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

Abstract

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 15II + 11I. 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 F, 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 15‐paired 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.