Feature

Meet My Twin and My Twin and My Twin and...

DNA studies of the endangered Chatham Island Black Robin show surprising results.

Wei Ma and David Lambert

As all New Zealanders know, the black robin (Petroica traversi) is an endangered bird which only survives on the Chatham Island. In 1979, there were only five birds left, and the famous "Old Blue" and "Old Yellow" are the ancestors of every black robin alive today. These miracle birds saved the species from the brink of extinction, with the help of conservationists like Don Merton.

The remarkable species recovery programme used inter-island transfers and cross fostering to increase the species' numbers. The black robin is currently maintaining a total population size of approximately 200 birds.

But the question has to be asked: has this population bottleneck reduced the genetic variation in black robins? This is of interest because it may have a bearing on the long-term viability of the species.

Modern DNA techniques can now be used to examine the genetic architecture of the surviving black robin population. We have used DNA fingerprinting techniques to answer this question.

Bird blood is a rich source of DNA, far richer than blood from humans, because bird red blood cells, unlike human ones, contain nuclei -- the cellular organelles that contain the chromosomes and their composite DNA. Importantly, taking blood from a bird is painless and not dangerous. Studies have shown that birds are not stressed by the experience, and only a few drops are needed to be able to harvest a large amount of DNA.

Our studies have shown that the black robin species is comprised of individuals that are about as genetically similar as identical, or monozygotic, twins. They all share essentially the same DNA profiles, as do identical twins.

There is a small amount of genetic variation in black robins, but it turns out to reflect differences between the two sexes. This is despite the fact that there are only very small morphological differences between the sexes. So there really are only two genetic types in the population -- males and females!

DNA Fingerprinting for the Birds

DNA fingerprinting, or DNA profiling as it is also called, is an invaluable tool to study bird species with. This technique involves the use of "minisatellite DNA" which consists of small stretches of DNA, about 10-60 base pairs in length. Such DNA sequences are scattered across all chromosomes of many animals and plants. If we radioactively label a piece of minisatellite DNA -- a DNA probe -- this can then be attached, or hybridised, to the DNA from other species to identify the original material.

This technique detects many different genetic loci, and consequently results in a complex "ladder" of DNA bands resembling a bar code of an item you might buy in the supermarket. Each band represents a fragment of DNA -- cut by the restriction enzymes -- containing minisatellite DNA.

Usually, the loci detected in this manner are extremely variable (that is individuals in a population are often different). Excluding identical twins, each individual's bar code is unique. On average, half the bands in the "bar codes" (the DNA fingerprint or profile) are inherited from each parent. The similarity of two profiles provides a measure of how genetically related individuals are.

In our current studies of the endangered black robin, we extracted high molecular weight DNA from 103 individuals and 67 from a closely related New Zealand bush robin (P. australis australis) population. An individual's DNA is isolated from avian erythrocytes (red blood cells) and cleaved with a restriction endonuclease. This enzyme cuts the DNA molecule at a particular position containing a specific short sequence of nucleotide bases. The resulting fragments are highly variable in length and are separated according to their size by the method of gel electrophoresis.

Once separated, the double-stranded DNA fragments are denatured into their component single strands and permanently transferred to a nylon filter membrane. Specific regions of the DNA are then detected and characterised by using a previously cloned DNA sequence as a probe.

The probe DNA is radioactively labelled, separated into single strands, and then hybridised to the complementary sequences in the restriction fragments on the nylon membrane. The radioactive probe reveals the presence of the different portions of the restriction fragments, producing identifying bands on X-ray film.

The molecular weight regions scored on DNA fingerprints of black robins and their North Island relatives were analysed. The degree of similarity among individuals can be calculated by looking at the number of bands shared between any two individuals as a proportion of the total number of bands scored in each individual. Genetically identical twins would be expected to have a bandsharing of 1.

In the sample of apparently unrelated North Island bush robins, the proportion of fragments detected by the 33.15 probe was 0.22 0.09 (SD). In contrast, when comparing chicks with their parents -- where genetic material would obviously be shared -- the result was 0.61 0.08 (SD).

The black robin population showed a considerable difference. The samples of 103 black robin individuals showed a remarkable degree of similarity. Almost every bird was shown to be virtually genetically the same. Specifically for black robins, the band-sharing coefficients is extremely high at 0.88 0.2 (SD) using the 33.15 probe; a second probe produced a score of 0.98 0.01 (SD).

Genetic Consequences

These findings show that all black robins are essentially twins, from a genetic point of view. But what do our results tell us about black robins and their possible survival in the future?

It is commonly argued by both conservationists and evolutionists alike that if a population is to survive over long periods of time, it needs to have a store of genetic variation. This pool of genes is required so that the population can adapt to an ever-changing environment. But evidence for this claim is indirect at best, even if it appeals to common sense.

Although this lack of variation in endangered black robins contradicts what is understood to be needed for a "healthy" population, the black robin population still continues to grow.

Of course it could be argued that if some new challenge to the environment appears -- a new virus or bacteria, or a new parasite or change in the environment -- then if one black robin is susceptible to this change, all the individuals will be.

This question forms the current focus of our research programme at Massey University. Whatever the outcome, the black robin will encourage us to rethink some of our assumptions about endangered species and their survival in today's world.

David Lambert conducts research in the Molecular Ecology Laboratory in the Department of Ecology at Massey University.
Wei Ma conducts research in the Molecular Ecology Laboratory in the Department of Ecology at Massey University.