Elevated genetic heterogeneity and Pleistocene climatic instability: inferences from nrDNA in New Zealand <i>Coprosma</i> (Rubiaceae)

Abstract

Aim To examine patterns of hybridization and genotype mixing within the genus Coprosma J.R.Forst. &amp; G.Forst. (Rubiaceae). Location New Zealand Methods Nucleotide sequence was determined for the internal transcribed spacer (ITS) and external transcribed spacer (ETS) regions of nuclear ribosomal DNA for fifty individuals from thirty‐six taxa within the New Zealand component of the genus Coprosma . Results Mixed sequences were found to be widespread in Coprosma . Direct sequencing of ITS polymerase chain reaction (PCR) products from seven polyploid taxa showed evidence of sequence mixtures. Cloning and sequencing of individual PCR products from two polyploids confirmed the presence of multiple templates, one of which corresponded to that of a diploid. Intra‐individual heterogeneity was also seen in a hybrid diploid taxon, with the mixed nucleotides corresponding to those of the parental lineages. Finally the ITS sequences of twenty‐two diploid taxa showed that eleven contained intra‐individual heterogeneity. Conclusions We conclude that the widespread occurrence of sequence mixtures in Coprosma results from of frequent hybridization. We also conclude that concerted evolution of the ITS and ETS regions is depressed. We propose that these characteristics evolved as a mechanism to maintain high levels of heterogeneity and suggest that this is adaptive for Coprosma in climatically unstable and physically complex New Zealand landscapes. These landscapes have been subjected to repeated oscillations between stadial and interstadial environments during the Pleistocene.