Donald W. Kyhos

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.

THE INDEPENDENT ANEUPLOID ORIGIN OF TWO SPECIES OF CHAENACTIS (COMPOSITAE) FROM A COMMON ANCESTOR

Very similar species are of great interest to students of evolution because such species probably have evolved recently, making it likely that much of the evidence of their origin can be reconstructed. In very few instances of closely similar non-polyploid plant species is there evidence to suggest that one is the ancestor of another. A notable example is the cytological evidence for the origin of Crepis fuliginosa (n-= 3) from C. neglecta (n = 4) or its near ancestor (Tobgy, 1943). Similarly, Sherman (1946) showed that Crepis kotschyana (n = 4) has been derived from an ancestor like C. foetida (n 5). Another convincing example of an aneuploid species with a living diploid ancestor was described by Lewis and Roberts (1956) in their studies of Clarkia biloba (n= 8) and C. lingulata (n -9). These two species are so similar that they can only be distinguished by petal shape and chromosome number. Cytogenetic analysis showed that C. lingulata has an additional chromosome composed of parts of two chromosomes of the C. biloba genome. The presence of duplicated chromosome material in C. lingulata clearly establishes it as a derivative of C. biloba. Recently, Jackson (1962) described a case in Haplopappus that also suggests the origin of one diploid species from another. What was considered Haplopappus gracilis was found to consist of two extremely similar species, one with a chromosome number of n = 4, the other with n = 2. Cytological evidence indicates in this instance, as in Crepis, that aneuploid reduction is the most probable explanation for the difference in chromosome number, with the n = 2 species the derived one. The purpose of this paper is to present a unique example in Chaenactis (Compositae) in which the cytogenetical evidence indicates beyond reasonable doubt that an extant species of relatively mesic habitats, C. glabriuscula DC., has given rise independently to two similar desert species, C. fremontii Gray, and C. stevioides H. & A., by aneuploid reduction in chromosome number. Chaenactis comprises approximately 24 herbaceous species, all endemic to western North America. There are at least nine annual species, and four of these, C. glabriuscula, C. stevioides, C. fremontii, and C. xantiana form a closely related assemblage,2 judging from their similarity and frequent natural hybridization. The first three will be considered in detail in this paper. The yellow-flowered C. glabriuscula is the most variable, and some of its intergrading variants have been treated as separate species.3 Chaenactis stevioides and C. fremontii have white flowers, but are otherwise very similar morphologically to certain populations of C. glabriuscula. Despite their overall similarity, the gametic chromosome number of C. glabrius-

CHROMOSOME NUMBERS AND RELATIONSHIPS IN ANNONIFLORAE

Summary First reports of chromosome numbers in the following families of Annoniflorae, suborder Laurineae, are presented: Gomortegaceae (Gomortega nitida, 2n = 24), Gyrocarpaceae (Gyrocarpus americanus, n = 15), and Lactoridaceae (Lactoris fernandeziana, 2n = c. 40). For Monimiaceae, the most important basic number for subfamily Monimioideae is x = 19, while other numbers must be confirmed. The recently segregated Siparunaceae and Atherospermataceae have x = 22. Further chromosomal work in Monimiaceae s. lat. is strongly urged. Recent reports (Raven and Kyhos, 1965; Ehrendorfer, et al. 1968) have added a great deal to our knowledge of the chromosomes and relationships of the Annoniflorae (Magnoliidae, Polycarpicae, Ranales s.1.), and offer strong evidence for n =7 as the original basic chromosome number of the angiosperms (see also Stebbins, 1966). Despite these advances, certain gaps in our knowledge still remain, particularly for the large and diverse group of families treated by Thorne (1968) as the suborder Laurineae. The purpose of the present paper is to record chromosome numbers for the families Gomortegaceae, Gyrocarpaceae, and Lactoridaceae, for which chromosomal information has not previously been available, and to comment upon chromosomal evidence as it bears on relationships in this group as a whole. The most diverse of these families, both morphologically and chromosomally, has been the Monimiaceae. Recently Schodde (1970) has made a valuable contribution to our understanding of this alliance with the segregation of two families from Monimiaceae sens. lat., Atherospermataceae and Siparunaceae. Atherospermataceae, corresponding to Monimiaceae subfamily Atherospermatoideae (Money, Bailey and Swamy, 1950), consist of seven genera of tall trees of Australasia and Chile. In this group, 2n-44 has been reported for one species each of Laurelia and Daphnandra, and we can now add the same chromosome count for a second species of Daphnandra, yet to be described by R. Schodde from the Illawarra district of New South Wales (from Schodde 5183, Yellow Rock Creek, near Albion Park, N.S.W.; vouchers in CANB, NSW, DS). Ehrendorfer et al. (1968) reported 2n=c. 82 for Doryphora sassafras Endl., one of the two species of an eastern Australian genus related to Laurelia and Dryadodaphne (R. Schodde, pers. comm.). Our own studies suggested a chromosome number of 2n-c. 86 for this species (from Schodde 5183, Lyons Road, Currowan Creek, Currowan State Forest, N.S.W.; vouchers in CANB, NSW, and DS). It seems likely that the chromosome number will ultimately prove to be 2n= 88, an exact multiple of the numbers in Laurelia and