The genus Libertia Sprengel: systematics, hybridisation and evolution

Abstract

Libertia Sprengel is the only member of the Iridaceae native to New Zealand. Species are also found in Australia, New Guinea and South America. The genus is considered to be primitive, and has been placed within the tribe Sisyrinchieae of the subfamily Iridoideae. The last revision of the genus for New Zealand was carried out in 1967, and recognised four species. That revision, while basically sound, did not resolve the status of a number of taxa. This study has used morphological, anatomical, cytological, hybridisation and molecular data to investigate the status of these taxa and elucidate the relationships of the genus. Hybridisation experiments were also used to improve the range of potential cultivars of Libertia available as ornamental plants. A wide range of morphological and anatomical characters including leaf morphology and anatomy, rhizome morphology and anatomy, inflorescence and floral structures, and capsule and seed characters were investigated for Libertia species and two species from the related genera, Orthrosanthus and Sisyrinchium. The resulting phylogenetic trees indicate that the larger New Zealand species form a single clade, related to the Australian L. paniculata, and the Chilean L. formosa. Chromosome numbers in the genus were found to range from the diploid level (2n=2x=38) to the dodecaploid (2n=12x=228), with tetraploids (2n=4x=76), hexaploids (2n=6x=114) and a nonaploid (2n=9x=171) in the series. The larger New Zealand species are found to be based on the hexaploid chromosome number, with nonaploid and dodecaploid derivatives. The closest relative of the New Zealand clade, the Ausralian L. paniculata, is tetraploid, suggesting that the New Zealand species are all derived from a tetraploid. Molecular studies used RAPDs and sequencing to elucidate species relationships. The RAPD study showed that the New Zealand species are all fairly homogeneous, with only one definite polymorphism found. Sequencing using the ITS regions of the 18S-26S rDNA was unsuccessful due to the presence of multiple loci of different sizes. Sequencing using the non-transcribed portions of the trnL cpDNA gave little resolution, but broadly separated the New Zealand species into groups. Sequencing using the gene spacer of 5S rDNA provided good resolution, and this data was used to produce several cladograms, which placed the larger New Zealand taxa in one clade, agreeing with the cladograms derived from morphological data. These tees placed L. paniculata as the sister clade to the New Zealand species. The three diploid species, L. micrantha, L. pulchella and L. caerulescens were also placed together. Hybridisation results matched those from other data sets. Hexaploid and dodecaploid New Zealand species were able to cross with each other, but not with tetraploid or diploid species. In all crosses the pollen tubes reached the micropyles of the ovules, even if the cross would subsequently fail. This was presumed to be due to either a late acting interspecific incompatibility reaction or a post-fertilization barrier. Unilateral interspecific incompatibility was found between L. caerulescens and L. formosa. Despite these difficulties, a number of novel hybrids were made between New Zealand species. Crosses between Libertia species and Orthrosanthus and Sisyrinchium species failed. The above data was used to revise the genus Libertia. Three of the species (L. grandiflora, L. ixioides and L. peregrinans) recognised in 1967 are maintained, while the fourth, (L. pulchella) is separated into two species, with Australian populations remaining as L. pulchella, and New Zealand populations reverting to L. micrantha. Three new species for New Zealand, L. cranwelliae, L. edgariae and L. mooreae, and one new variety, L. mooreae var. magna, are named here. A neotype is designated for L. peregrinans. Biogeography, species concepts and phylogenetic conclusions are discussed.