The Genetics of the Extinct Toromiro Tree

Summary

Sophora toromiro is "Extinct in the Wild" sensu IUCN (ref.) and is endemic to the Pacific island of Rapa Nui, a territory of Chile. Thought to have become extinct the species has survived as a number of individuals in private and botanic gardens. The species is the subject of an international study to support its conservation ex situ and eventual reintroduction. As a contribution to this study the genetic diversity of the surviving stocks and herbarium samples has been assessed by means of random amplified polymorphic DNA (RAPDs and inter-SSR primers). The analysis indicates very low levels of genetic diversity confirming the species historical persistence at very low population numbers and suggesting a very low number of founders, probably between one and three.

The Genetic Status of the Toromiro Tree

The Toromiro tree, Sophora toromiro Skottsb. (Leguminosae) is endemic to the remote Pacific island of Rapa Nui (2705'S, 10920'W). It is one of an increasing number of taxa that now survive only in cultivation following extirpation of the wild populations. This study has been undertaken to ascertain the genetic status of a species that has survived as a low number of scattered individuals in botanic gardens and private collections. Currently Toromiro trees are scattered through botanic gardens, arboreta and private gardens in Europe, Australia, Chile and North America. The genetic status, and accordingly the conservation value, of these cultivated stocks is obscured through poor documentation with regard to confusion of stocks with morphologically similar congenerics and a lack of documentation pertaining to the pedigree of the stocks. There is further concern that cultivated specimens may be of hybrid origin due to open pollination.

The effective conservation of the Toromiro is dependent upon a knowledge of the taxon's surviving genetic diversity and the distribution of that genetic diversity within and between cultivated stocks. An initial isozyme study (Ricci & Eaton, 1997) assessed the taxonomic identity and relationships of some cultivated Toromiro trees in cultivated in Chile. The selection of appropriate molecular techniques was guided by the following constraints: (1) restricted access to fresh tissue and the need to utilise herbarium specimens as a source of DNA, (2) need to use parental leaf material rather than seed derived from uncontrolled pollination within a botanic garden or nursery, and (3) small volumes of leaf material available. These constraints suggested a PCR based technique. Two techniques were selected for application, namely RAPD (randomly amplified polymorphism DNA) and anchored simple sequence repeat (inter-SSR). RAPDs are a relatively cheap technique utilising agarose gels, that are regularly used in the development of species specific markers, the management and documentation of germplasm collections, genetic diversity studies of threatened species and the identification of founders for recovery planning. Inter-SSR techniques, whilst relatively expensive, have been successfully utilised as a quick technique for identifying cultivars.

Fundamental to the successful management of the Toromiro is the location of all surviving trees to allow an assessment of the remnant diversity for subsequent incorporation into a planned breeding programme. This process of "founder salvage" from ex situ collections involved an initial interrogation of the Botanic Gardens Conservation International (BGCI) database and subsequently direct correspondence with botanic gardens around the world. The search was expanded by CONAF beyond botanic gardens to cover private gardens/collections in Chile. Leaf samples were obtained from as any specimens as possible and priority for screening given to trees of unknown lineage from private collections, as it was considered most likely that these trees represented a different genetic line to the European botanic garden stocks.

The RAPD and inter-SSR data suggest that several introductions to cultivation have occurred. The phenograms indicate the following pattern of introduction: 1) the Chilean trees (Behn M28, Sudsuki M13, Vina del Mar M9 and Pitze M27) which appear to be derived from wild seed and may represent several introductions all of which may be derived from M33 through outcrossing with a conspecific individual; 2) the European botanic garden stock derived from seeds collected by Heyerdahl from the last tree in Rano Kao crater; 3) the Melbourne trees of unknown origin. The last group could be derived from early introduction to horticulture made by Skottsberg and Routledge in the early 1920s.

Phenograms

The current Toromiro stocks are closely related, however, the level and impact of in-breeding depression and/or genetic drift is unassessed. It is probable that the Toromiro has been subject to a population bottleneck for centuries, further aggravated after the collapse of traditional land use patterns in the nineteenth century. It is likely that this extended population bottleneck will have profound implications for the taxon's long term survival and required management. In contrast with other ex-situ propagation projects; the cultivated stocks of Toromiro are derived from a wild population that has not been at, or close to, a mutation-selection balance for over a century.

To ensure the successful conservation management of Toromiro a number of issues should be further investigated: (1) the most efficient use of surviving Toromiro trees to minimise the loss of current genetic diversity; (2) assessment of the impact of a prolonged population bottleneck and any associated inbreeding depression on Toromiro, and (3) ensurance of the successful reintroduction of Toromiro to Rapa Nui.

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