New paper explores the origin of Chatham Islands native flora

Chatham Island forget-me-not ([i]Myosotidium hortensia[/i]) (left) shares a common ancestry with a northern hemisphere Mediterranean herb called navel wort ([i]Omphalodes verna[/i]) (right)Chatham Island forget-me-not (Myosotidium hortensia) (left) shares a common ancestry with a northern hemisphere Mediterranean herb called navel wort (Omphalodes verna) (right)A major paper (Heenan et al. 2010) examining the origin of the Chatham Islands native flora has just been published in the June 2010 issue of the New Zealand Journal of Botany. The paper written jointly by staff of Landcare Research, Department of Conservation, and Universities of Auckland, Lincoln and Otago explores the origin and diversification of 31 endemic land plants (ferns and flowering plants) of the Chatham Islands archipelago using DNA sequences pooled from a variety of independent unpublished studies.

Currently the Chatham Islands flora comprises about 900 flowering plants, ferns and fern allies of which about 412 are native. That native flora is spread over 75 plant families and 139 genera (this compares with a "mainland" New Zealand indigenous flora of 140 families and 451 genera). Currently there are 36 formally described and officially accepted plants endemic to the Chathams (meaning that they occur naturally nowhere else in the world), of these two genera, Embergeria (Chatham Islands sow thistle) and Myosotidium (Chatham Islands forget-me-not) are endemic. Both these genera are also monotypic, meaning that they are represented by just the single species each. We also recognised that there are other as yet unnamed potentially distinct endemic plants on the islands, and so it is possible that the final endemic figure for the islands may reach 50 making the islands one of the key biodiversity hot spots in the New Zealand Botanical Region!

In the past there has been considerable debate about the origins and age of the Chatham Islands Flora with botanists suggesting that the flora was derived from two parts of New Zealand "proper" - the far north and south of the two main islands. Further some scientists suggested that the flora was "ancient" (i.e. > 30 million years old) whilst others felt that it was "recent" (i.e. < 1 million years old (if not younger)).

Heenan et al.. (2010) used multiple DNA sequence data and three different molecular clock models to date the origin of and determine the age of 19 of the 36 Chatham Island endemics, and also for a further four unnamed, potentially distinct and endemic flowering plants. The authors concluded that the 19 endemics mostly diverged from their western New Zealand ancestors < 2.4 million years ago but that four (Chatham Island sow thistle, Chatham Island forget-me-not, mahoe (Melicytus chathamicus) and Chatham Island bamboo rush (Sporadanthus traversii) diverged much earlier, with molecular clock dates consistently recovered for these plants of > 3.54 million years. Of all the endemics analysed only bamboo rush had a clear relationship to northern New Zealand where its sister species S. ferrugineus is endemic, the rest had origins scattered throughout western New Zealand, except for one startlingly result - the Chatham Islands forget-me-not.

The origins and relationship of the Chatham Islands forget-me-not have been a matter of puzzlement ever since it was drawn to western scientist's attention by its formal description 164 years ago. While all botanists agree that the forget-me-not is part of the forget-me-not family (the Boraginaceae) most have suggested that its closest relative is the genus Cynoglossum. Cynoglossum are found throughout the world, indeed one species Cynoglossum amabile - the so called Chinese forget-me-not - is a common garden plant in New Zealand and is sparingly naturalised on the Chatham Islands. However there are no native species of Cynoglossum in New Zealand. Despite this opinion, Heenan et al. (2010) showed that the closest ancestors to Chatham Islands forget-me-not are species of Omphalodes and Lappula. Omphalodes in particular is recovered as a close relative; this genus has two centres of distribution, Mexico and the Mediterranean, and of those species in the genus, the closest match is Omphalodes verna - a Mediterranean species!

The authors stress that this does not necessarily mean that Chatham Island forget-me-nots arose in the Mediterranean and hitched a lift to the Chathams but rather that Myosotidium is obviously a well diverged and "old" genus with no close modern relatives bridging the gap between it and Omphalodes. It also highlights just how singular this plant is.

The results from the study also suggest that part of the Chatham endemic flora has persisted in the area for longer than geologists can confirm that emergent land has been present. While it is possible that the endemics evolved in western New Zealand first, and dispersed from there to the modern Chatham Islands, the authors favour a sequence of island hopping that they suggest has been going on in the area for the last six million years when the first clear Cenozoic contender for emergent land in the Chatham Rise appeared, the Mangere Volcano.

What is also interesting is the origins of the non-endemic native flora, which in contrast to the endemic plants shows a clear northern and southern New Zealand influence. This is especially evident in the islands mosses, liverworts and lichens. In this regard it is perhaps noteworthy that the islands only endemic moss Macromitrium ramsayae is very closely related to M. longirostre a widespread New Zealand moss; but one which is only common in southern New Zealand. Investigating the levels of genetic divergence in the non-endemic Chatham Islands flora would be a very worthwhile future study indeed.

Reference
Heenan, P.B.; Mitchell, A.D.; de Lange, P.J.; Keeling, J.; Paterson, A.M. 2010: Late Cenozoic origin and diversification of Chatham Islands endemic plant species revealed by analyses of DNA sequence data. New Zealand Journal of Botany 48: 83-136.