Local effects of inbreeding on embryo number and consequences for genetic diversity in Kerguelen mouflon

A classical paradigm in population genetics is that homozygosity or inbreeding affects individual fitness through increased disease susceptibility and mortality, and diminished breeding success. Using data from an insular population of mouflon (Ovis aries) founded by a single pair of individuals, we compare embryo number of ewes with different levels of inbreeding. Contrary to expectations, ewes with the highest levels of homozygosity showed the largest number of embryos. Using two different statistical approaches, we showed that this relationship is probably caused by heterozygosity at specific genes. The genetics of embryo number coupled with cyclic dynamics could play a central role in promoting genetic variation in this population.     

Genetic rescue and inbreeding depression in Mexican wolves

Although inbreeding can reduce individual fitness and contribute to population extinction, gene flow between inbred but unrelated populations may overcome these effects. Among extant Mexican wolves (Canis lupus baileyi), inbreeding had reduced genetic diversity and potentially lowered fitness, and as a result, three unrelated captive wolf lineages were merged beginning in 1995. We examined the effect of inbreeding and the merging of the founding lineages on three fitness traits in the captive population and on litter size in the reintroduced population. We found little evidence of inbreeding depression among captive wolves of the founding lineages, but large fitness increases, genetic rescue, for all traits examined among F1 offspring of the founding lineages. In addition, we observed strong inbreeding depression among wolves descended from F1 wolves. These results suggest a high load of deleterious alleles in the McBride lineage, the largest of the founding lineages. In the wild, reintroduced population, there were large fitness differences between McBride wolves and wolves with ancestry from two or more lineages, again indicating a genetic rescue. The low litter and pack sizes observed in the wild population are consistent with this genetic load, but it appears that there is still potential to establish vigorous wild populations.     

Salmonid inbreeding: a review

We review the published literature oninbreeding and its consequences in salmonidfishes. Inbreeding reduces genetic variationwithin populations by decreasingheterozygosity, either through an increasedchance of sharing parental genes or a loss ofalleles from random genetic drift. Increasedinbreeding is often associated with a reductionin mean phenotypic value of one or more traitswith respect to fitness (inbreedingdepression). We identify several sources ofinbreeding in salmonids. Although inbreedingoccurs naturally, much of the evidence forinbreeding stems from direct or indirectresults of human activity. The potentialconsequences of inbreeding highlight theimportance of maintaining genetic diversity insalmonid populations. Our weak understandingof genetic interactions between cultured andwild salmonids has allowed widespread practicesthat can reduce genetic variability in naturalpopulations. Although studies have detectedinbreeding depression in salmonids, its geneticbasis has rarely been addressed in wild,anadromous salmon. The genetic basis ofinbreeding depression is complex, andevaluating its effects over the entire lifecycle remains challenging. The experimentalevidence nevertheless reinforces the importanceof maintaining genetic variation withinpopulations as a primary goal of conservationand management.     

Genetic analysis, breed assignment and conservation priorities of three native Danish horse breeds

A genetic analysis was performed on three indigenous Danish horse breeds using 12 microsatellite markers from a standard kit for parental testing. These three breeds are all considered endangered based on their small population sizes. Genetic variation in these three breeds was comparable to other horse breeds in Europe, and they do not seem to be at immediate danger of extinction caused by genetic deterioration. The Knabstrupper breed had more genetic variation, as measured by expected heterozygosity and allelic richness, than the other two breeds (Frederiksborg and Jutland). F(ST) statistics and population assignments confirmed population differentiation into three distinct breeds. The Frederiksborg and Knabstrupper breeds were closer to each other than to the Jutland breed. When establishing conservation priorities for the breeds, the priorities will depend on the conservation goals. Different methods for establishing conservation priorities are also discussed.     

Effect of outbreeding on weight and growth rate of captive infant rhesus macaques

The introduction of unrelated conspecifics into captive groups of primate species is desirable to inhibit inbreeding. However, for many species, such groups increasingly represent highly inbred isolate populations that are analogous to regionally isolated populations of the same species among whom the effects of outcrossing on fitness are unknown. A study of potentially adverse effects of cross-breeding between regionally isolated populations of rhesus macaques was conducted to assess the maximum advantage or disadvantage to be expected for cross-breeding such populations of other cercopithecoid primates. For this purpose, the infant weights and rates of growth of hybrid Chinese/Indian rhesus macaques were compared to those of their nonhybrid Indian peers and to those of consanguineously inbred Indian rhesus macaques from several other captive breeding groups. Neither adverse nor advantageous effects of such hybridization were detected, suggesting that outcrossing between isolates of other cercopithecoid primates with a similar social structure and mating pattern should not affect the fitness of resulting offspring. Since levels of inbreeding are roughly inversely proportional to levels of genetic diversity within populations, such outcrossing should be intensified by zoos and other captive breeding centers to ensure the continued survival of captive species, especially those that are endangered or otherwise irreplaceable.     

A comparison of management strategies for conservation with regard to population fitness

Computer simulations have been carried out tocompare, under realistic genetic models, twomethods proposed in the literature to retaingenetic diversity in conservation programmes.In a two-step method, contributions of parentsare set up to produce minimum coancestry(kinship) among the offspring, and this isindependent of the mating system subsequentlyapplied. In a single-step method,contributions and matings are decidedsimultaneously in order to minimise coancestry.The comparison is made in terms of maintainedgenetic diversity and in terms of populationfitness. We conclude that the two methodsmaintain approximately the same geneticdiversity but the latter induces higher levelsof inbreeding, reducing the fitness of thepopulation. Avoidance of close relatives'matings improves this latter method, but thefitness levels do not reach those of thetwo-step scheme. We also investigate theperformances of different mating strategies incombination with minimum coancestry (two-stepmethod), concluding that these mating systemsdo not substantially affect the effectivenessof the management. Finally, we illustrate howminimum group coancestry can be restrictedto a minimum loss of fitness, if a measure ofthis is available for the individuals.     

The relationship between genetic variability and growth rate among populations of the pocket gopher, Thomomys bottae

Perhaps the oldest unresolved debate inconservation genetics is whether geneticvariability matters – in other words, whetherrelatively low average genetic variationcontributes to deficits in individual andpopulation level vigor and fitness. Using astatistically powerful paired sampling designin which each of three pairs of populationsconsisted of one high genetic variability andone low genetic variability population from aparticular subspecies of the pocket gopher,Thomomys bottae, we tested the hypothesisthat individuals from populations with lowergenetic variability have lower growth rates (acommonly used surrogate for fitness) than thosefrom populations with higher variability. Wemeasured genetic variability using averageallozyme heterozygosity and two measures of DNAfingerprint band sharing (Jeffreys 33.15 andMS1 probes). The population rankings of thelevels of genetic variability among the threemeasures were concordant. The least squaresmean growth rate (controlling for sex,subspecies and initial mass) of gophers fromlow variability populations (0.41 ± 0.06g/day, n = 48) was less than half that ofgophers from high variability populations (1.04± 0.07 g/day, n = 45). This result lendscredence to the premise that differences inpopulation level genetic variability havesignificant fitness consequences andunderscores the importance of maintaininggenetic variability in managed populations.