Translocations

Once we have identified which populations are most at risk from inbreeding depression, management strategies can be drawn up that will help to increase their chances of long-term survival. One of the most effective ways of slowing the decline of small, genetically depauperate populations is through the introduction of immigrants.

Genetic rescue

When migrants are translocated from one population to another, they will often introduce new alleles into the recipient population. If this results in a reduction of inbreeding depression, it is known as genetic rescue (Thrall et al., 1998). Genetic rescue will increase the growth rate of a population over multiple generations from the time when the novel genes were introduced. This is usually attributed to heterosis, which is elevated fitness in the offspring of genetically divergent individuals (sometimes known as hybrid vigour). Recall that inbreeding depression can be attributable to either dominance or overdominance. As we might expect from a process that effectively reverses inbreeding depression, heterosis can result from either the production of relatively fit heterozygous individuals or, more likely, the masking of deleterious alleles.

There are several success stories in which genetic restoration has dramatically improved the fitness of populations, possibly saving them from extinction. One extreme example of this occurred in the last remaining population of the Florida panther (Felis concolor coryi). In recent decades the effective size of this population has been around 25 and it is therefore not surprising that genetic variation, as revealed by microsatellite loci, is much lower than that found in populations of any other North American subspecies of F. concolor (Culver et al., 2000). More importantly, the population became fixed for a number of deleterious traits, including a kinked tail, extremely poor quality semen, and unilaterally undescended testicles (Roelke, Martenson and O'Brien, 1993). In 1995 a plan was initiated to introduce Texas cougars (Felis concolor stanleyana), one of the Florida panther's closest relatives, in a bid to reverse the decline of the Florida population. Eight females were introduced, and by the year 2000 four of these were still alive and a minimum of 25 Florida panther x Texas cougar descendants were thriving. By that time the population numbered around 60-70 individuals. Of the animals with Texas ancestry that have so far been evaluated, only 7 % have a kinked tail and three out of five males had two descended testicles. Semen quality was evaluated in one male and was remarkably improved. Although it is early days yet, these preliminary results are highly encouraging and suggest that the genetic rescue of the Florida panther has been successful (Land and Lacy, 2000).

Another success story is an adder (Vipera berus) population in southern Sweden that is restricted to a coastal strip of grassy meadow and has been isolated from other adder populations for at least a century. Inbreeding depression began to manifest itself in the early 1980s in the form of a number of deleterious traits that led to a high proportion of deformed or stillborn offspring. The low survival rates meant that by 1992 there were only about ten males in the population. At this time, 20 adult males from a large, genetically diverse population further north were introduced and left for four breeding seasons, after which time the surviving immigrants were recaptured and returned to their natal population. In 1999 an assessment of MHC variability showed that genetic diversity within the population had increased substantially following this introduction (Madsen, et al., 1999). The latest report was from 2003 when 39 adult males were collected, which is the largest number since the population was first monitored 23 years ago (Madsen, Ujuari and Olsson, 2004). Once again, the introduction of novel genes has ameliorated inbreeding depression and therefore increased the chance of a population's survival.

Perhaps the most striking example of genetic rescue comes from an isolated wolf pack that was discovered in southern Scandinavia more than 900 km from the nearest known wolf packs in Finland and Russia. In the 1980s there were fewer than ten individuals in this pack, but in 1991 the population started to grow exponentially and by 2001 it had expanded to between 90 and 100 individuals distributed among 10--11 packs. Researchers collected a total of 94 tissue or blood samples between 1984 and 2001 and genotyped them using mtDNA, Y-chromosome and nuclear (autosomal and X-chromosome) markers. All animals born before 1991 had the same mtDNA haplotype and the same Y-chromosome haplotype. These data, combined with very low allelic diversity at the nuclear loci, strongly suggest that the pack was founded by a single female and a single male. Between 1991 and 1992 the genetic diversity of this pack increased dramatically, when ten new alleles across 19 loci appeared in six sibling wolves that were born during that time. These siblings also carried a new Y-chromosome haplotype, showing that they had been fathered by an immigrant male who was new to the population. This single individual increased the heterozygosity of the pack from a mean of 0.49 in eight wolves that were born between 1985 and 1990 to a mean of 0.62 in 16 wolves that were born between 1991 and 1995. This increase in genetic diversity coincided with rapid population growth, and is testimony to the contribution that a single individual can make to the genetic rescue of a population (Vila et al., 2003a).

Source populations

Although translocations are often successful, care must be taken when moving individuals across large geographical distances. If the extant source population at one site shows substantial genetic differences from the extinct or endangered population at the destination site, then there is the risk that the introduction will bring with it the hazards that are often associated with invasive species. This is why biologists screened several potential donor populations in China and Russia before identifying an appropriate source of Oriental white storks (Ciconia boyciana) for re-establishing the Japanese population, which became extinct in 1986 (Murata et al., 2004). The lack of extant Japanese storks made genetic comparisons somewhat challenging, but researchers circumvented this problem by cutting small pieces of skin from 17 Japanese storks that had been stuffed and mounted on display in Toyooka City, Japan, and two nearby villages. They extracted DNA from these samples and identified mitochondrial haplotypes by sequencing a variable portion of the control region. These were compared to haplotypes from Chinese and Russian storks that had been previously used in captive breeding programmes. The maximum divergence between Japanese, Chinese and Russian storks was only 2.6 %, which is much lower than the levels of intraspecific control region sequence divergence that have been found in some other bird species; furthermore, one haplotype was found in both a Japanese and a Chinese stork, suggesting a relatively recent historic connection between the Japanese and continental populations. Finally, a maximum likelihood phylogenetic tree showed no distinction between the evolutionary lineages in Japan, China and Russia. The authors of this study therefore concluded that translocation of storks from the continent to Japan would be appropriate, at least on the basis of genetic compatibility.

Biologists may be more concerned about the genetic compatibility of source and destination populations if they are separated by large distances, although distance is not necessarily an accurate predictor of genetic differentiation. The Egyptian vulture (Neophron percnopterus) is an endangered species in Europe that is declining rapidly in many areas. Populations in the Canary and Balearic Islands are particularly vulnerable, and it has been suggested that they may benefit from future translocations. However, the island populations show significant levels of genetic differentiation from the continental European populations, and in fact the continental populations are genetically more similar to a subspecies that lives in India (N. p. ginginianus) than they are to the geographically closer island populations (Figure 7.9). It therefore seems appropriate to treat the mainland and island European populations as ESUs, and to avoid translocating birds from the mainland to the islands if possible (Kretzmann et al., 2003).

Founder effects

Successful translocation programmes must also ensure that the translocated individuals collectively harbour adequate levels of genetic diversity. This will depend partially on the number of immigrants that are introduced at any given time. Most extant populations of North American bison (Bison bison) were founded from a small number of individuals and, although genetic diversity is not universally low, it is correlated with the number of founders in each population and in some cases has left populations susceptible to inbreeding depression (Wilson and Strobeck, 1999). A relationship between inbreeding depression and number of founding individuals also emerged from a comparison

Figure 7.9 Genetic differentiation (FST) between populations of the Egyptian vulture (Neophron percnopterus) from mainland Spain, the Canary and Balearic islands and the subspecies N. p. ginginianus from India. Because populations from continental Europe are more similar to the Indian subspecies than they are to populations on the Canary and Balearic islands, translocations from Spain to these islands may be inappropriate. Data from Kretzmann et al. (2003)

Figure 7.9 Genetic differentiation (FST) between populations of the Egyptian vulture (Neophron percnopterus) from mainland Spain, the Canary and Balearic islands and the subspecies N. p. ginginianus from India. Because populations from continental Europe are more similar to the Indian subspecies than they are to populations on the Canary and Balearic islands, translocations from Spain to these islands may be inappropriate. Data from Kretzmann et al. (2003)

of 22 species of native New Zealand birds. Species that had not passed through population bottlenecks had an average hatching failure of 3.0 per cent, whereas species that had experienced bottlenecks of <150 individuals, some of which occurred during translocations, had an average hatching failure of 25.3 per cent (Briskie and Mackintosh, 2004). This finding has important practical implications because conservation programmes usually establish populations of endangered species from substantially fewer than 150 founding individuals.

It is not just the number of founders that influences the success of translocation programmes. Numerous attempts have been made to restore koala (Phascolarctos cinereus) populations to southeastern Australia following their disappearance from this region in the 1930s after they had been hunted extensively for their fur. Further north in Queensland the koalas remained common, and a well-managed translocation using individuals from Queensland populations should have minimized any accompanying founder effect. Instead, populations in mainland South Australia were restocked using individuals from Kangaroo Island, which is just off the south coast of Australia. This was unfortunate because the Kangaroo Island population was founded in the 1920s by a few individuals taken from a population on French Island that itself had been founded in the late 19th century by only two c

Re-established populations in NSW

Undisturbed population in NSW

Figure 7.10 Inbreeding coefficients in three translocated koala populations in New South Wales (NSW), Australia, compared with a single undisturbed population. Inbreeding depression in translocated populations has resulted in testicular aplasia in as many as 23.9 per cent of males. Adapted from Seymour et al. (2001)

Re-established populations in NSW

Undisturbed population in NSW

Figure 7.10 Inbreeding coefficients in three translocated koala populations in New South Wales (NSW), Australia, compared with a single undisturbed population. Inbreeding depression in translocated populations has resulted in testicular aplasia in as many as 23.9 per cent of males. Adapted from Seymour et al. (2001)

or three individuals. The South Australia population was therefore restocked by individuals whose recent genetic history featured three substantial bottlenecks. It is therefore not surprising that populations in southern Australia have significantly lower levels of genetic diversity than those further north. They are also suffering from testicular aplasia, a unilateral or bilateral failure in testicular development that is evidence of inbreeding depression (Houlden et al., 1996; Seymour et al., 2001) (Figure 7.10).

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