INTERGENERIC HYBRIDIZATION, INDUCED POLYPLOIDY, AND THE ORIGIN OF THE HAWAIIAN ENDEMIC LIPOCHAETA FROM WEDELIA (COMPOSITAE)

The genetic relationships of the Hawaiian endemic genus Lipochaeta and the nearly cosmopolitan but extra-Hawaiian genus Wedelia were assessed by way of experimental hybridization. Hybrids between Wedelia biflora and diploid species of Lipochaeta consistently exhibited 15 pairs of chromosomes at meiosis, whereas the modal and maximum configuration seen in hybrids between W. biflora and tetraploid species of Lipochaeta was 1511 + 111. Meiotic pairing in microsporocytes of the intergeneric hybrid combinations involving Wedelia trilobata and both sections of Lipochaeta is lower than in intergeneric hybrids involving W. biflora. All of the intergeneric F1 hybrids produced had relatively low pollen stainabilities ranging from less than 1-16% and, although they were vegetatively vigorous, they failed to produce viable achenes. The effects of chromosome doubling by colchicine in one species and several hybrids in Lipochaeta and in one intergeneric hybrid between Lipochaeta and Wedelia were studied. Chromosome doubling of the diploid species caused a decrease in pollen stainability and resulted in lack of achene production, whereas doubling the chromosomes of sterile intersectional and intergeneric hybrids restored female fertility and effected a dramatic increase in pollen stainability. Cytological analysis of the microsporocytes at diakinesis and metaphase I revealed pairs and quadrivalents in the induced polyploids. Pollen measurements indicated that pollen size in Lipochaeta is correlated with doubling of the chromosomes. Statistical analyses revealed that there is no significant difference in pollen size between the diploids and natural tetraploids, whereas both groups have pollen grains that are significantly smaller than the induced autotetraploid, the intergeneric allotetraploid, and the intersectional allohexaploid. The cytogenetic evidence indicates a relatively close genetic relationship between Wedelia and Lipochaeta and supports the view that the ancestry of the diploid section Aphanopappus of Lipochaeta (n = 15) is to be found in a species akin to Wedelia biflora (n = 15) and that this or a similar 15paired wedelioid species and an unknown 11 -paired wedelioid taxon are involved in the alloploid hybrid origin of the tetraploid section Lipochaeta (n = 26) of Lipochaeta. LIPOCHAETA DC. (COMPOSITAE: HELIANTHEAE), a genus considered endemic to the Hawaiian Islands (Gardner, 1979) comprises 22 shrubby perennial species and one annual species which collectively occupy a highly diverse array of habitats ranging from the low coastal zone to steep dry or mesic slopes. Gardner (1979) recognized two sections of Lipochaeta, Aphanopappus and Lipochaeta, 1 Received for publication 26 December 1979; revision accepted 31 March 1980. This paper is part of a dissertation submitted to the Graduate Division of the University of Hawaii in partial fulfillment of the requirements for the Ph.D. degree in Botanical Sciences. The authors thank Ms. R. Leilani Stemmermann, Ms. Winona P. Char, Mr. Rick Warshauer, and Mr. Rene Sylva for their assistance in the collection of materials from the field. Financial support from the African American Institute, AFGRAD program to the senior author is gratefully acknowledged. 2 Present address: Universite de Madagascar, Establissement d'enseignement Superieur des Sciences, Service de Botanique, B.P. 906, Tananarive, Madagascar. containing 18 taxa in 17 species and nine taxa in six species, respectively. He further demonstrated that species of section Aphanopappus have 15 pairs of chromosomes, five-lobed disk corollas and produce flavonols but not flavones, whereas species of section Lipochaeta have 26 pairs of chromosomes, mostly four-lobed disk corollas and produce both flavonols and flavones. Lipochaeta has long been considered closely related to Wedelia Jacq. (Gray, 1861; Bentham and Hooker, 1873; Jones, 1905; Sherff, 1935; Becker, 1972; Gardner, 1976, 1977, 1979). The latter author pointed to the widespread Pacific basin species, Wedelia biflora, as the species most closely related to Lipochaeta. He made this suggestion on the basis of comparisons of morphology, leaf flavonoid chemistry, and chromosome number. Carr (1978) noted the wide array of chromosome numbers, ranging from n = 11 to n = 33 that have been reported for Wedelia, and argued for the origin of each section of Lipochaeta from independent intro-