number of founders, 97% of the heterozygosity of the wild panda population has been retained, or captured, in the ex situ population. This is equivalent to the proportion of heterozygosity contained in a population founded by almost 17 completely unrelated individuals (i.e. 17 founder genome equivalents; Lacy, 1989). Under ideal genetic management, which includes successfully breeding all additional potential founders (Lacy, 1995), the proportion of heterozygosity retained could be increased to almost 99%. To date, inbreeding has been avoided, and there are only two inbred animals in the population (with inbreeding coefficients of 6.3%, i.e. their parents are first cousins). The level of kinship among all individuals (average mean kinship; see also 'Priorities for the future', p. 512) is only 3%. Overall, the captive population is genetically quite well off, much more so than other highprofile species in ex situ breeding programmes, for example, the black-footed ferret (Wisely et al., 2002) and California condor (Ralls & Ballou, 2004).
Nonetheless, there is a serious challenge with significant genetic substructure from a pedigree perspective. Very few animals have been exchanged between breeding institutions, and each of the three primary breeding centres (at Beijing, Chengdu and the Wolong Nature Reserve) has founder lineages that are more- or- less unique to that institution. The Beijing and Chengdu facilities, each with representation from 12 founders, share representation from only three founders (SB 152, 174 and 202). The Chengdu and Wolong facilities, with 20 founders represented, share representation from only three founders (SB 231, 253 and 298). The Beijing and Wolong centres do not share any common founders.
This genetic fragmentation results in low levels of relatedness among institutions but high relatedness within institutions. Although the average kinship in the total population is 0.03, it is 0.08, 0.10 and 0.13 in the Wolong, Chengdu and Beijing centres, respectively. At the Beijing Zoo, animals are related on average more than half-sibs. The implication of high within-institution relatedness is that the number of genetically suitable breeding pairs at each institution is becoming limited. The proportion of all possible male/female pairings that involve related individuals (which would produce inbred offspring) is 60%, 56% and 33% at the Beijing, Chengdu and Wolong facilities, respectively. Limiting future pairings to within these institutions will only increase the level of relatedness, eventually resulting in inbreeding. Although data are not yet available on the effects of inbreeding in giant pandas, there is overwhelmingly strong scientific evidence in many other species that it is detrimental in small populations. In almost every species studied, inbreeding adversely affects population health, invariably increasing mortality and decreasing reproductive success (Ralls et al., 1988; Lacy, 1997; Keller & Waller, 2002).
An additional risk associated with this genetic fragmentation is the potential loss in genetic diversity given a catastrophic loss of animals at any one of the breeding centres, e.g. due to disease, natural disaster or other unforeseen calamity. With many founder lineages present in only one facility, loss of individuals at that facility could cause the extinction of those lineages. If offspring were transferred among centres (and other zoos), founder representation would be distributed, decreasing the risk of being lost.
In summary, while the overall genetic status of the captive population is good, gene flow among breeding centres is needed in the near future to avoid inbreeding and, thus, inbreeding depression. We next examine the potential effect of inbreeding depression in unmanaged, fragmented populations versus the benefits of a managed cooperative breeding programme.
Impact of inbreeding in managed versus unmanaged populations
To examine the influence of inbreeding depression on the probability of extinction, we modelled the ex situ giant panda population using the population viability analysis (PVA) software VORTEX (Miller & Lacy, 1999) and SIMPOP (Lacy & Ballou, 2002). Demographic and genetic parameters used for VORTEX, derived from the demographic and genetic analyses of the studbook, are presented in Table 21.2. The scenarios modelled were:
1. A small isolated population with an initial population size set at 15 individuals. This scenario was used to model the long-term effects of inbreeding in a single, small, closed institution (with no immigration or new founders) of a size similar to the current Beijing Zoo collection.
2. A larger isolated population with an initial population size set at 45 individuals. This example was developed to resemble a closed population of size similar to the current collections of the larger
Genetics and demography of the ex situ population Table 21.2. Parameters used in VORTEX and SIMPOP population viability analysesa
Proportion of females breeding Maximum breeding age Age (years) for first reproduction (males/females) Male mortality Age 0 Age 1 Age 2 Age 3 Age 4 Adult Female mortality Age 0 Age 1 Age 2 Age 3 Age 4 Adult Litter size
% litters with 1 cub % litters with 2 cubs % litters with 3 cubs Inbreeding depression
Was this article helpful?