. Kobayashi, 1973a). It is thus, under normal conditions, an essentially herbaceous, monocarpic plant. The floral heads of sword plants are characteristically large (>3 cm) and radiate. Argyroxiphium occurs primarily as a pioneer plant on volcanic pumice or in bogs and is restricted to the islands of Hawaii and Maui. In marked contrast to Argyroxiphium the related endemic genus Dubautia of this alliance (sensu Keck, 1936) is comprised largely of woody, branched shrubs that produce small (<1.5 cm), discoid heads year after year. However, the genus shows a truly remarkable spectrum of variation from somewhat herbaceous, low-growing forms through woody shrubs to trees. Moreover, the recently rediscovered D. latifolia (Gray) Keck from Kauai is a large liana. Altogether there are about 25 species of Dubautia, some of which, like Argyroxiphium may also be found as pioneers on volcanic pumice or in bogs, but others are found in dry forests or in rain forests. Its distribution extends throughout the entire system of major islands from Hawaii to Kauai. A third, related Hawaiian endemic genus, Wilkesia, is comprised of two species, one of which is often monocarpic and produces a rosette of leaves at the summit of a usually unbranched woody stem (cf. St. John, 1971). The second species branches rather freely from the base and presumably flowers repeatedly. The heads of Wilkesia are fairly large and discoid. Wilkesia is found on dry slopes, primarily in Waimea Canyon, only on the island of Kauai. The remarkable array of life forms within the silversword alliance exploits almost every conceivable terrestrial habitat in Hawaii. Its representatives occur from near sea level to timberline and from areas that receive less than 45 cm of annual precipitation to perhaps the wettest place on earth, receiving about 1,300 cm of annual precipitation. These habitats range from very recent lava flows on Hawaii to mature rain forests on Kauai. In spite of the spectacular morphological and ecological diversity exhibited by the members of the silversword alliance, the occurrence of many natural intergeneric, intersubgeneric, and interspecific hybrids (Sherff, 1935, 1944; Kobayashi, 1973b; and Carr, 1978a) attests to the fact that they form a natural, genetically cohesive group that has in all probability resulted from rapid evolutionary differentiation after a single colonization by a progenitor, possibly in less than 10 million years (see Macdonald and Abbott, 1970 for a discussion of Hawaiian geology). Collectively, these plants constitute what may be considered an unparalleled ex-

Adaptive radiation in the Hawaiian silversword alliance (Compositae-Madiinae). II: Cytogenetics of artificial and natural hybrids

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.

Chromosome numbers in Compositae. XVIII.

Chromosome numbers and other cytogenetic data were determined from microsporocytes in 316 collections including 13 tribes of Compositae, mostly from Africa, Australia, Mexico, Central America, and South America. First reports are provided for 66 species and the genera Cassinia (2n ≈ 14(II)), Feldstonia (2n = 11(II)), Gochnatia (2n ≈ 23(II)), and Pseudoconyza (n = 10). In addition, new chromosome numbers are established at the generic level in Acourtia, Calea, Craspedia, Gnaphalium, Helipterum, Liabum, Leucheria, Smallanthus, Trixis, and Viguiera and at the specific level in 13 additional species.

Beggar's ticks and tarweeds: Masters of adaptive radiation

The beggars ticks (Bidens) and tarweeds (Madiinae) are very much more diversified morphologically and ecologically in the Hawaiian islands than in continental areas. Isozyme data and preliminary genetic analyses suggest that the adaptive features exhibited by these groups in Hawaii are based on relatively few gene loci. Each group has mechanisms promoting outcrossing: gynodioecy in the beggars ticks, self-incompatibility in the tarweeds. While hybridization is not rare in the beggars ticks, it is very common in the tarweeds. The tarweeds provide the only known instance of chromosomal evolution among studied examples of adaptive radiation. A number of other qualitative and quantitative differences exhibited by the tarweeds suggests the possibility of their greater antiquity in Hawaii compared to beggars ticks and other groups.

ARTIFICIAL HYBRIDIZATION IN THE HAWAIIAN ENDEMIC GENUS LABORDIA (LOGANIACEAE)

Cross-pollinations were performed within and among eight species of Labordia from Oahu, Molokai, and Hawaii, and one species from Guam of the closely related genus Geniostoma. Detailed floral examination confirmed that the species are functionally dioecious, i.e., a given individual lacks either ovules or pollen grains. Female inflorescences bagged to prevent pollination never produced seed, but intraspecific crosses between male and female individuals nearly always yielded fruits with viable seed (>80%). Interspecific crosses between species from different islands and separate taxonomic sections of the genus also yielded good fruit set (>55%). Interspecific F1 hybrids were vigorous and appeared to be morphologically intermediate to their parents. Intergeneric crosses between Geniostoma and Labordia failed. Chromosome counts from Labordia species were found to be 2n = 80 or 2n =~80, twice the chromosome complement of two collections examined from its postulated ancestor, Geniostoma rupestre. Labordia species are distinct morphologically, ecologically, and geographically but apparently lack genetic barriers to interbreeding. This suggests that geographical and ecological isolation, recentness of colonization, and/or rapid speciation have been important factors in the origin of species of Labordia.

CHROMOSOME EVOLUTION AND ANEUPLOID REDUCTION IN CALYCADENIA PAUCIFLORA (ASTERACEAE)

The striking paucity of detailed studies of chromosome evolution of diploid plants is especially surprising when one considers the great wealth of information such studies can provide. In addition to their elucidation of basic mechanisms of evolution, chromosome studies sometimes provide concrete evidence of phylogenetic relationships which may serve as critical checks on more speculative trends of evolution of morphological or physiological features. In a recent paper on chromosome evolution in Chaenactis, Kyhos (1965) adequately reviewed the earlier work in this field so that it need not be repeated here. Jackson (1971) stressed the need for many more sophisticated cytogenetic analyses of karyotype change involving studies of hybrids as well as parental types in order to substantiate or correct many of the generalizations about karyotype evolution. The present investigation examines chromosome evolution in the annual tarweed genus Calycadenia to exemplify the phenomena associated with chromosomal repatterning in self-incompatible plants. Calycadenia is comprised of about 12 species (d. Keck, 1960), most of which are California endemics. Chromosome numbers of n = 4, 5, 6, 7, and 9 occur in the genus (see Carlquist, 1959), but polyploidy in natural populations has not been reported. The five species at the n = 7 level are very strongly differentiated from each

Experimental evidence for saltational chromosome evolution in Calycadenia pauciflora gray (Asteraceae)

SummaryThe fertility of the F1 structural heterozygote formed by crossing two aneuploid chromosome races of Calycadenia pauciflora is high despite the fact they are differentiated by the equivalent of three chromosome translocations. This and the fact that ancestral and derived structural homozygotes were recovered in experimental F2 and F3 progenies support the hypothesis that the derived race could have originated directly from the ancestral race in nature through a single saltational event involving multiple chromosome breaks. Two individuals with structurally unique, recombined chromosomes were also recovered in the F2 and the evolutionary potential of such products of meiosis in structural heterozygotes is considered to be significant.

Calycadenia hooveri (Asteraceae), a new tarweed from California

In accord with its cytological and morphological distinctiveness and its geographical disjunction, the Sierran element ofC. pauciflora A. Gray is described as a new species,C. hooveri. Its chromosome number ofn = 7 is documented for the first time. Evidence from cytogenetic analysis of synthetic hybrids is presented to support the conclusion thatC. hooveri is more closely related toC. villosa DC. than toC. pauciflora. Characteristics of ligule morphology and the distribution of the peculiar saucer-shaped glands on the peduncular and receptacular bracts of these species also support the new interpretation.