Phylogenetic relationships of the New Zealand Pittosporum (Pittosporaceae) inferred from ITS sequences of rDNA
Permanent link to Research Commons versionhttps://hdl.handle.net/10289/14689
Sequence data from the internal transcribed spacer (ITS) region of nuclear ribosomal DNA was collected from almost all of the New Zealand Pittosporum species, as well as representatives from other areas and- other genera within the Pittosporaceae and the outgroup Pseudopanax discolor. The variation in this region was used to examine the phylogenetic relationships of these species. The New Zealand Pittosporum taxa did not form a monophyletic group, instead they were found in two distinct clades. One clade contained P. cornifolium, P. pimeleoides subsp. pimeleoides and P. pimeleoides subsp. maius. All other taxa formed a second monophyletic group. The clade comprised of P. cornifolium and the two P. pimeleoides taxa appears to be a result of a dispersal event from New Caledonia. There is no clear indication of the ancestral source for the clade containing the rest of the New Zealand species. Pittosporum bracteolatum from Norfolk Island was found either within the main New Zealand clade, or as the sister group to it giving rise to the possibility that the main New Zealand clade and P. bracteolatum share a common Australian ancestor. Both of the New Zealand radiations appear to be relatively recent events. The sequences of P. comifolium and the two P. pimeleoides taxa are identical. The level of sequence differentiation within the main New Zealand clade . is also relatively low, especially when compared to the New Caledonian and Australian Pittosporum taxa. A nucleotide substitution rate of approximately 1 bp per million years gives an age of around 22 million years for the main New Zealand radiation, which is consistent the fossil record. The results of this study does not support previous hypothesises on relationships within the New Zealand Pittosporum based on morphology. The bivalved and trivalved species do not form monophyletic groups. Within the main New Zealand clade bivalve is ancestral and trivalve capsules have evolved twice. The species with a papery endocarp, the small leaved bivalved species and the species thought to represent a closely related group showing a reduction in size do not form monophyletic groups, and are spread out throughout the New Zealand species. The identical sequences found in Pittosporum cornifolium and P. pimeleoides, which was unexpected as these species have never been grouped based on morphology. Identical ITS sequences found in P. pimeleoides subsp. pimeleoides and P. pimeleoides subsp. maius and in P. tenuifolium subsp. tenuifolium and P. tenuifolium subsp. colensoi, and their similar morphology, supports their classification as subspecies. Pittosporum rigidum 1, P. turneri and P. anomalum had identical ITS sequences, but due to their distinct morphological difference, should retain their specific status. Two individuals of P. rigidum· were sequenced and differed by one nucleotide. Based on this the South Island, smaller leaved individuals currently called P. rigidum may need to reinstated as P. crassicaule. The Pittosporum taxon found on Kermadec Islands identified as P. crassifolium, was most closely related to P. fairchildii. Pittosporum bracteolatum from Norfolk Island is closely related to the New Zealand species, however its placement in relation to the New Zealand species varies. Four of the New Caledonian species form a distinct clade, with the fifth having no real indication of if its relationships to the other species. The Pacific species formed two clades, one appearing to be the result of a dispersal from Australia and the other from New Caledonia. Citriobatus was consistently found within the Pittosporum. Character mapping indicates that both heteroblasty and the divaricating habit have each evolved at least three times independently in the New Zealand species. The darker flower colour is the more derived character in the New Zealand Pittosporum. Because of the low level of sequence variation between most of the New Zealand taxa many relationships were unresolved. Hopefully future studies using a faster evolving marker and morphological characters will clarify these relationships.
The University of Waikato
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